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Detergents, soaps and surface tension

In association with Nuffield Foundation

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Various experiments to observe the effects of detergents and soaps on the surface tension of purified and hard water

A fine insoluble powder, such as talcum powder, is sprinkled on a clean water surface in a beaker, a drop of detergent or soap solution added to the centre, and the effect observed as the surface tension of the water is changed. This can be repeated with other powders after cleaning the beaker and using fresh water samples. A needle can be carefully floated on a clean water surface and the effect of adding detergent or soap observed. Finally the same experiments can be repeated using samples of hard water to compare the effects.

This is a series of quick, simple, class experiments which can be extended or shortened as desired. Each experiment should take no more than two minutes, though the cleaning of the beaker between experiments may take up more time than expected. If a full range of experiments is desired, the time taken could amount to 30 minutes, but this may not be justified in terms of the learning objectives concerned.

• Beaker (250 cm 3 )
• Glass stirring rod
• Clean sewing needle (note 1)

Apparatus notes

• The sewing needle should be a fine needle, and for safety issued to students with the pointed end inserted into a piece of card bearing a safety warning about handling the needle.
• Talcum powder, in pepper pot or similar dispenser
• Other powders (see technical notes)
• Liquid detergent in a dropping bottle
• Liquid soap in a dropping bottle
• Access to a supply of purified water (distilled or deionised), about 1 dm 3  per working group
• Access to a supply of hard water

Health, safety and technical notes

• Read our standard health and safety guidance
• Other powders – Any powders used other than talcum powder, such as lycopodium powder or carbon powder, should be subject to a risk assessment. Lycopodium powder is a potential allergen.
• Liquid detergent – Any washing-up liquid or multipurpose detergent will suffice.
• Liquid soap - Genuine liquid soap or soap flakes from which the liquid can be made, is increasingly difficult to obtain. Wanklyn’s and Clarke’s soap solutions should still be available from chemical suppliers. Lux soap flakes are ideal for making liquid soap if you can source them. Granny’s Original and other non-branded soap flakes work fine but need to be used in solution as soon as they are made. They do not form a stable emulsion and precipitate out overnight. Note that most liquid hand washes are based on the same detergents as washing-up liquids and do not contain soap. To obtain soap solution from soap flakes – dissolve soap flakes (or shavings from a bar of soap) in ethanol – use IDA (Industrial Denatured Alcohol) (HIGHTLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040a and CLEAPSS Recipe Book RB000. Do not dilute with water.
• Hard water – A supply of hard water can be made by stirring solid calcium sulfate into a large volume of tap water, allowing to stand for some time then, after the undissolved solid has settled out, decanting the clear solution into a container suitable for students to collect their samples as required. Label as ‘Hard Water’. Allow about 1 dm 3  for each working group in the class.
• Half fill the beaker with purified water.
• Sprinkle the water surface carefully with a fine layer of powder.
• Add one drop only of detergent in the middle of the water surface. Observe what happens. Does the talcum powder stay on the surface, or does it sink?
• Clean the beaker thoroughly, half-fill again with purified water, and repeat steps two and three using a drop of liquid soap instead of detergent. Compare what happens to what happened in the previous experiment.
• Repeat steps three   and four, only this time use hard water instead of purified water. Are the results different from those obtained with purified water? If so, in what ways?
• Other powders may be available to test instead of talcum powder, to see whether the type of powder makes any difference. If you do test any of these, what differences do you find?
• Again using a clean beaker with purified water, try to float a fine sewing needle on the surface by carefully lowering it into the beaker, avoiding breaking the surface with your fingers, and dropping it from as close above the surface as possible. Once you have a needle floating, add a small drop of detergent to the water, but away from the needle. What happens?

Teaching notes

This series of brief experiments on the surface tension of water, and the effects of detergents and soaps on this, can serve as an introduction to the phenomenon of surface tension, with a discussion of results leading into simple theory. Alternatively, it could be used to illustrate prior teaching of the topic, leading to discussion of what is happening when detergents and soaps are added, including the differences found with hard water.

A diagram of the forces between water molecules at the surface and centre of a liquid.

There is a net force of attraction between the molecules of water (or any other liquid) holding the molecules together. For a molecule in the middle of the liquid, these forces, acting equally in all directions, more or less balancing out. For a molecule in the surface layer of the liquid, the forces do not balance out, and all the molecules in the surface layer are pulled towards each other and towards the bulk of the liquid. This brings these molecules closer to their neighbours until increasing forces of repulsion create a new balance, and gives rise to the phenomenon of surface tension.

When an object falls onto the surface, it has to push the water molecules apart. If the effect of the weight of the object is insufficient to match the attractive forces between molecules in the surface layer, the object will not enter the surface. Careful observation of the floating needle will show that the water surface is bent down under the weight of the needle, the surface tension causing it to behave as if the needle was supported by a flexible skin.

A diagram of the forces enabling a needle to float on water

Molecules of most detergents and soaps are long chain hydrocarbon molecules with an ionic group at one end, usually carrying a negative charge, thus making it an anion. This charge is balanced by the opposite charge of a soluble cation, for example Na + . The long hydrocarbon chains do not interact well with water molecules, and many of them are effectively ‘squeezed out’ to the interfaces between the water and the air or the glass sides of the beaker. The effect of these molecules on the water surface is to considerably weaken the forces between water molecules there, thus lowering the surface tension.

A diagram of a detergent or soap molecule, which is responsible for breaking down surface tension

When the drop of detergent is added to the powdered surface, the initial effect is to draw the powder back to the edges very rapidly as the detergent molecules form their own surface layer with a lower surface tension than the water. As the detergent gradually mixes with the water, the powder begins to sink, and a needle will now pass through the surface with ease under its own weight. However, if lycopodium powder is used, which is less dense than water, it remains at the edges. Other powders may clump into nodules if they are not wetted by the detergent solution.

A diagram showing detergent molecules in a beaker of water, some lining the surfaces and other forming clumps

In hard water there is a significant concentration of calcium, Ca 2+ , and/or magnesium, Mg 2+ , cations. These cations form an insoluble compound with soap anions, so instead of forming a surface layer, they are precipitated out, leaving the surface tension largely unchanged.

2COO − (aq) + Ca 2+ (aq) → (COO) 2 Ca(s)

However, the calcium and magnesium salts of many detergent molecules are soluble, so detergents lower the surface tension of hard water.

Additional information

This is a resource from the  Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany  Practical Physics  and  Practical Biology . 

© Nuffield Foundation and the Royal Society of Chemistry

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• Published: 30 March 2021

AIR–WATER INTERFACE

Faster chemistry at surfaces

• Robert A. Walker   ORCID: orcid.org/0000-0002-0754-6298 1  

Nature Chemistry volume  13 ,  pages 296–297 ( 2021 ) Cite this article

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The physical properties of a liquid at an interface differ from bulk solution limits, but how this affects chemical reactivity is unclear. Now, ultrafast, surface-sensitive vibrational spectroscopy has revealed that the light-induced reaction of phenol with water is four orders of magnitude faster at the water surface than in bulk.

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Understanding the enhanced catalytic activity of high entropy alloys: from theory to experiment

First published on 25th June 2021

High-entropy alloys (HEAs) have evolved to be one of the most popular materials in the last decade. Their unique configuration and attractive properties make HEAs one of the most promising catalysts. Although a very limited amount of work has been reported, higher activities of HEAs than traditional catalysts have been confirmed. This review firstly summarizes the current synthetic methods of nanostructured HEA catalysts. Then, four core effects of HEAs, namely high entropy, cocktail effect, lattice distortion and sluggish diffusion, are briefly introduced, and their impacts on the catalytic properties of HEAs are highlighted. The research progress in the application of HEAs in heterogeneous catalysis is subsequently reviewed from the perspectives of both theory and experiment. The relationships among metastable microstructures, substitution effects ( i.e. strain, ligand and ensemble effects) and d-band center are discussed in detail, and their impacts on the adsorption energy of intermediates in catalytic reactions are emphasized. We conclude the review with the discussion of the challenges and opportunities of HEA catalysts. Several directions of future HEA research are put forward. The review provides a valuable resource for those interested in these exciting catalytic materials.

1. Introduction

A promising area of research concerns the role of HEAs as heterogeneous catalysts in chemical and electrochemical reactions. The catalytic reactions are expected to be controlled by tuning the composition, surface atomic coordination and electronic configuration of HEAs. These microstructures can directly affect the interaction between intermediate species and catalysts, thus determining the catalytic activity and selectivity. Therefore, engineering the microstructures is a feasible and reasonable approach to make HEAs a promising catalyst with desirable functions. Although the field of HEAs began in 2004, 3,16 only in recent years has attention been paid to the catalytic application of HEAs. 17 Minimizing the HEA particle size to the micron or even nanometer scale is the key and challenge to the development of efficient HEA catalysts. In 2018, a thermal shock method was reported to alloy dissimilar elements into HEA nanoparticles (NPs), which greatly stimulated the blossom of HEAs in catalysis. A wide range of multicomponent nanoscale HEAs with the desired composition and size can be achieved by controlling the carbothermal shock parameters. 18

Based on the results of the limited investigations performed so far, HEAs are expected to be emerging catalytic materials in the future. In this review, the advantages and potential of HEAs as catalysts are showcased based on the current theoretical calculation and experimental results. First, we summarize the current preparation methods of nanostructured HEA catalysts in Chapter 2. The drawbacks of the current synthesis methods are highlighted. Next, the core effects of HEAs ( i.e. high entropy, cocktail effect, lattice distortion and sluggish diffusion), which play crucial roles in improving the catalytic performance, are emphasized in Chapter 3. Some unique properties of HEAs resulting from the core effects are then introduced in Chapter 4, including metastable microstructures, substitution effects ( i.e. strain, ligand and ensemble effects), d band center and adsorption energy. How these effects modify the d-band is emphasized. In particular, we discuss the mechanisms behind the remarkable catalytic activity and stability induced by these properties. Challenges associated with HEA catalysts are finally clarified, and several perspectives for future research directions and development of HEAs in the field of catalysis are suggested in Chapter 5. This work aims to be a comprehensive and critical review of general interest to communities who focus on efficient catalysts. It can provide guidance for synthesizing high-quality HEA nanoparticles with improved catalytic performance.

2. Synthesis strategies of nanostructured HEAs

2.1 carbothermal method.

Although the carbothermal shock method can craft HEA NPs with controllable composition and ultrafine size, it only produces NPs immobilized on a carbon substrate due to the extreme synthetic conditions. It is incompatible with thermally sensitive substrates such as metal, glass, and polymers.

2.2 Electrosynthesis method

Despite many advantages, this method can only produce HEA NPs in the amorphous phase. In addition, it is difficult to achieve uniform immobilization of HEA NPs on granular supports.

2.3 Solvothermal-pyrolysis method

Lu et al. developed a fast-moving bed pyrolysis (FEBP) strategy to synthesize HEA NPs on granular supports including carbon, γ-Al 2 O 3 and zeolite ( Fig. 3 ). This method was conducted at 923 K with a population speed of 20 cm s −1 , resulting in the formation of a small size (∼2 nm) of HEA NPs within 5 s. The critical size of HEA nucleus is small enough to reduce excess free energy used for forming nanoalloys, thus avoiding phase separation. 23 It is demonstrated that when the time for the precursors to reach the heating zone increases from 5 s to 20 s, obvious phase separation has been found in NPs. 24 In the fast moving bed pyrolysis strategy, all metal precursors with different reduction potentials can be decomposed at 923 K. 24 The formation of HEA NPs without phase separation is thermodynamically favored because of the low critical free energy. 23 In contrast, a slow moving bed pyrolysis causes a larger radius of nuclei, and thus results in phase separation. 25

Iversen et al. reported a solvothermal autoclave synthesis method to synthesize HEA NPs at 200 °C in an acetone–ethanol solution with metal precursors. 26 Metal chloride salts are prone to yielding the HCP phase, whereas the acetylacetonate precursors craft the FCC phase, due to their different pre-nucleation structures. 27 Homogeneous HEA NPs were generated at a temperature lower than the reduction temperature of the metals in HEAs due to autocatalyzed metal reduction at the (111) facets of the FCC phase. 28 A Pd core is initially formed which would autocatalyze the reduction of the other metals on the (111) facets. Growth along the [111] direction occurs in the initial 20 min, whereas the growth rate along the perpendicular direction is small, inducing elongation of the HEA NPs. 28 The alloying of HEAs by solvothermal reactions, governed by the reduction rates, is a kinetically driven process rather than thermodynamic energy gain of mixing. 28 However, products of solvothermal reactions are inevitably inhomogeneous comprising particles with agglomerated crystallites deviating from the main HEA phase.

