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Convection Science Experiment – How Heat Moves through Liquid

Can heat cause movement? With a few drops of food coloring, cooking oil, and a candle you can find out! In this simple yet exciting science experiment, kids can explore the concepts of density and convection as they watch convection currents in motion!

A demonstration video, printable instructions, and a supplies list are included as well as an easy to understand scientific explanation of how this experiment works.

Note: Because this experiment uses a fire component, adult supervision is required.

JUMP TO SECTION:   Instructions  |  Video Tutorial  |  How it Works | Purchase Lab Kit

Supplies Needed

• Large heat safe glass bowl
• Cooking Oil
• Food Coloring
• Two 2×4 blocks
• Match or Lighter

Convection Science Lab Kit – Only $5

Use our easy Convection Science Lab Kit to grab your students’ attention without the stress of planning!

It’s everything you need to  make science easy for teachers and fun for students  — using inexpensive materials you probably already have in your storage closet!

Convection Science Experiment Instructions

Step 1 – Begin by filling a large glass bowl with cooking oil.

Step 2 – Next, add between 5-10 drops of food coloring into the oil. Take a moment to make some observations. What happens to the food coloring? Does it mix with the oil?

Helpful Tip: Place the drops near the center of the bowl.

Step 3 – Prop the bowl up off the table using two 2×4 blocks. Position the blocks so there is a space between them to put a candle.

Step 4 – Light a candle and carefully place it under the bowl. The flame of the candle should touch the bottom of the glass bowl. 

Step 5 – Look through the side of the glass bowl and watch carefully to observe what happens. Write down what happens. Helpful Tip: It will likely take 5 minutes before you see anything happen to the liquid/food coloring.

Do you know why the food coloring moves around in the oil? Find out the answer in the how does this experiment work section below.

Video Tutorial

How Does the Science Experiment Work

Heat can move in three ways: conduction, convection, and radiation. In this experiment, heat is transferred by means of convection. Convection is the transfer of heat by the movement of currents within a fluid.

In our experiment, the oil at the bottom of the bowl was heated by the candle. The particles of oil at the bottom of the pot began to move faster and further apart. As a result, these oil particles became less dense than the rest of the oil particles in the bowl, so these heated, less dense oil particles began to rise. (Less dense fluids rise and more dense fluids sink). The surrounding, cooler oil particles flow in to take its place. This flow creates a circular motion known as a convection current . A convection current is caused by the rising and sinking of heated and cooled fluids.

You can see evidence of the convection current if you look at the food coloring in the bowl. Notice bubbles of food coloring rise from the center of the bowl, drift to edges of the bowl, and sink back to the bottom.

Convection currents are all around us and responsible for heating many things! Our homes are heated in the winter through convection currents. The troposphere of the atmosphere (the layer closest to Earth) is heated through convection currents. The mantle inside of Earth is heated through convection currents, which causes Earth’s crust to drift in a process called continental drift.

I hope you enjoyed the experiment. Here are some printable instructions:

Convection Science Experiment

• Two 2×4 blocks

Instructions

• Begin by filling a large glass bowl with cooking oil.
• Next, add between 5-10 drops of food coloring into the oil. Helpful Tip: Place the drops near the center of the bowl.
• Prop the bowl up off the table using two 2×4 blocks.
• Light a candle and carefully place it under the bowl. The flame of the candle should touch the bottom of the glass bowl.
• Look through the side of the glass bowl and watch carefully to observe what happens. Helpful Tip: It will likely take 5 minutes before you see anything happen to the liquid/food coloring.

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Convection Currents Made Easy

June 13, 2022 By Emma Vanstone Leave a Comment

When part of a liquid or gas is heated, it expands and becomes less dense. The warmer, less dense liquid rises upwards, and the cooler liquid falls to take its place. This cycle of a liquid or gas rising and falling is called a convection current .

We set up a very simple convection current demonstration using hot and cold water with food colouring to show the movement of warm water through cold water.

Convection Current Demonstration

You’ll need.