2.4 Mechanical milling approach

2.5 Wet chemical synthesis

Notably, the synthesis of HEA NPs by the chemical reduction strategy is challenging, particularly, when the redox potentials of individual components differ substantially. It is prone to yielding alloy NPs with severe phase separation.

2.6 Liquid metal dealloying

It has been proven that nano-porous HEAs prepared by the dealloying technique (see Fig. 6 ) are stable under both annealing and electrochemical cycling conditions due to a thin conformal oxide coating. 38,39 Moreover, such a coating improves the catalytic activity of CO oxidation by increasing the efficiency of O 2 dissociation which is the rate-limiting step for CO oxidation. Nevertheless, the liquid metal dealloying process for the preparation of the precursor alloy is a high-cost technique due to the requirement of high temperature and an inert atmosphere.

2.7 Reactive sputter deposition

A major drawback of reactive sputtering is its complexity. Some fundamental aspects of the process have not been elucidated yet, which makes it difficult to understand the properties of the obtained HEA NPs as a function of the deposition conditions.

2.8 Pulsed laser ablation

Based on the above discussion, we summarize each synthetic strategy in Table 1 , including the driving force, experimental conditions, elements contained in HEAs, crystal phase and morphology, substrate applicability, duration time and elemental distribution. Most of the current methods require rigorous conditions including low pressure, temperature and inert atmospheric protection. Under such experimental conditions, HEA NPs are only immobilized on limited thermally resistant substrates ( e.g. carbon, γ-Al 2 O 3 , zeolite) rather than thermally sensitive ones ( e.g. metal foam, glass, polymer, which are often used in many applications). While some synthesis methods, such as electrosynthesis and pulsed laser ablation, yield HEAs under mild conditions, they either require targets with the same composition as the HEA NPs, or only craft amorphous NPs, narrowing the varieties of HEAs. On the other hand, it is clearly found that HEAs with good elemental distribution are generally synthesized in a short period of time, even microseconds. It is understandable that the high mixing entropy property of HEAs contributes to solid solution formation at high temperatures due to T Δ S mix . In the slow cooling process, segregation or precipitation of the second phase may occur due to decreased T Δ S mix , thus forming heterogeneous microstructures. A faster cooling rate can suppress the formation of secondary phases, and tend to create single-phase HEAs with good elemental distribution.

3. Core effects of HEAs

3.1 high entropy effect.

The characteristic of high mixing entropy enhances the mutual solubility among elements and facilitates the formation of simple FCC, BCC or HCP solid solution phases within HEAs during solidification ( Fig. 7c ). 50 Cantor et al. manufactured transition-metal-rich HEAs with six to nine components (the same five elements of Fe, Co, Ni, Cr, and Mn together with other elements such as Cu, Ti, Nb, Ni, Mo, Ta and Ge) in equal atomic ratio, which also forms a single FCC solid solution. 52

In principle, HEAs with a solid solution phase have many merits that can justify them as great potential catalysts. Firstly, they can be produced with wide composition ranges not available in the crystalline form, permitting the fine tuning of their electronic properties to meet catalytic reaction demands. 53 Under the definition of HEAs which consist of more than five elements, we can obtain a total of 7099 possibilities for designing equal-mole HEA systems at an arbitrary choice of a group of 13 metallic elements. 49 Unequal-mole HEAs may also be designed with minor alloying elements like AlCo 0.5 CrCuFe 1.5 Ni 1.2 B 0.1 C 0.15 for further modification of the microstructure and electronic properties. As a result, HEAs offer researchers even more room for design in terms of their composition and electronic properties than traditional alloys. Secondly, the isotropic and homogeneous characters of the alloys allow the active sites in a chemically identical environment. For HEAs, their single-phase character and lack of surface segregation of the alloying elements ensure that the catalytically active species are dispersed uniformly, which would benefit the development of the HEA catalysts with high and exclusive selectivity. 54 Thirdly, the high entropy effect has been considered to be the main reason for the stability of HEAs. The increase of the number of components significantly increases the configurational entropy, and leads to phase stability via a decrease of the Gibbs free energy. 3 It has been reported that the HCP phase of HEA catalysts (IrOsReRhRu) is still retained after heat treatment up to 1500 K and compression to 45 GPa, achieving a record temperature and pressure stability for a single-phase HEA. 55 Due to the large difference in compressibility between Os and other metals, the HEA has higher thermal expansion and lower bulk modulus in comparison with the pure metals in the compositions. 55 In the electrocatalytic oxidation of methanol, the simple-phase character and the high mixing entropy of the elements in HEAs ensure that the active sites are in a uniform dispersion in a homogeneously chemical environment, thus showing pronounced electrocatalytic activity. 55 In the nanostructures of HEA-CoMoFeNiCu, the five principal components are initially randomly assigned to each lattice site, forming a simple FCC phase. Such a solute–solution mixing phase prevents a large miscibility gap which presents in a bimetallic Co–Mo alloy. 56 As shown in Fig. 8 , the randomly mixing surface with uniform distribution of Co and Mo sites optimizes both the dehydrogenation of NH 3 molecules and the desorption of the product N 2 from the HEA surface in the catalytic decomposition reaction of NH 3 . 56

3.2 Cocktail effect

The multi-metallic cocktail effect can optimize the electronic structures of catalysts. In HEA systems containing Ni and Pd, the electron transfer from Ni to Pd could occur due to the smaller electronegativity of Ni than Pd, which can decrease the Pd–CO binding energy and enhance the catalytic oxidation of methanol molecules. 63 Moreover, the electronic states of metal atoms are altered by alloying with the d-band center of Pd shifting down and the d-band center of Ni shifting up, promoting the electrocatalytic activity of Pd towards methanol/ethanol oxidation and enhancing the adsorption of adsorbates on the Ni sites. 64 The cocktail effect can also alter the charge transfer and chemical ordering of HEAs, as shown in Fig. 9 . In an AlCoCrCuFeNi HEA, the occupied and empty Ni 3d states shift away from the Fermi level, whereas the Cr 3d empty states shift towards the Fermi level, compared to the corresponding pure metals. 65 The charge transfer between the elements in HEAs is negligible due to the compensation of the 3d state occupancy change by the redistribution of delocalized 4s and 4p states of the transition metals. 65 These properties play important roles in the optimization of the adsorption energy of HEAs during catalytic reactions.

In a word, the cocktail effect in HEAs is generally considered as a complex synergetic mechanism that is responsible for the outstanding catalytic performance of HEAs. The synergistic interactions of the multi-element compositions in HEAs have resulted in a huge divergence of the properties as compared to atoms in single-element metals. But the remarkable thing is that the mechanism of action of such multi-component synergy in HEAs remains largely unknown. The underlying synergistic mechanisms from the view point of lattice distance and electron distribution should be further explored.

3.3 Sluggish diffusion effect

The sluggish diffusion effect of HEAs can suppress the degradation caused by the coarsening of nanostructured HEA catalysts. 35,66 Despite the poor corrosion resistance of pure transition metals in strong acid or alkali solution, the high entropy alloy Ni 20 Fe 20 Mo 10 Co 35 Cr 15 composed of such elements showed high corrosion resistance in both acidic and basic electrolytes for the hydrogen evolution reaction (HER). 66 The reasons for the sluggish diffusion effect of HEAs are still controversial. Most of the researchers hold the view that multiple components are responsible for the sluggish diffusion due to low lattice-potential-energy sites provided by small atoms. 35,67 HEAs with multiple principal elements have larger fluctuations in lattice potential energy than pure metals or traditional alloys ( Fig. 10 ). Many low lattice-potential-energy sites always serve as atomic traps and blocks. 67 Thus, if an atom jumps to a low-local-potential state, it would have a low possibility to jump out. In contrast, if an atom jumps to a high-local-potential state, it would have a high chance to hop back to the initial site. Both cases can hinder atomic diffusion and particle coarsening due to the increased energy barrier and activation energy for diffusion. Meanwhile, some other researchers think that certain elements, such as Mn in CoCrFeMnNi alloy, produce a deep potential wall and thus cause a sluggish effect. 67 Therefore, a thorough study should be performed to clarify the origins of the sluggish diffusion effect of HEAs, which in turn helps researchers better design the overall architecture of high-performance HEA catalysts.

3.4 Lattice distortion effect

It was reported that the lattice distortion in an Fe-enriched alloy promoted a higher density of active electrons around the Fermi level ( Fig. 11c ). 69 The higher density of activated electrons results in faster electron transfer. In contrast, the electron transfer in alloys with a pristine lattice is quite limited due to the absence of activated electrons. Thus, the alloys with lattice distortion showed improved catalytic performance than alloys with a pristine lattice in catalysis, such as the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). 69 Moreover, some metastable structures (namely fragmented domains such as short-range order, critical defects, amorphous structures, etc. ) may form to accommodate the lattice distortion effect during the preparation of HEAs. 1 Such metastable microstructures are believed to play a key role in enhancing the catalytic performance of HEAs. Lattice distortion can also induce a residual strain field with atomic scale fluctuation, shift the d-band center of catalysts and ultimately affect the catalytic selectivity. The effects of these microstructures and strain on the catalytic performance of HEA catalysts are discussed in detail in Section 3.2 and 3.3, respectively.

In brief, the high-entropy effect simplifies the microstructures of HEAs to form simple solid solution phases, leading to the homogeneous distribution of active site configurations in catalysis. The cocktail effect causes a synthetic effect on properties, wherein the interactions among the different elements may optimize the adsorption energy of the intermediates, thereby enhancing the catalytic activity. The sluggish diffusion effect increases the activation energy and reduces the coarsening kinetics in the grain growth process. It enhances the thermal and chemical stability of HEAs in catalysis. The lattice distortion effect has a great impact on the physical and chemical properties of HEAs, which often results in severe strain due to the atomic size mismatch. Such a strain potentially shifts the d-band center of alloys, and affects the binding modes of intermediates as well as the catalytic selectivity. Moreover, some possible metastable microstructures in the solid-state solution phase of HEAs may form during the preparation process due to the lattice distortion effect, which holds promise for providing a variety of active sites in catalysis.

4. Core effect-induced properties for highly efficient HEA catalysts

4.1 Metastable microstructures

An interesting issue arises regarding the functions of the metastable microstructures in catalytic reactions. Alloys with these microstructures have coordinatively unsaturated sites which are essential for the bonding and activation of the reactants. HEAs have a high concentration of coordinatively unsaturated metal centers (active sites), which makes adsorption 83 and surface reactions 84 easier than on the corresponding crystalline catalysts. Although the effect of such microstructures on the catalytic properties of HEAs has not yet been reported in the literature, their functions in traditional metal/alloy-based catalysts have been demonstrated in detail. 85–87 It has been reported that the presence of unsaturated Ni( II ) binding sites in a nano-porous hybrid material significantly improved hydrogen sorption quantity compared with that of similar materials without unsaturated metals sites. 83 A silver catalyst with stacking faults (defects) and a low coordination number showed superior activity of the hydrogen evolution reaction that outperforms commercial platinum on carbon which is usually considered as the best catalyst. 88 These sites make the adsorption and surface reactions of catalytic intermediates easier than conventional crystalline catalysts, ensuring a high catalytic activity. Catalytic doping of alloys is an effective avenue to improve the hydrogen-storage property of MgH 2 . Compared with crystalline HEAs, amorphous counterparts exhibited better kinetics and lower activation energies during MgH 2 catalysis due to a more uniform distribution of highly refined nanostructures in the amorphous phase. 77 The amorphous alloy catalysts become brittle by absorbing hydrogen, which is beneficial to the formation of refined nanostructures. Consequently, the presence of an amorphous HEA catalyst with the refined nanostructures accelerates hydrogen diffusion in the Mg/MgH 2 matrix, thus enhancing the hydrogen storage properties of MgH 2 . In addition, as the metastable structures are nonporous, the surface reaction would not be affected by diffusion limitations, which is often a problem in traditional heterogeneous catalysis. 89 All these features make HEAs with metastable microstructures attractive in heterogeneous catalysis.

4.2 Relationships of strain, ligand and ensemble effects with the d-band of HEAs

The mechanical behavior of catalysts is one of the most important factors for the reliable and efficient catalytic reactions, and understanding the role of surface strain in tuning the reaction is critical for catalyst design. 92–97 For instance, as-exfoliated monolayered WS 2 nanosheets exhibited enhanced electrocatalytic activity for hydrogen evolution due to the high concentration of the strained metallic 1T phase. 98 In principle, strain modifies the physical/chemical properties of catalyst surfaces by changing the average energy of the d band. 99 The width of the surface d band was found to be proportional to the interatomic matrix element that describes bonding interactions. The misfit strain changes the width of the d band through changes in the d-orbital interactions between the d orbitals of a metal atom and the d orbitals of its nearest neighbors that are quite sensitive to interatomic spacing. 100 As a consequence of the d width change, the average energy of the d band (the d-band center) moves down or up relative to the Fermi energy in order to maintain a constant d-band filling, resulting in modifications of the strained surface properties. On the basis of the theoretical simulations, the d-band center of catalysts play an important role in their catalytic activity because the d-orbital electrons determine both bond formation and breaking of the intermediate species. 101,102 According to the d-band model, 103 the position of the d-band center determines the adsorption energies and activation energy barriers. The shift of the d-band center influences the bonding and anti-bonding states of adsorbates and reactants on the alloy catalyst surface, and further determines the activity and selectivity of catalytic reactions. 104 To achieve optimal catalytic activity, the d-band center must not be too close or too far from the Fermi level. Notably, the shift trend of the d-band center is reverse for late transition metals (LTMs, for which the d bands are more than half filled) and early transition metals (ETMs) with a less than half-filled d band. Fig. 14 shows the influence of tensile strain on the position of the d band in ETMs and LTMs. 105 ETMs exhibited lower adsorption energies under tensile strain because the expanded lattice reduced the overlap of the wavefunctions and therefore narrowed the metal d band, in contrast to what is observed in LTMs. 105,106 The band narrowing results in an increased population of the d band of LTMs, upshifting the d-band center to preserve the degree of d-band filling. Yan et al. proved that the influence of externally applied elastic strain on the catalytic activity of metal films in the hydrogen evolution reaction (HER) is in a controlled and predictable way. 107 The activities of Ni and Pt were accelerated by compression, while that of Cu was accelerated by tension. 107 Pt-based catalysts exhibited intensive adsorption for catalytic intermediates in the HER and ORR, which was weakened by introducing compressive strain through interface mismatch or size reduction. 108–111 It has been reported that a surface strain of −2.0% would be the best for Pt-based alloy catalysts toward the highest ORR activity. 112,113 DFT has proven that a Pt layer with −2.0% strain would lead to 30 times higher ORR activity than pure Pt. 114 In the experiment, the 3ML-Pt/Pt 25 Ni 75 (111) alloy with −1.7% strain (close to the best strain of −0.2% in theoretical predictions) displayed enhanced activities for the ORR, consistent with the theoretical results. 83 For the FeCoPdIrPt system in the HER, 24 Fe, Co and Pd could downshift the d-band center of Pt, and more electrons could occupy the antibonding states, facilitating the desorption of hydrogen species to produce more hydrogen than the commercial Pt/C catalyst.

Considering that catalytic materials are usually nanoscale and the absolute magnitude of the induced strain is very small, strain along a certain direction is defined as ( l f − l i )/ l i , where l i and l f represent the atomic bond length in the initial and final states, respectively. 105 It has been proven that the maximum catalytic activity required an optimum atomic bond length in catalysts. 115 A larger atomic bond length would cause oxygen dissociation before the adsorption in ORRs, whereas a smaller value would generate strong repulsive forces for dual-site adsorption. 115 Furthermore, an optimized surface strain can contribute to a high HER performance due to the small hydrogen adsorption Gibbs free energy on the catalytic sites. 116 Strain-induced changes in the atomic bond length can be viewed as the bulk lattice distortions in catalysts. 105 For HEAs, a serious lattice distortion effect gives rise to changes in the surface strain. The surface strain of HEAs can be tuned by selecting principal components with different atom radii. A seven-component FeNiCoSiCrAlTi HEA coating with BCC solid solution phase was prepared by laser cladding on a low carbon steel substrate. 117 The presence of the small-atomic-radius Si and large-atomic-radius Al and Ti increased the lattice packing density and crystal distortion. The segregation of Ti atom caused the different lattice expansion and growth stress between Ti-depleted polygonal grains and Ti-enriched interdendrites, which can lead to increase of the contraction stress at their interfaces. 117 A higher concentration of Al, which has a larger atomic radius than Co, Ni, Fe, and Cu elements, resulted in a larger lattice distortion of Co 25 Ni 25 Fe 25 A l7.5 Cu 17.5 with transformation of the HEA phase from FCC into a BCC structure. 118–120 This result revealed that the distribution of random elements with various atomic radii in HEAs can induce volume contraction or expansion along the specific direction. In addition, the local lattice distortion can result in the fluctuation of stacking fault (SF) energy. 121,122 The SF energy can change the number of SFs in catalysts, and thus tune their catalytic properties. 16 A high density of SFs in silver NPs caused a low coordination number and high tensile strain, transforming the non-active Ag into a highly active catalyst towards the HER. 116 The different atomic radii of the principal elements in HEAs can induce local strain effects due to the variation of SF energy. 34 SF–SF intersections produced a local strain field, leading to dislocation accumulation and SF formation in order to release the local strain concentration. 34 Based on the above mentioned analysis, it is expected that HEAs can optimize the surface strain, surface d-band width and SF density by selecting principal components with different atomic radii to achieve high catalytic activity. 123

Owing to the ligand effect, the reactivity of one metal can be varied substantially by depositing it on another due to the adjustment of the d-band. Fig. 15b displays the d-band center change of a given metal when it is deposited on another metal, calculated by DFT. 125 This helps us to design HEAs with suitable components for a specific catalytic reaction. For instance, Pt is generally used as an anode catalyst for PEM fuel cells. However, the strong binding of CO on the Pt surface leads to poisoning of the catalyst. Finding a catalyst surface that weakly binds CO is desirable. As shown in Fig. 15b , a surface with weaker CO bonds than Pt(111) can be obtained by positioning Pt on top of atoms such as Ir, Rh, Ru, Cu, Fe, and Co, due to the down-shift of the Pt d-band. 125 Likewise, if a surface with stronger adsorption energy of intermediates is required, Pt can be put on atoms such as Ag or Au to up-shift the d-band center.

As lattice strain often changes the electronic structure of alloys, the strain effect and the ligand effect are difficult to be distinguished in practical HEA catalysts. To understand the contribution of the ligand effect, Rossmeisl et al. have isolated the electronic ligand effect of the HEA catalyst (IrPdPtRhRu) from the strain effect by creating an unstrained environment in DFT to investigate its ORR activity ( Fig. 16 ). 126 After statistical analysis of 2000 DFT calculations and subsequent host/guest calculations, it has been found that selected atoms among the fourth nearest neighboring positions in the third layer of an FCC (111) metallic structure have more impact on the bond strength of an adsorbate in the ORR than any second or third nearest atomic positions. 126 It is found that the ligand effect affects both the d-band center and d-band shape which correlates closely with the bond strength of the adsorbate.

Alloys with distinct atomic ensembles result in different absorber binding strengths. When Au alloys with Pt as the catalyst in allyl alcohol hydrogenation, the H atoms are only adsorbed onto Pt sites. 127 Au atoms, which simply act as a surface diluent, have no impact on the H binding energy. PtAu catalysts exhibit a linear increase in activity with increasing Pt ratios. 127 However, when Pt atoms are replaced by Pd, H can interact with both Pd and Au atoms. Thus, the binding energy strength of H atoms on the Pd–Au atomic surface ensemble can be tuned to achieve improved hydrogenation activity. The contrasting behavior of PtAu and PdAu alloys is because different effects work in these two systems. The ensemble effect dominates the PtAu system, whereas PdAu alloys exhibit both ensemble and ligand effects. The ligand effect results in the direct charge transfer from Au to Pd atoms, leading to the d-band perturbation. These results provide valuable guidance for tuning the absorber binding energy by screening suitable components in HEAs.

Overall, the strain, ligand, ensemble effects and the d-band perturbation can significantly affect the adsorption energy of intermediates in catalytic reactions. The following section mainly discusses the relationship between the adsorption energy and catalytic activity of HEA catalysts.

4.3 Adsorption energy

The optimization of adsorption energy has also been found when a nanocrystalline AuAgPtPdCu HEA was used for the electrocatalytic reduction of CO 2 . 9 The faradaic efficiency is near 100% with respect to gaseous products at −0.3 V vs. reversible hydrogen electrode (RHE) due to a large number of catalytic sites present randomly on the surface of the HEA NPs, highlighting the uniqueness of the HEAs. 9 In CO 2 reduction, the conversion of *OCH 3 into the *O intermediate, which is an endoergic reaction, has been considered the rate-determining step due to the high barrier. DFT calculation, based on the free-energy calculations of intermediates, demonstrated that the barrier is much lower for the HEA system (1.35 eV) than for the pristine Cu(111) (1.95 eV), as shown in Fig. 18 . 9 The fact that the CO 2 reduction on the HEA NPs over the Cu(111) is thermodynamically favored should be attributed to the easier destabilization of *OCH 3 intermediates and the stronger stabilization of *O intermediates on the HEA surface than the Cu(111) surface. For stabilization of *O, both Pd 11 and Cu 7 atoms can bond O atoms on the HEA NP surface. 9 Notably, despite the presence of five elements in the HEA catalyst, the electrocatalytic activity is predominantly described by Cu atoms, and other atoms only provide a synergetic effect. 9

Electrochemical HER is a classic two-electron-transfer reaction occurring through the Volmer–Heyrovsky mechanism. This mechanism involves two steps: Volmer step (H + + e − + * → H*) and Heyrovsky step (H + + e − + H* → 0.5H 2 + *). The corresponding free energy changes of the two steps are given by Δ G Volmer = E H ads + Δ E ZPE − T Δ S H and Δ G Heyrovsky = − E H ads − (Δ E ZPE − T Δ S H ), where E H ads is the hydrogen adsorption energy, Δ E ZPE and Δ S H are the difference in zero-point energy and the entropy difference between the adsorbed state and H 2 , respectively. 133 Thus, E H ads determines the overall HER activity. It has been found that the catalytic activity shows a volcano trend as a function of the hydrogen adsorption strength on catalyst sites ( Fig. 19a ). 134 The activity can be optimized when the adsorption energy of hydrogen species on catalysts is close to 0 V due to the balance of adsorption and desorption. 97 Although Pt is a high-activity catalyst for the HER, its scarcity limits its application. HEAs are expected to decrease the loading of noble metals without losing their electrocatalytic efficiencies as the cocktail effect can optimize the adsorption strength of hydrogen. In the FeCoPdIrPt system, the combination of Co with strong adsorption and Ir with weak adsorption can moderate the free energy of H species. 24 Pd, despite its poor HER activity, can modulate the hydrogen binding energy on the Pt surface. 97 Thus, the synergic effect of the atoms in the HEA NPs leads to excellent activity towards the HER. 24 In another work, an amorphous HEA of CoFeLaNiPt has also been found to show improved electrocatalytic performance compared to the individual components during the HER due to the elemental synergisms between Pt and other components on the atomic scale ( Fig. 19b ). 6