Tall glass or vase

A smaller glass or cup

Food colouring

Instructions

Fill the tall glass or jar with cold water.

Fill the smaller container with hot ( but not boiling water ) and add a few drops of food colouring.

Carefully place the small container into the container with the cold water.

Watch what happens to the warmer, coloured water.

The hot, coloured water rises upwards and collects at the top of the cold water. It then cools and sinks downwards. Eventually, all the water will be at the same temperature.

How does convection work?

Particles of warm water move more quickly and spread out. They rise upwards through denser, cooler water, which sinks to the bottom, where it warms up. Eventually, all the liquid is at the same temperature.

Warm water rises because when liquids and gases are heated, they expand. This means they take up more room but have the same mass, so their density is less than when they are cool. Substances with lower densities float on substances with higher densities.

A hot air balloon is another example of convection currents in action.

Ask an adult to help with this activity

Extra Challenge

Repeat the activity with cold water instead of hot and watch what happens to the coloured water.

You can see that the cold water isn’t moving upwards like the warm water.

How is heat transferred?

Heat can be transferred in three different ways.

• Conduction.
• Radiation .

Heat convection occurs when warmer molecules of a liquid or gas move from a warmer to a cooler area, taking the heat with them.

Water being heated in a pan is an example of convection. This is the type of heat transfer we demonstrated above.

Another way to demonstrate convection is with a spinning convection snake .

Conduction of heat occurs when vibrating particles pass any extra vibrational energy to nearby particles. The more energy the particles have, the hotter the object gets. An example of this type of heat transfer is when metal pans are heated on a hob. Heat travels through the pan. If the pan handle is also metal, it will get hot, too. This is why metal pans often have plastic or wooden coverings on their handles. Plastic and wood are not good conductors of heat.

Radiation of heat is when heat is radiated to the surrounding area by heat waves. Particles are not involved in this kind of heat transfer.

The heat from the sun travelling through space is an example of heat transfer by radiation. Waves transfer this type of heat.

The campfire and pan example above shows all three kinds of heat transfer.

Heat travels by radiation from the campfire to the metal pan. Heat travels through the pan’s metal by conduction to warm the lower layers of water. The water is then heated by convection as the less dense warmer water rises through the cooler water, creating a convection current !

Remember – heat is only transferred if there is a temperature difference.

Science concepts

• Heat transfer

Last Updated on April 20, 2024 by Emma Vanstone

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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.

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Convection Experiments for Kids

How to Make a 3D Model of a Typhoon

Convection is the cycle of transferring heat. It is a fascinating topic to tackle when attempting scientific experiments with kids, because it’s something that occurs in liquid and the air on a daily basis. Convection is also something that can be tested and understood without using expensive laboratory equipment and tools.

Convection Snake

To perform the convection snake experiment, you will need a piece of paper and scissors. Using the scissors, cut the piece of paper into a spiral shape that is 6 cm long. Attach one end of a 15-cm piece of yarn to the middle of the spiral using clear tape. Take a table lamp and hang the paper spiral about 10 cm above the table lamp. You will notice that the heat from the lamp causes the spiral to twirl. This is due to a convection current. The energy from the lamp's light source warms up the air above it. Because hot air is lighter than cool air, as the air heats up and the hot air rises above the lamp, colder air moves where the warmer air was previously.

Convection Currents Experiment: Materials

If you watch the news, you see weather forecasters talking about natural weather phenomena such as El Nino and La Nina. El Nino and La Nina are caused by convection, because convection currents form when warm and cold air come together in the atmosphere. This creates warm water currents in the ocean. To create your own convection current, you will need four plastic soda bottles that are the same size, two different colors of food coloring, warm and cold water and an index card.

Convection Currents Experiment: Procedure and Results

Completely fill two of the soda bottles with cold water and the other two bottles with warm water. Use the food coloring to color the cold water one color and the warm water the other color. Put your index card over the top of one of the bottles of warm water. Holding the index card in place, inverse the bottle and put it on top of one of the bottles of cold water. Slide the index card from in between the bottles. You will see that the cold water, which is heavier, stays in the bottom bottle and the warm water stays in the top bottle. However, if you perform the experiment placing the bottle of cold water on top and the bottle of warm water on the bottom, the warm water would rise to the top and the cold water would move to the bottom bottle.