The ORR in fuel cells usually involves four-proton–electron transfer to form H 2 O. 135 During this process, the adsorption energy of intermediates such as O, OH and OOH on the surface of catalysts determines the catalytic activity of the ORR. The activity of metals with strong adsorption of the intermediates is limited by the proton transfer to the intermediates. For metals with weak adsorption of the intermediates, oxygen is unstable on the catalyst surface, thus no transfer of protons and electrons to oxygen occurs. A theoretical volcano-like trend between the activity and intermediate adsorption energy is also observed ( Fig. 20a and b ). Although Pt is near the top of the volcano-like trend, there still exists an overpotential of ca. 0.4 V for the ORR. 136,137 Pt-based alloys can reduce the overpotential by optimizing the intermediates' adsorption energy related to the pure Pt catalyst. 138,139 A first principles study has proven that transition metals can weaken the adsorption of chemical species to the Pt, leading to higher ORR activity. 140 Fig. 20c compares the calculated free energies of the steps in the ORR on CuNiPt and CuNiPtMn HEA surface systems. 79 The generation of the OOH* intermediate is the rate-limiting step of the ORR on both catalyst surfaces. Compared with CuNiPt, the addition of Mn in CuNiPtMn modulates the electronic properties of the catalyst surface, and thus optimizes the binding of OOH through the cocktail effect, resulting in enhanced activity ( Fig. 20d and e ). 79 The cocktail effect has also been found to be applicable to design advanced ORR catalysts without Pt, such as AlCuNiAgMn, AlCuNiAgMo, and AlCuNiAgCo, which even outperform the pure Pt catalyst. 79 The fact that the cocktail effect can optimize the adsorption energy of intermediates in the ORR is also proven by density functional theory (DFT), as shown in Fig. 21 . 141 In order to achieve a highly efficient ORR reaction, *OH and *O intermediates must not be too stable on the surface of catalysts nor be easily removed. While Pt(111) is the best crystal surface of pure metal for the ORR, the adsorption energy of *OH (∼0.1 eV) on it is still too strong. 136 According to the DFT results, the surface of an HEA catalyst can offer a near-continuous distribution of adsorption energy due to the complicated surface configurations. 141 Thus, a high-efficiency ORR can be achieved by tuning the compositions of the HEA catalyst with optimal adsorption energy close to the peak of the Sabatier volcano curve. 142 The HEA materials become a design platform for new alloys by increasing sites with superior catalytic activity to pure Pt(111).

5. Conclusions and perspectives

Firstly, scientific theories for the construction of HEAs is scarce. There are a huge number of possible compositions and combinations of properties in the HEA field. Wise element design strategies for suitable compositions and structures to fit the requirements in heterogeneous catalysis thus become especially important. The rational design of HEA nanomaterials from fundamental principles has the potential to create catalysts with high activity and selectivity. 147 Until now, designing new HEA materials is based on the traditional trial-and-error method which becomes very difficult due to the large number of possible compositions for HEAs. As the combinations of composition and process for producing HEAs are numerous, each HEA has its own microstructure and properties to be identified and understood. It is very important to present basic concepts relating to HEAs in advance. Using principles of materials science is the most basic way to design a new material. This route can be used at the beginning to develop new HEA nanomaterials for desired properties by fully understanding the properties of components in materials, such as the crystal structure, atomic size, atomic weight, redox potential, electronegativity, melting and boiling points, density, electron configuration, etc. Rapid-throughput screening approaches are required to vet potential HEA compositions. For example, if we want to design low-density HEAs, more light elements should be used. If HEAs with oxidation resistance are needed, more oxidation-resistant elements such as Al, Cr, and Si should be selected. If HEAs with BCC phases are desired, Al can be added by strong binding with other elements to promote the formation of a BCC phase. In catalysis, Pd instead of Pt can be chosen as one of the components in HEAs to promote the catalytic activity due to the high electro-oxidation catalytic activity and lower price of Pd. In addition, both leverage neural network (NN) and machine learning can be employed to provide insights for rational design of HEA catalysts. The NN strategy can account for the three substitution effects of HEAs (mentioned in Fig. 13 ) for predicting the adsorption energy of catalysts. 148 Machine learning can help calculate adsorption energies of all surface sites on catalysts, allowing the optimization of the HEA compositions. 9,76 Nevertheless, there is also a risk that promising alloys might be dismissed in the early stage by these methods, due to the absence of exploration of the complex links between microstructures and properties. Certainly, careful experimental assessment of HEAs is the prerequisite to obtain the microstructure characteristics and stability.

Secondly, moderate and scalable synthesis strategies are urgently needed. Although current methods mentioned in Chapter 4 have successfully crafted HEA NPs, they generally require rigorous conditions such as high pressure, temperature and inert atmospheric protection. NPs can be only immobilized on limited thermally resistant substrates rather than thermally sensitive ones. It may incur great stoichiometric deviation due to the high vapor pressure of metal elements under these extreme conditions. The mild electrosynthesis method can only craft amorphous NPs (electrosynthesis method 6 ) or is limited by the corresponding targets (PLA approach 44 ). It is urgent to develop a more convenient technology with low energy consumption under mild conditions for synthesizing a library of HEA nanostructures.

Thirdly, although HEAs have shown great potential in catalysis applications, the understanding of the whole HEA world and how HEA materials behave in complex catalysis environments is still at the infant stage. Several future trends are pointed out: more fundamental studies on HEA structures are required . In most cases, the catalytic performance of HEA catalysts is simply explained by the synergetic effect between multiple elements. Very limited studies aim to establish an in-depth understanding of the structure–performance relationship. HEA structures with mutual interactions between different atoms, lattice distortion, metastable structures, stacking fault energy, electrical and thermal conductivity, diffusion coefficients, corrosion and oxidation are desired to fully understand the relationship between the structure and performance. Moreover, a theoretical model including microstructure details in HEAs will be instructive for establishing an accurate structure–performance relationship over HEA catalysts. Architectures in the atomic structures of HEAs can be explored in a systematic and site-specific manner. Otherwise, the possibility of comprehensively exploring such systems would be precluded. Accurate models for catalyst selections need to be built . The lack of such models makes the design of high-activity HEA catalysts difficult. The chosen descriptors determined the catalytic activities of catalysts. For example, the Sabatier principle shows a volcano-type relationship between catalytic activities and adsorption energy, whereas it is a linear relationship in the Brønsted–Evans–Polanyi behavior. 149,150 Due to the interactions of elements in alloys, the properties in terms of adsorption energy may be more complex. Establishing a descriptor suitable for HEA materials will contribute to guiding the selection of high-activity catalysts. The behavior of HEA NPs in real service environments is investigated . In the application of industrial catalysis, critical environments, such as high temperature/pressure, oxidizing/reducing conditions, and extreme pH solution, are usually involved. 151 Since it is challenging to obtain structural and compositional information of such nanoscale HEAs at spatial and temporal resolution, there is still very limited knowledge of the behavior of HEA NPs in these environments. Shahbazian-Yassar et al. employed in situ gas-cell TEM to investigate the oxidation behavior of Fe 0.28 Co 0.21 Ni 0.20 Cu 0.08 Pt 0.23 HEA NPs at 400 °C and in an atmospheric pressure air environment ( Fig. 22 ). 152 Although the overall oxidation kinetics of the HEA NPs are slower than that of both monometallic and bimetallic alloy NPs with similar principal elements, the oxidation of HEA NPs governed by the Kirkendall effect has been found involving outward diffusion of Fe, Co, Ni, and Cu to form an oxide layer with a concentration gradient. Pt stays in the HEA core region during the oxidation process due to its nonreactivity. Studies on other extreme conditions such as oxidizing/reducing conditions and extreme pH solution are also needed, yet are still scarce, but are crucial for understanding HEA behavior in these environments and providing insights in designing chemical/thermal corrosion-resistant alloys for various applications.

Fourthly, more research can be focused on high-entropy ceramics (HECs) such as nitrides, carbides, oxides, and sulfides. A few studies have indicated that four core effects of HEAs are also applicable in HECs. As some conventional phosphides/sulfides/oxides/carbides/nitrides have been proven as efficient catalysts, 153–157 HECs are expected to have promising applications in catalysis via their four core effects. Unfortunately, there are few reports on the synthesis of HEC nanomaterials, limiting their research in the field of catalysis.

Based on these deliberations, HEAs have thrust us into a new world of seemingly infinite possibility. Research on HEA-based catalysis has been attracting increasing attention. Recent studies indeed offer significant potential to improve our fundamental understanding of the catalytic behavior of HEAs. Nevertheless, future efforts should focus on key features presented by the microstructures of HEAs, rather than wandering in its limitless expanse.

Conflicts of interest

Acknowledgements.

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1st Edition

Colloid and Surface Chemistry A Laboratory Guide for Exploration of the Nano World

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Description

With principles that are shaping today’s most advanced technologies, from nanomedicine to electronic nanorobots, colloid and interface science has become a truly interdisciplinary field, integrating chemistry, physics, and biology. Colloid and Surface Chemistry: Exploration of the Nano World- Laboratory Guide explains the basic principles of colloid and interface science through experiments that emphasize the fundamentals. It bridges the gap between the underlying theory and practical applications of colloid and surface chemistry. Separated into five chapters, the book begins by addressing research methodology, how to design successful experiments, and ethics in science. It also provides practical information on data collection and analysis, keeping a laboratory notebook, and writing laboratory reports. With each section written by a distinguished researcher, chapter 2 reviews common techniques for the characterization and analysis of colloidal structures, including surface tension measurements, viscosity and rheological measurements, electrokinetic methods, scattering and diffraction techniques, and microscopy. Chapters 3–5 provide 19 experiments, each including the purpose of the experiment, background information, pre-laboratory questions, step-by-step procedures, and post-laboratory questions. Chapter 3 contains experiments about colloids and surfaces, such as sedimentation, exploration of wetting phenomena, foam stability, and preparation of miniemulsions. Chapter 4 covers various techniques for the preparation of nanoparticles, including silver, magnetic, and silica nanoparticles. Chapter 5 demonstrates daily-life applications of colloid science, describing the preparation of food colloids, body wash, and body cream.

Table of Contents

Seyda Bucak, Ph.D. , has more than 15 years of experience in chemistry laboratory practices, predominantly in colloids and surface chemistry. She has been working as an associate professor in the Department of Chemical Engineering at Yeditepe University, Istanbul, Turkey, since 2011. She is involved in teaching colloid and surface chemistry at the undergraduate and graduate levels. Currently, her main research areas are in the synthesis and applications of magnetic nanomaterials and peptide self-assembly. She has numerous publications on self-assembly, colloidal material, and nanoparticles. She has also coauthored General Chemistry Laboratory Book and Physical Chemistry Laboratory Book published by Yeditepe University Press. Deniz Rende, Ph.D. , has nearly ten years of experience in undergraduate-level chemical engineering laboratory courses. She actively participated in establishing and coordinating various undergraduate laboratories to be used in teaching general chemistry, physical chemistry, and unit operations courses in the Department of Chemical Engineering, Yeditepe University, Istanbul, Turkey. She is currently teaching as an adjunct professor and appointed as proximal probe laboratory manager in the Department of Materials Science and Engineering, Rensselaer Polytechnic Institute. Her current research involves viability and reactivity of cells in response to activated magnetic nanoparticles.

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April 28, 2016

React Fast: How Size Determines Rate

A fizzy science project from Science Buddies

By Science Buddies

Bubble up with this fun test of reaction times. See which size makes the biggest fizz!

George Retseck

Key concepts Chemistry Physics Reaction Surface area

Introduction Did you know that flour can explode? Luckily, this does not happen spontaneously on your kitchen counter, but only if the conditions are right. You need a very fine powder of flour to make an explosion happen. In fact, any solid flammable material that is dispersed in the air as a dust cloud will explode if it comes into contact with flame (a reason extreme caution must be used where there is a large amount of grain dust, such as in storage facilities). Why is that? It has to do with the particle size of the solid material, which determines how rapidly a chemical reaction takes place. In this activity, you can try this for yourself—skipping the explosion and creating a big fizz instead!

Background Some chemical reactions happen very fast (think vinegar and baking soda), whereas others take a very long time (such as rust forming on metal). In chemical reactions that include a solid as one of the reactants, you can actually change the reaction rate by varying the size of the solid that reacts with the liquid or the gas. How does this work? For a chemical reaction to happen, the molecules or atoms of the reactants need to collide with each other. This can only happen at the surface of the solid, as all the molecules trapped within the body of the solid cannot react until they meet the molecules of the other reactant. However, if you take the same material and break it into smaller pieces, there is much more surface area exposed that can interact with the other components—allowing the chemical reaction to occur much more quickly.

Surface Tension - Definition and Experiments

Understand Surface Tension in Physics

• Physics Laws, Concepts, and Principles
• Quantum Physics
• Important Physicists
• Thermodynamics
• Cosmology & Astrophysics
• Weather & Climate

Causes of Surface Tension

Examples of surface tension, anatomy of a soap bubble, pressure inside a soap bubble, pressure in a liquid drop, contact angle, capillarity, quarters in a full glass of water, floating needle, put out candle with a soap bubble, motorized paper fish.

• M.S., Mathematics Education, Indiana University
• B.A., Physics, Wabash College

Surface tension is a phenomenon in which the surface of a liquid, where the liquid is in contact with a gas, acts as a thin elastic sheet. This term is typically used only when the liquid surface is in contact with gas (such as the air). If the surface is between two liquids (such as water and oil), it is called "interface tension."

Various intermolecular forces, such as Van der Waals forces, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid, as shown in the picture to the right.

Surface tension (denoted with the Greek variable gamma ) is defined as the ratio of the surface force F to the length d along which the force acts:

gamma = F / d

Units of Surface Tension

Surface tension is measured in SI units of N/m (newton per meter), although the more common unit is the cgs unit dyn/cm (dyne per centimeter).

In order to consider the thermodynamics of the situation, it is sometimes useful to consider it in terms of work per unit area. The SI unit, in that case, is the J/m 2 (joules per meter squared). The cgs unit is erg/cm 2 .

These forces bind the surface particles together. Though this binding is weak - it's pretty easy to break the surface of a liquid after all - it does manifest in many ways.

Drops of water. When using a water dropper, the water does not flow in a continuous stream, but rather in a series of drops. The shape of the drops is caused by the surface tension of the water. The only reason the drop of water isn't completely spherical is that the force of gravity pulling down on it. In the absence of gravity, the drop would minimize the surface area in order to minimize tension, which would result in a perfectly spherical shape.

Insects walking on water. Several insects are able to walk on water, such as the water strider. Their legs are formed to distribute their weight, causing the surface of the liquid to become depressed, minimizing the potential energy to create a balance of forces so that the strider can move across the surface of the water without breaking through the surface. This is similar in concept to wearing snowshoes to walk across deep snowdrifts without your feet sinking.

Needle (or paper clip) floating on water. Even though the density of these objects is greater than water, the surface tension along the depression is enough to counteract the force of gravity pulling down on the metal object. Click on the picture to the right, then click "Next," to view a force diagram of this situation or try out the Floating Needle trick for yourself.

When you blow a soap bubble, you are creating a pressurized bubble of air which is contained within a thin, elastic surface of liquid. Most liquids cannot maintain a stable surface tension to create a bubble, which is why soap is generally used in the process ... it stabilizes the surface tension through something called the Marangoni effect.

When the bubble is blown, the surface film tends to contract. This causes the pressure inside the bubble to increase. The size of the bubble stabilizes at a size where the gas inside the bubble won't contract any further, at least without popping the bubble.

In fact, there are two liquid-gas interfaces on a soap bubble - the one on the inside of the bubble and the one on the outside of the bubble. In between the two surfaces is a thin film of liquid.

The spherical shape of a soap bubble is caused by the minimization of the surface area - for a given volume, a sphere is always the form which has the least surface area.

To consider the pressure inside the soap bubble, we consider the radius R of the bubble and also the surface tension, gamma , of the liquid (soap in this case - about 25 dyn/cm).

We begin by assuming no external pressure (which is, of course, not true, but we'll take care of that in a bit). You then consider a cross-section through the center of the bubble.

Along this cross section, ignoring the very slight difference in inner and outer radius, we know the circumference will be 2 pi R . Each inner and outer surface will have a pressure of gamma along the entire length, so the total. The total force from the surface tension (from both the inner and outer film) is, therefore, 2 gamma (2 pi R ).

Inside the bubble, however, we have a pressure p which is acting over the entire cross-section pi R 2 , resulting in a total force of p ( pi R 2 ).

Since the bubble is stable, the sum of these forces must be zero so we get:

2 gamma (2 pi R ) = p ( pi R 2 ) or p = 4 gamma / R

Obviously, this was a simplified analysis where the pressure outside the bubble was 0, but this is easily expanded to obtain the difference between the interior pressure p and the exterior pressure p e :

p - p e = 4 gamma / R

Analyzing a drop of liquid, as opposed to a soap bubble , is simpler. Instead of two surfaces, there is only the exterior surface to consider, so a factor of 2 drops out of the earlier equation (remember where we doubled the surface tension to account for two surfaces?) to yield:

p - p e = 2 gamma / R

Surface tension occurs during a gas-liquid interface, but if that interface comes in contact with a solid surface - such as the walls of a container - the interface usually curves up or down near that surface. Such a concave or convex surface shape is known as a meniscus

The contact angle, theta , is determined as shown in the picture to the right.

The contact angle can be used to determine a relationship between the liquid-solid surface tension and the liquid-gas surface tension, as follows:

gamma ls = - gamma lg cos theta

• gamma ls is the liquid-solid surface tension
• gamma lg is the liquid-gas surface tension
• theta is the contact angle

One thing to consider in this equation is that in cases where the meniscus is convex (i.e. the contact angle is greater than 90 degrees), the cosine component of this equation will be negative which means that the liquid-solid surface tension will be positive.

If, on the other hand, the meniscus is concave (i.e. dips down, so the contact angle is less than 90 degrees), then the cos theta term is positive, in which case the relationship would result in a negative liquid-solid surface tension!

What this means, essentially, is that the liquid is adhering to the walls of the container and is working to maximize the area in contact with solid surface, so as to minimize the overall potential energy.

Another effect related to water in vertical tubes is the property of capillarity, in which the surface of liquid becomes elevated or depressed within the tube in relation to the surrounding liquid. This, too, is related to the contact angle observed.

If you have a liquid in a container, and place a narrow tube (or capillary ) of radius r into the container, the vertical displacement y that will take place within the capillary is given by the following equation:

y = (2 gamma lg cos theta ) / ( dgr )

• y is the vertical displacement (up if positive, down if negative)
• d is the density of the liquid
• g is the acceleration of gravity
• r is the radius of the capillary

NOTE: Once again, if theta is greater than 90 degrees (a convex meniscus), resulting in a negative liquid-solid surface tension, the liquid level will go down compared to the surrounding level, as opposed to rising in relation to it.

Capillarity manifests in many ways in the everyday world. Paper towels absorb through capillarity. When burning a candle, the melted wax rises up the wick due to capillarity. In biology, though blood is pumped throughout the body, it is this process which distributes blood in the smallest blood vessels which are called, appropriately, capillaries .

Needed materials:

• 10 to 12 Quarters
• glass full of water

Slowly, and with a steady hand, bring the quarters one at a time to the center of the glass. Place the narrow edge of the quarter in the water and let go. (This minimizes disruption to the surface, and avoids forming unnecessary waves that can cause overflow.)

As you continue with more quarters, you will be astonished how convex the water becomes on top of the glass without overflowing!

Possible Variant: Perform this experiment with identical glasses, but use different types of coins in each glass. Use the results of how many can go in to determine a ratio of the volumes of different coins.

• fork (variant 1)
• piece of tissue paper (variant 2)
• sewing needle

Place the needle on the fork, gently lowering it into the glass of water. Carefully pull the fork out, and it is possible to leave the needle floating on the surface of the water.

This trick requires a real steady hand and some practice, because you must remove the fork in such a way that portions of the needle do not get wet ... or the needle will sink. You can rub the needle between your fingers beforehand to "oil" it increase your success chances.

Variant 2 Trick

Place the sewing needle on a small piece of tissue paper (large enough to hold the needle). The needle is placed on the tissue paper. The tissue paper will become soaked with water and sink to the bottom of the glass, leaving the needle floating on the surface.

• lit candle ( NOTE: Do not play with matches without parental approval and supervision!)
• detergent or soap-bubble solution

Place your thumb over the small end of the funnel. Carefully bring it toward the candle. Remove your thumb, and the surface tension of the soap bubble will cause it to contract, forcing air out through the funnel. The air forced out by the bubble should be enough to put out the candle.

For a somewhat related experiment, see the Rocket Balloon.

• piece of paper
• vegetable oil or liquid dishwasher detergent
• a large bowl or loaf cake pan full of water

Once you have your Paper Fish pattern cut out, place it on the water container so it floats on the surface. Put a drop of the oil or detergent in the hole in the middle of the fish.

The detergent or oil will cause the surface tension in that hole to drop. This will cause the fish to propel forward, leaving a trail of the oil as it moves across the water, not stopping until the oil has lowered the surface tension of the entire bowl.

The table below demonstrates values of surface tension obtained for different liquids at various temperatures.

Experimental Surface Tension Values

Edited by Anne Marie Helmenstine, Ph.D.

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7 Surface Tension Experiments To Try With Kids

• November 2, 2022
• Science Experiments

Here is a list of easy and fun surface tension experiments for kids. These surface tension experiments with water can help kids learn about static water and the forces within it.

Do you love the 4th of July milk fireworks?

What if you can create them using milk?

Have you noticed crazy little balls in your coffee mug while stirring it?

Well, it’s possible to recreate them! There are lots of other factors to know. So, let’s have a look at the seven science experiments that will help to understand physics in a better way while having fun at home.

1. Milk Fireworks        

This is an easy science activity that needs only a few raw materials and can prove to be a great boredom buster. 

Raw materials 

• A dish 
• Food coloring agents 
• Liquid dish soap

Required Steps 

• Take the dish and pour some milk into it.
• Now drop a few drops of coloring agents in the middle.
• Now take an earbud soaked in liquid dish soap and dip it into the middle with food coloring agents. 
• The colors get scattered in all directions like fireworks! 

Observation

Experiment Observing that adding little soap to the milk weakens its surface tension by pushing the milk molecules with its hydrophobic ends. Also, the food coloring agents are pushed along with them, and end up having a spectacular sight of  fireworks on liquids !

Note: You can conduct this experiment with milk at different temperatures such as warm and very cold to see whether this will make any difference to the behavior of the milk molecules.

2. Water BBS

This experiment demonstrates how crazy little balls notice in the coffee mug while stirring it.

• One cup of coffee
• One coffee stirrer
• Few drops of liquid soap

Required steps

• Take the coffee mug and stir it with the stirrer
• Maybe nothing will happen, and then mix a few drops of liquid soap

In this experiment, notice some little balls in the coffee mug, which are nothing but anti-bubbles. These bubbles are formed when a liquid is dropped turbulently into the same or another liquid.

These are thin films of gas enclosing a sphere of liquid that can appear and then get fully submerged in the liquid.

Unlike ordinary air bubbles, these anti-bubbles do not rise quickly on the top. Patient to see them as they are quite mesmerising.

3. Soap Boat

This science activity video on a  soap boat experiment  is all about the surface tension of water and the impact of soap on water.

• 1 dish containing water
• 1 little boat with a notch out of a card
• A few cotton buds
• Liquid soap
• Take the dish and place the little paper boat on the surface of the water
• Now, soak the cotton bud in liquid soap and touch its tip into the water to power your paper boat

In this experiment, the boat will start moving swiftly! Now, this happens when you touch the soap on the surface of the water. Soap weakens its surface tension and creates enough force to push the lightweight paper boat. Interesting to notice it!

4. Floating Card

• 1 open jar with a mesh screen on its mouth  
• 1 card 
• 1 jug of water
• Take the open jar with the mesh screen and pour water into it from the jug
• Now, take the card and place it gently in the mouth of the jar
• Invert the jar, and you will see that it will uphold the card!
• Next, gently remove your hand from the card
• Slid out the card from beneath the jar
• The jar will hold up the water mysteriously!

 Observation

Observations help to notice the  mysterious water suspension.  So, the science behind this  floating water trick  is nothing but the surface tension across the screen, which holds up the water.

There is also a role of cohesion to play in this science activity. It is the cohesion that causes surface tension. Here, water molecules remain joined together between each tiny opening of the screen mesh and form a thin invisible membrane that is strong enough to hold the water when the jar is inverted.

You can even stick some needles inside the jar! Interestingly, the surface tension will successfully prevent the water from falling in that case too! 

You can use this experiment as a magic trick before your friends and can, later on, explain to them the science behind the water suspension.

5. Suddenly sinking paper clips

This science activity video on  paper clip floating  and sinking is again about the surface tension of water.

• 1 glass containing water
• 1 paper clip
• 1 piece of tissue paper
• A small quantity of liquid soap
• Take the paper clip and place it on top of the water surface of the glass
• Try to balance it on the water surface
• If it sinks, take it out from the glass
• Now, place the piece of small tissue paper on the water surface and then put the paper clip on it
• Next, gently remove the tissue paper from beneath the paper clip as it will start floating on the water surface
• Now take the Q-tip and soak it in liquid soap and touch its tip into the water
• The paper clip will again sink at the bottom! 

Now, wondering why is the  paper clip floating on water soap?  Well, the reason is again the humble surface tension! 

In the second step, then try to make the paper clip float on the water surface, it sinks because the metal with which the clip is made is denser than the water. 

However, when placing it on a piece of floating tissue paper, it does not sink because now the surface tension of the water is supporting it.

Again, when you touch the water with soap, this surface tension gets reduced. So, the clip sinks like a brick into the glass. 

Also, let’s experiment with this interesting activity with different lightweight objects to see whether the same thing is happening again. 

6. Penny Dropper

Have you ever wondered  how many drops of water can fit on a penny ?

Well, this super fun science activity will give all the answers. 

• 1 plastic dropper 
• Take the penny and place it on a flat surface
• Now take the dropper, fill it with water, and put a drop of water at the center of your penny
• Keep on adding water drops to the penny and count
• A dome shape made up of water drops will form on the penny 

The experiment makes us observe that a penny can hold several water drops before it eventually starts spilling over the coin. Here, it is the surface tension of water that prevents the water molecules from falling apart. So, the water molecules remain together and form a dome shape. Even Experimenting with other liquids such as saltwater, milk, and soapy water to figure out whether they yield the same result or not.

7. Leidenfrost Effect        

Have you ever heard about the  Leindenfrost effect ?

Well, it is a phenomenon where liquids, instead of getting evaporated, glided on the surface of a pan. This happens when the pan is heated beyond the boiling point of that liquid. 

This effect was named after the German doctor Johann Gottlob Leidenfrost (1715-1794), who described this effect. 

However, to do this exciting science experiment, you will need adult supervision as this involves heat hazards! 

• 1 empty pan
• One dropper
• Take the empty pan and put it on a stove.
• Next, add some water droplets into the pan one by one with the help of the dropper, and the water droplets will quickly evaporate. 
• Keep on adding the water droplets but now increase your speed. 
• Water droplets will now not evaporate. They will instead make small spheres gliding on the hot surface of the pan.

The observation makes us wonder how  does water dance on a hot pan . See, when heating the pan more than the boiling point of water, which is 100-degree Celsius, water drops vaporize quickly that it forms a layer of steam that insulates the rest of the water droplets are added from the hot surface of the pan. As a result, you end up watching the dancing water droplets. 

All the above activities can be done at home to develop a better understanding of some key concepts of physics. 

Dianna Cowern, also called physics girl presented a video of seven experiments or science tricks that offer surface tension, anti-bubble, cohesion, and lienenforst effect.

Courtesy: Physics Girl

https://www.youtube.com/watch?v=WsksFbFZeeU&feature=emb_logo

https://www.antibubble.org/

https://www.stevespanglerscience.com/lab/experiments/water-screen/

https://msdlt.instructure.com/courses/108/files/2571/download?wrap=1

https://www.rookieparenting.com/how-many-drops-of-water-can-you-put-on-a-penny/

https://www.sociologygroup.com/water-drops-dance-hot-plate/

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Surface Tension of Water Demonstration

April 17, 2019 By Emma Vanstone 3 Comments

These super simple investigations are great for demonstrating the surface tension of water .

What is surface tension?

Surface tension  is a force which causes a layer of liquid to behave like an elastic sheet or skin.

Molecules of water are more attracted to each other than other molecules, as water is a polar molecule. The positive hydrogen end of one molecule is attracted to the negative oxygen end of another water molecule. The surface water molecules only have air above them, so they are pulled down, creating surface tension.

The high surface tension of water allows insects to walk over it. Pond skaters have long, hairy legs, allowing them to spread their weight over a wide area. They press very gently on the water’s surface so as not to break through it.

In a container of water, molecules below the surface are pulled together ( or attracted to each other ) equally in all directions, but those on top are pulled together more tightly, as they don’t have water molecules above them; this draws them together to form a ‘skin’. It is this skin ( surface tension ) that stops items on the surface from sinking.

Surface Tension Holes Experiment

You’ll need.

A big bowl of water

Some ground pepper (black so you can see it) or any other ground product with colour

Washing up liquid ( dish soap )

Once the water settles, sprinkle the ground pepper over the top.

Drip some washing-up liqu id in the middle of the bowl and watch what happens.

A hole appears in the centre as the pepper moves outwards. This is your surface tension hole !

If you want to repeat the demonstration, you’ll need to wash out the bowl thoroughly to remove any traces of the dish soap ( washing up liquid ), or the effect won’t be as dramatic.

Why does this happen?

The surface tension hole is caused by the washing up liquid reducing the surface tension of the water. This allows the particles of water at the surface to spread out, starting from where the washing-up liquid was added.

More Surface Tension Experiments for Kids

Frugal Fun for Boys has an excellent surface tension investigation using a coin and different liquids !

You can use washing-up liquid to disrupt the surface tension of water to race lolly sticks .

In a magic milk experiment , the washing up liquid disrupts the surface tension of the milk, which makes food colouring spread out just like the pepper and water.

Another surface tension experiment is where you make a shape on the surface of the water with cocktail sticks and drop some washing-up liquid in the centre to force the sticks apart.

Watch how water behaves on the space station with this NASA video.

Try filling a bowl half full with water and carefully placing a paperclip on the top, so it floats. Mix a little washing-up liquid in a cup with water and gently pour it into the bowl; the paper clip will sink as the water can no longer support the weight of the paper clip after the washing-up liquid disrupts the surface tension of the water.

Science concepts

Surface tension

Last Updated on July 8, 2023 by Emma Vanstone

Safety Notice

Science Sparks ( Wild Sparks Enterprises Ltd ) are not liable for the actions of activity of any person who uses the information in this resource or in any of the suggested further resources. Science Sparks assume no liability with regard to injuries or damage to property that may occur as a result of using the information and carrying out the practical activities contained in this resource or in any of the suggested further resources.

These activities are designed to be carried out by children working with a parent, guardian or other appropriate adult. The adult involved is fully responsible for ensuring that the activities are carried out safely.

Reader Interactions

October 16, 2011 at 3:00 pm

Great activity, I am going to try it with my daughter! I love how you call it “washing up liquid” – I call it that too. 🙂

October 16, 2011 at 9:26 pm

Thanks, glad you like it!

October 21, 2011 at 6:01 pm

So many great ideas come form this blog! Thank you for linking up to the The Sunday Showcase

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68 Best Chemistry Experiments: Learn About Chemical Reactions

Whether you’re a student eager to explore the wonders of chemical reactions or a teacher seeking to inspire and engage your students, we’ve compiled a curated list of the top 68 chemistry experiments so you can learn about chemical reactions.

While the theories and laws governing chemistry can sometimes feel abstract, experiments bridge the gap between these concepts and their tangible manifestations. These experiments provide hands-on experiences illuminating the intricacies of chemical reactions, molecular structures, and elemental properties.

1. Covalent Bonds

By engaging in activities that demonstrate the formation and properties of covalent bonds, students can grasp the significance of these bonds in holding atoms together and shaping the world around us.

Learn more: Covalent Bonds

2. Sulfuric Acid and Sugar Demonstration

Through this experiment, students can develop a deeper understanding of chemical properties, appreciate the power of chemical reactions, and ignite their passion for scientific exploration.

3. Make Hot Ice at Home

Making hot ice at home is a fascinating chemistry experiment that allows students to witness the captivating transformation of a liquid into a solid with a surprising twist.

4. Make a Bouncing Polymer Ball

This hands-on activity not only allows students to explore the fascinating properties of polymers but also encourages experimentation and creativity.

Learn more: Thought Co

5. Diffusion Watercolor Art

This experiment offers a wonderful opportunity for students to explore the properties of pigments, observe how they interact with water, and discover the mesmerizing patterns and textures that emerge.

Learn more: Diffusion Watercolor Art

6. Exploding Baggie

The exploding baggie experiment is a captivating and dynamic demonstration that students should engage in with caution and under the supervision of a qualified instructor.

Learn more: Exploding Baggie

7. Color Changing Chemistry Clock

This experiment not only engages students in the world of chemical kinetics but also introduces them to the concept of a chemical clock, where the color change acts as a timekeeping mechanism.

Learn more: Color Changing Chemistry Clock

8. Pipe Cleaner Crystal Trees

By adjusting the concentration of the Borax solution or experimenting with different pipe cleaner arrangements, students can customize their crystal trees and observe how it affects the growth patterns.

Learn more: Pipe Cleaner Crystal Trees

9. How To Make Ice Sculptures

Through this experiment, students gain a deeper understanding of the physical and chemical changes that occur when water freezes and melts.

Learn more: Ice Sculpture

10. How to Make Paper

Through this hands-on activity, students gain a deeper understanding of the properties of cellulose fibers and the transformative power of chemical reactions.

Learn more: How to Make Paper

11. Color Changing Chemistry

Color changing chemistry is an enchanting experiment that offers a captivating blend of science and art. Students should embark on this colorful journey to witness the mesmerizing transformations of chemicals and explore the principles of chemical reactions.

12. Gassy Banana

The gassy banana experiment is a fun and interactive way for students to explore the principles of chemical reactions and gas production.

Learn more: Gassy Banana

13. Gingerbread Man Chemistry Experiment

This hands-on activity not only introduces students to the concepts of chemical leavening and heat-induced reactions but also allows for creativity in decorating and personalizing their gingerbread creations.

Learn more: Gingerbread Man Chemistry Experiment

14. Make Amortentia Potion

While the love potion is fictional, this activity offers a chance to explore the art of potion-making and the chemistry behind it.

Learn more: How to Make Amortentia Potion

15. Strawberry DNA Extraction

This hands-on experiment offers a unique opportunity to observe DNA, the building blocks of life, up close and learn about its structure and properties.

16. Melting Snowman

The melting snowman experiment is a fun and whimsical activity that allows students to explore the principles of heat transfer and phase changes.

Learn more: Melting Snowman

17. Acid Base Cabbage Juice

The acid-base cabbage juice experiment is an engaging and colorful activity that allows students to explore the pH scale and the properties of acids and bases.

By extracting the purple pigment from red cabbage leaves and creating cabbage juice, students can use this natural indicator to identify and differentiate between acidic and basic substances.

Learn more: Acid Base Cabbage Juice

18. Magic Milk

The magic milk experiment is a mesmerizing and educational activity that allows students to explore the concepts of surface tension and chemical reactions.

By adding drops of different food colors to a dish of milk and then introducing a small amount of dish soap, students can witness a captivating display of swirling colors and patterns.

Learn more: Magic Milk

19. Melting Ice with Salt and Water

Through this hands-on activity, students can gain a deeper understanding of the science behind de-icing and how different substances can influence the physical properties of water.

Learn more: Melting Ice with Salt and Water

20. Barking Dog Chemistry Demonstration

The barking dog chemistry demonstration is an exciting and visually captivating experiment that showcases the principles of combustion and gas production.

21. How to Make Egg Geodes

Making egg geodes is a fascinating and creative chemistry experiment that students should try. By using common materials like eggshells, salt, and food coloring, students can create their own beautiful geode-like crystals.

Learn more: How to Make Egg Geodes

22. Make Sherbet

This experiment not only engages the taste buds but also introduces concepts of acidity, solubility, and the chemical reactions that occur when the sherbet comes into contact with moisture.

Learn more: Make Sherbet

23. Hatch a Baking Soda Dinosaur Egg

As the baking soda dries and hardens around the toy, it forms a “shell” resembling a dinosaur egg. To hatch the egg, students can pour vinegar onto the shell, causing a chemical reaction that produces carbon dioxide gas.

Learn more: Steam Powered Family

24. Chromatography Flowers

By analyzing the resulting patterns, students can gain insights into the different pigments present in flowers and the science behind their colors.

Learn more: Chromatography Flowers

25. Turn Juice Into Solid

Turning juice into a solid through gelification is an engaging and educational chemistry experiment that students should try. By exploring the transformation of a liquid into a solid, students can gain insights of chemical reactions and molecular interactions.

Learn more: Turn Juice into Solid

26. Bouncy Balls

Making bouncy balls allows students to explore the fascinating properties of polymers, such as their ability to stretch and rebound.

 27. Make a Lemon Battery

Creating a lemon battery is a captivating and hands-on experiment that allows students to explore the fundamentals of electricity and chemical reactions.

28. Mentos and Soda Project

The Mentos and soda project is a thrilling and explosive experiment that students should try. By dropping Mentos candies into a bottle of carbonated soda, an exciting eruption occurs.

29. Alkali Metal in Water

The reaction of alkali metals with water is a fascinating and visually captivating chemistry demonstration.

30. Rainbow Flame

The rainbow flame experiment is a captivating and visually stunning chemistry demonstration that students should explore.

31. Sugar Yeast Experiment

This experiment not only introduces students to the concept of fermentation but also allows them to witness the effects of a living organism, yeast, on the sugar substrate.

32. The Thermite Reaction

The thermite reaction is a highly energetic and visually striking chemical reaction that students can explore with caution and under proper supervision.

This experiment showcases the principles of exothermic reactions, oxidation-reduction, and the high temperatures that can be achieved through chemical reactions.

33. Polishing Pennies

Polishing pennies is a simple and enjoyable chemistry experiment that allows students to explore the concepts of oxidation and cleaning methods.

34. Elephant Toothpaste

The elephant toothpaste experiment is a thrilling and visually captivating chemistry demonstration that students should try with caution and under the guidance of a knowledgeable instructor.

35. Magic Potion

Creating a magic potion is an exciting and imaginative activity that allows students to explore their creativity while learning about the principles of chemistry.

36. Color Changing Acid-Base Experiment

Through the color changing acid-base experiment, students can gain a deeper understanding of chemical reactions and the role of pH in our daily lives.

Learn more: Color Changing Acid-Base Experiment

37. Fill up a Balloon

Filling up a balloon is a simple and enjoyable physics experiment that demonstrates the properties of air pressure. By blowing air into a balloon, you can observe how the balloon expands and becomes inflated.

38. Jello and Vinegar

The combination of Jello and vinegar is a fascinating and tasty chemistry experiment that demonstrates the effects of acid on a gelatin-based substance.

Learn more: Jello and Vinegar

39. Vinegar and Steel Wool Reaction

This experiment not only provides a visual demonstration of the oxidation process but also introduces students to the concept of corrosion and the role of acids in accelerating the process.

Learn more: Vinegar and Steel Wool Reaction

40. Dancing Rice

The dancing rice experiment is a captivating and educational demonstration that showcases the principles of density and buoyancy.

By pouring a small amount of uncooked rice into a clear container filled with water, students can witness the rice grains moving and “dancing” in the water.

Learn more: Dancing Rice

41. Soil Testing Garden Science

Soil testing is a valuable and informative experiment that allows students to assess the composition and properties of soil.

By collecting soil samples from different locations and analyzing them, students can gain insights into the nutrient content, pH level, and texture of the soil.

Learn more: Soil Testing Garden Science

42. Heat Sensitive Color Changing Slime

Creating heat-sensitive color-changing slime is a captivating and playful chemistry experiment that students should try.

Learn more: Left Brain Craft Brain

43. Experimenting with Viscosity

Experimenting with viscosity is an engaging and hands-on activity that allows students to explore the flow properties of liquids.

Viscosity refers to a liquid’s resistance to flow, and this experiment enables students to investigate how different factors affect viscosity.

Learn more: Experimenting with Viscosity

44. Rock Candy Science

Rock candy science is a delightful and educational chemistry experiment that students should try. By growing their own rock candy crystals, students can learn about crystal formation and explore the principles of solubility and saturation.

Learn more: Rock Candy Science

45. Baking Soda vs Baking Powder

Baking soda and baking powder have distinct properties that influence the leavening process in different ways.

This hands-on experiment provides a practical understanding of how these ingredients interact with acids and moisture to create carbon dioxide gas.

46. Endothermic and Exothermic Reactions Experiment

The endothermic and exothermic reactions experiment is an exciting and informative chemistry exploration that students should try.

By observing and comparing the heat changes in different reactions, students can gain a deeper understanding of energy transfer and the concepts of endothermic and exothermic processes.

Learn more: Education.com

47. Diaper Chemistry

By dissecting a diaper and examining its components, students can uncover the chemical processes that make diapers so effective at absorbing and retaining liquids.

Learn more: Diaper Chemistry

48. Candle Chemical Reaction

The “Flame out” experiment is an intriguing and educational chemistry demonstration that students should try. By exploring the effects of a chemical reaction on a burning candle, students can witness the captivating moment when the flame is extinguished.

49. Make Curds and Whey

This experiment not only introduces students to the concept of acid-base reactions but also offers an opportunity to explore the science behind cheese-making.

Learn more: Tinkerlab

50. Grow Crystals Overnight

By creating a supersaturated solution using substances like epsom salt, sugar, or borax, students can observe the fascinating process of crystal growth. This experiment allows students to explore the principles of solubility, saturation, and nucleation.

Learn more: Grow Crystals Overnight

51. Measure Electrolytes in Sports Drinks

The “Measure Electrolytes in Sports Drinks” experiment is an informative and practical chemistry activity that students should try.

By using simple tools like a multimeter or conductivity probe, students can measure the electrical conductivity of different sports drinks to determine their electrolyte content.

52. Oxygen and Fire Experiment

The oxygen and fire experiment is a captivating and educational chemistry demonstration that students should try. By observing the effects of oxygen on a controlled fire, students can witness the essential role of oxygen in supporting combustion.

53. Electrolysis Of Water

The electrolysis of water experiment is a captivating and educational chemistry demonstration that students should try.

Learn more: Electrolysis Of Water

54. Expanding Ivory Soap

The expanding Ivory Soap experiment is a fun and interactive chemistry activity that students should try. By placing a bar of Ivory soap in a microwave, students can witness the remarkable expansion of the soap as it heats up.

Learn more: Little Bins Little Hands

55. Glowing Fireworks

This experiment not only introduces students to the principles of pyrotechnics and combustion but also encourages observation, critical thinking, and an appreciation for the physics and chemistry behind.

Learn more: Glowing Fireworks

56. Colorful Polymer Chemistry

Colorful polymer chemistry is an exciting and vibrant experiment that students should try to explore polymers and colorants.

By combining different types of polymers with various colorants, such as food coloring or pigments, students can create a kaleidoscope of colors in their polymer creations.

Learn more: Colorful Polymer Chemistry

57. Sulfur Hexafluoride- Deep Voice Gas

This experiment provides a firsthand experience of how the density and composition of gases can influence sound transmission.

It encourages scientific curiosity, observation, and a sense of wonder as students witness the surprising transformation of their voices.

58. Liquid Nitrogen Ice Cream

Liquid nitrogen ice cream is a thrilling and delicious chemistry experiment that students should try. By combining cream, sugar, and flavorings with liquid nitrogen, students can create ice cream with a unique and creamy texture.

59. White Smoke Chemistry Demonstration

The White Smoke Chemistry Demonstration provides an engaging and visually captivating experience for students to explore chemical reactions and gases. By combining hydrochloric acid and ammonia solutions, students can witness the mesmerizing formation of white smoke.

60. Nitrogen Triiodide Chemistry Demonstration

The nitrogen triiodide chemistry demonstration is a remarkable and attention-grabbing experiment that students should try under the guidance of a knowledgeable instructor.

By reacting iodine crystals with concentrated ammonia, students can precipitate nitrogen triiodide (NI3), a highly sensitive compound.

61. Make a Plastic- Milk And Vinegar Reaction Experiment

Through the “Make a Plastic – Milk and Vinegar Reaction” experiment, students can gain a deeper understanding of the chemistry behind plastics, environmental sustainability, and the potential of biodegradable materials.

Learn more: Rookie Parenting

62. Eno and Water Experiment

This experiment not only introduces students to acid-base reactions but also engages their senses as they witness the visible and audible effects of the reaction.

63. The Eternal Kettle Experiment

By filling a kettle with alcohol and igniting it, students can investigate the behavior of the alcohol flame and its sustainability.

64. Coke and Chlorine Bombs

Engaging in this experiment allows students to experience the wonders of chemistry firsthand, making it an ideal choice to ignite their curiosity and passion for scientific exploration.

65. Set your Hand on Fire

This experiment showcases the fascinating nature of combustion and the science behind fire.

By carefully following proper procedures and safety guidelines, students can witness firsthand how the sanitizer’s high alcohol content interacts with an open flame, resulting in a brief but captivating display of controlled combustion.

66. Instant Ice Experiments

The Instant Ice Experiment offers an engaging and captivating opportunity for students to explore the wonders of chemistry and phase changes.

By using simple household ingredients, students can witness the fascinating phenomenon of rapid ice formation in just a matter of seconds.

67. Coke Cans in Acid and Base

Engaging in this experiment allows students to gain a deeper understanding of the chemical properties of substances and the importance of safety protocols in scientific investigations.

68. Color Changing Invisible Ink

The Color Changing Invisible Ink experiment offers an intriguing and fun opportunity for students to explore chemistry and learn about the concept of chemical reactions.

Learn more: Research Parent

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Science Projects > Physics & Engineering Projects > Water Experiments  

Water Experiments

Surface tension experiments.

Surface tension is one of the most important properties of water .

It is the reason that water collects in drops, but it’s also why plant stems can “ drink water ,” and cells can receive water through the smallest blood vessels.

You can test multiple surface tension experiments using just a few household items.

What You Do:

1. Start with a cup of water and some paperclips. Do you think a paperclip will float in the water? Drop one in the cup to find out. Since the paperclip is denser than the water, it will sink to the bottom of the cup.

Now find out if you can use surface tension to float the paperclip. Instead of dropping the paperclip into the cup, gently lay it flat on the surface of the water.

(This is tricky — it may help to place a piece of paper towel slightly bigger than the paperclip in the water. Then lay the paperclip on top of it. In a minute or so, the paper towel will sink, leaving the paperclip floating on top of the water.)

2. Even though the paperclip is still denser than the water, the strong attraction between the water molecules on the surface forms a type of ‘skin’ that supports the clip.

3. Now put a drop of dish soap in the water. This will bind with the water molecules, interfering with the surface tension .

The paper clip will sink. You can try floating other things on top of the water also – pepper floats well until you add dish soap. Can you find any other light items that will float?

Surface tension creates the ‘skin’ on top of the water, but it is also what causes water to stick together in drops.

Observe how these drops stick together by experimenting with water and a penny. All you need is a cup of water, a penny, and a medicine dropper .

First make a prediction: how many drops of water do you think you can fit on the top surface of the penny? Add one drop. After seeing how much room it takes, do you want to rethink your first prediction?

Now continue carefully adding drops until the water spills off the penny. Try this three times, recording the number of drops each time, and then find the average number of drops that can fit.

Surface tension is the reason you can fit so much water on the penny. The water molecules attract each other, pulling together so the water doesn’t spill.

Try this experiment with different-sized coins. Predict how many drops you can fit on a quarter compared with the penny.

For one final surface tension experiment, start with a full glass of water. Predict how many pennies you can add to the water without the glass overflowing. Gently add pennies one by one. Because of surface tension, the water will rise above the rim of the glass before it spills! Compare your original prediction with the number of pennies you were able to add.

Freezing Point

Have you ever wondered why rivers and lakes freeze in the winter, but oceans do not? In this experiment we will see that it is the presence of salt in the ocean that makes it less likely to freeze.

What You Need:

• 1-gallon freezer bag
• 1-quart freezer bag
• crushed ice
• thermometer

1. Fill the gallon freezer bag half full with crushed ice. Add one cup of salt and seal the bag. Put on some gloves and knead the ice and salt until the ice has completely melted.

2. Use the thermometer to record the temperature of the saltwater mixture. Even though the ice has melted, the temperature should be less than 32°F (0°C).

3. Now put about an ounce of water in the quart freezer bag. Seal the quart bag and then put it in the saltwater mixture in the larger bag. Seal the larger bag also and leave it until the water inside the quart bag freezes.

How did the water freeze when surrounded only by saltwater?

The salt broke apart the bonds between the water molecules in the ice, causing it to melt, but the temperature remained below the freezing point for pure water.

Salt (and other substances dissolved in water) will always lower the freezing point .

This is why water in the ocean rarely freezes.

• Find out more about salt water by making a Solar Purifier

More Water Projects:

•   Liquid Density
•   Hot Water: Convection Science
•  Water Wheel

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45 Cool Chemistry Experiments, Demos, and Science Fair Projects

Don’t forget your safety equipment!

Bunsen burners, colorful chemicals, and the possibility of a (controlled) explosion or two? Everybody loves chemistry experiments! We’ve rounded up the best activities, demos, and chemistry science fair projects for kids and teens. Try them in the classroom or at home.

Easy Chemistry Experiments and Activities for All Ages

Chemistry science fair projects.

These chemistry experiments and activities are all easy to do using simple supplies you probably already have. Families can try them at home, or teachers and students can do them together in the classroom.

Mix up some magic milk

Kids love this colorful experiment, which explores the concept of surface tension. This is one of our favorite chemistry experiments to try at home, since the supplies are so basic and the results are so cool!

Taste the Rainbow

Teach your students about diffusion while creating a beautiful and tasty rainbow. You’ll definitely want to have extra Skittles on hand so your class can enjoy a few as well!

Learn more: Skittles Diffusion

Crystallize sweet treats

Crystal science experiments teach kids about supersaturated solutions. This one is easy to do at home, and the results are absolutely delicious!

Learn more: Candy Crystals

Make elephant-sized toothpaste

This fun project uses yeast and a hydrogen peroxide solution to create overflowing “elephant toothpaste.” You can also add an extra fun layer by having kids create toothpaste wrappers for their plastic bottles.

Blow the biggest bubbles you can

Add a few simple ingredients to dish soap solution to create the largest bubbles you’ve ever seen! Kids learn about surface tension as they engineer these bubble-blowing wands.

Learn more: Giant Soap Bubbles

Demonstrate the “magic” leakproof bag

So simple and so amazing! All you need is a zip-top plastic bag, sharp pencils, and some water to blow your kids’ minds. Once they’re suitably impressed, teach them how the “trick” works by explaining the chemistry of polymers.

Learn more: Leakproof Bag

Use apple slices to learn about oxidation

Have students make predictions about what will happen to apple slices when immersed in different liquids, then put those predictions to the test! Finally, have them record their observations.

Learn more: Apple Oxidation

Float a marker man

Their eyes will pop out of their heads when you “levitate” a stick figure right off the table. This experiment works due to the insolubility of dry-erase marker ink in water, combined with the lighter density of the ink.

Learn more: Floating Marker Man

Discover density with hot and cold water

There are a lot of easy science experiments you can do with density. This one is extremely simple, involving only hot and cold water and food coloring, but the visuals make it appealing and fun.

Learn more: Layered Water

Layer more liquids

This density demo is a little more complicated, but the effects are spectacular. Slowly layer liquids like honey, dish soap, water, and rubbing alcohol in a glass. Kids will be amazed when the liquids float one on top of the other like magic (except it is really science).

Learn more: Layered Liquids

Grow a carbon sugar snake

Easy science experiments can still have impressive results. This eye-popping chemical reaction demonstration only requires simple supplies like sugar, baking soda, and sand.

Learn more: Carbon Sugar Snake

Make homemade bouncy balls

These homemade bouncy balls are easy to make since all you need is glue, food coloring, borax powder, cornstarch, and warm water. You’ll want to store them inside a container like a plastic egg because they will flatten out over time.

Learn more: Make Your Own Bouncy Balls

Create eggshell chalk

Eggshells contain calcium, the same material that makes chalk. Grind them up and mix them with flour, water, and food coloring to make your very own sidewalk chalk.

Learn more: Eggshell Chalk

Make naked eggs

This is so cool! Use vinegar to dissolve the calcium carbonate in an eggshell to discover the membrane underneath that holds the egg together. Then, use the “naked” egg for another easy science experiment that demonstrates osmosis .

Learn more: Naked Egg Experiment

Turn milk into plastic

This sounds a lot more complicated than it is, but don’t be afraid to give it a try. Use simple kitchen supplies to create plastic polymers from plain old milk. Sculpt them into cool shapes when you’re done.

Test pH using cabbage

Teach kids about acids and bases without needing pH test strips. Simply boil some red cabbage and use the resulting water to test various substances—acids turn red and bases turn green.

Learn more: Cabbage pH

Clean some old coins

Use common household items to make old oxidized coins clean and shiny again in this simple chemistry experiment. Ask kids to predict (hypothesize) which will work best, then expand the learning by doing some research to explain the results.

Learn more: Cleaning Coins

Blow up a balloon (without blowing)

Chances are good you probably did easy science experiments like this when you were in school yourself. This well-known activity demonstrates the reactions between acids and bases. Fill a bottle with vinegar and a balloon with baking soda. Fit the balloon over the top, shake the baking soda down into the vinegar, and watch the balloon inflate.

Learn more: Balloon Experiments

Assemble a DIY lava lamp

This 1970s trend is back—as an easy science experiment! This activity combines acid/base reactions with density for a totally groovy result.

Explore how sugary drinks affect teeth

The calcium content of eggshells makes them a great stand-in for teeth. Use eggs to explore how soda and juice can stain teeth and wear down the enamel. Expand your learning by trying different toothpaste and toothbrush combinations to see how effective they are.

Learn more: Sugar and Teeth Experiment

Mummify a hot dog

If your kids are fascinated by the Egyptians, they’ll love learning to mummify a hot dog. No need for canopic jars ; just grab some baking soda and get started.

Extinguish flames with carbon dioxide

This is a fiery twist on acid-base experiments. Light a candle and talk about what fire needs in order to survive. Then, create an acid-base reaction and “pour” the carbon dioxide to extinguish the flame. The CO2 gas acts like a liquid, suffocating the fire.

Send secret messages with invisible ink

Turn your kids into secret agents! Write messages with a paintbrush dipped in lemon juice, then hold the paper over a heat source and watch the invisible become visible as oxidation goes to work.

Learn more: Invisible Ink

Set popcorn dancing

This is a fun version of the classic baking soda and vinegar experiment, perfect for the younger crowd. The bubbly mixture causes popcorn to dance around in the water.

Learn more: Dancing Popcorn Experiment

Shoot a soda geyser sky-high

You’ve always wondered if this really works, so it’s time to find out for yourself! Kids will marvel at the chemical reaction that sends diet soda shooting high in the air when Mentos are added.

Learn more: Mentos and Coke Experiment

All of these chemistry experiments are perfect for using the scientific method. Form a hypothesis, alter the variables, and then observe the results! You can simplify these projects for younger kids, or add more complexity for older students.

Break apart covalent bonds

Difficulty: Medium / Materials: Medium

Break the covalent bond of H 2 O into H and O with this simple experiment. You only need simple supplies for this one. Turn it into a science fair project by changing up the variables—does the temperature of the water matter? What happens if you try this with other liquids?

Learn more: Breaking Covalent Bonds

Measure the calories in various foods

Are the calorie counts on your favorite snacks accurate? Build your own calorimeter and find out! This kit from Home Science Tools has all the supplies you’ll need.

Detect latent fingerprints

Forensic science is engrossing and can lead to important career opportunities too. Explore the chemistry needed to detect latent (invisible) fingerprints, just like they do for crime scenes!

Learn more: Fingerprints Project

Use Alka-Seltzer to explore reaction rate

Difficulty: Easy / Materials: Easy

Tweak this basic concept to create a variety of high school chemistry science fair projects. Change the temperature, surface area, pressure, and more to see how reaction rates change.

Determine whether sports drinks really have more electrolytes than other beverages

Difficulty: Medium / Materials: Advanced

Are those pricey sports drinks really worth it? Try this experiment to find out. You’ll need some special equipment for this one; buy a complete kit at Home Science Tools .

Turn flames into a rainbow

You’ll need to get your hands on a few different chemicals for this experiment, but the wow factor will make it worth the effort. Make it a science project by seeing if different materials, air temperature, or other factors change the results.

Discover the size of a mole

The mole is a key concept in chemistry, so it’s important to ensure students really understand it. This experiment uses simple materials like salt and chalk to make an abstract concept more concrete. Make it a project by applying the same procedure to a variety of substances, or determining whether outside variables have an effect on the results.

Learn more: How Big Is a Mole?

Cook up candy to learn mole and molecule calculations

This edible experiment lets students make their own peppermint hard candy while they calculate mass, moles, molecules, and formula weights. Tweak the formulas to create different types of candy and make this into a sweet science fair project!

Learn more: Candy Chemistry

Make soap to understand saponification

Take a closer look at an everyday item: soap! Use oils and other ingredients to make your own soap, learning about esters and saponification. Tinker with the formula to find one that fits a particular set of parameters.

Learn more: Saponification

Uncover the secrets of evaporation

Explore the factors that affect evaporation, then come up with ways to slow them down or speed them up for a simple science fair project.

Learn more: Evaporation

More Chemistry Experiment Science Fair Ideas

These questions and prompts can spark ideas for unique chemistry experiments:

• Compare the properties of sugar and artificial sweeteners.
• Explore the impact of temperature, concentration, and seeding on crystal growth.
• Test various antacids on the market to find the most effective product.
• What is the optimum temperature for yeast production when baking bread from scratch?
• Compare the vitamin C content of various fruits and vegetables.
• How does temperature affect enzyme-catalyzed reactions?
• Investigate the effects of pH on an acid-base chemical reaction.
• Devise a new natural way to test pH levels (such as cabbage leaves).
• What’s the best way to slow down metal oxidation (the form of rust)?
• How do changes in ingredients and method affect the results of a baking recipe?

Like these chemistry experiments? Don’t miss STEM Activities for Kids of All Ages and Interests .

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