Boiling Water

A simple illustration of convection is a pot or kettle of boiling water. When you boil water to make beverages or cook meals, though you start off with cool water, as the water is warmed by an external heating source, it begins to expand. As the hotter water rises, colder water in other parts of the pot or kettle moves in to replace the warmer water. As this process continues, the water gets hot and the convection currents cause the liquid to move in a circular fashion. Eventually, the action becomes very strong and the water begins to boil.

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About the Author

Mike Jones is an Atlanta native who has been writing professionally since 2000. He has written a number of entertainment, health and how-to articles for online publications such as eHow and Answerbag. He holds a master's degree in journalism from Regent University.

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Science Projects > Earth & Space Projects > Hot Water: Convection  

Hot Water: Convection

Geothermal science projects, water convection science project.

Try this experiment to get an idea of where the hot water for hot springs comes from.

What You Need:

• 4 identical, clear, wide-mouthed jars (or just reuse 2 identical jars)
• Blue food coloring
• Red food coloring
• Two small pieces of thin tag board, index cards, or wax paper
• A place that is okay to get wet

What You Do:

1. Take your materials to the place that is okay to get wet.

2. Fill two of the jars to the rim with cold tap water. Place a couple of drops of blue food coloring in each jar (enough to make the water noticeably blue). Add a few more drops of cold water so that a bulge of water forms over the rim.

3. Fill the other two jars to the rim with hot water from the tap. Place a couple of drops of red food coloring in each jar (enough to make the water noticeably red). Add a few more drops of hot water so that a bulge of water forms over the rim.

4. Take one of the red jars and place the tag board on top, letting the water seal the tag board to the jar. Using one hand to keep the tag board on the mouth of the jar, quickly turn the jar over. The water seal will keep the tag board stuck to the rim and will prevent water from leaking out.

5. Place the upside-down jar on top of a blue jar. Align the two mouths of the jars together and then, holding both jars steady, have someone else carefully remove the tag board, keeping the mouths of the jars together as much as possible.

6. You should now have the red jar sitting upside down on top of the blue jar, both filled with their respective water. What do you notice about the water?

7. Now, take a new piece of tag board and place it on the second blue jar. Using the same method as before, place the second blue jar on top of the second red jar, making sure the mouths are aligned.

8. Remove the tag board and watch the water in the two jars. What happens to the water?

What Happened:

When the red jar was placed on top of the blue jar, the distinction between red water and blue water stayed fairly clear. But when the blue jar was placed on top of the red jar, there was a very rapid mixing of colors. Why is this?

Well, simply put, cold water is “heavier” than hot water. When the hot water is heated, the water molecules start moving around pretty fast and move apart from each other. The water molecules in the cold water, on the other hand, are packed closer together. So, in two equal size jars, more cold water molecules can fit in their jar than hot water molecules can fit in their jar. In scientific terms, the cold water is more dense than the hot water. So when hot water is placed beneath cold water, it will rise up while the cold water sinks down. This causes the mixing of the water you saw earlier. However, when the hot water is placed on top of the cold water, nothing moves because the hot water is already where it wants to be – at the top.

The water in hot springs generally originates as cold rain water or snow melt. This cold water sinks into the ground until it reaches a layer of rock that is being heated by a chamber of magma. The hot rock heats the water, and the hot water rises back up to the surface of the Earth in the form of hot springs. This cycle of cold water sinking and hot water rising is known as convection. (The same is true of air – hot air rises while cold air sinks.)

Fumaroles Science Project

Ever wonder why some fumaroles produce large amounts of steam, while others produce very little? Try out this experiment to find out one of the reasons! This experiment requires adult help and supervision.

• Medium saucepan
• Disposable tin pie pan
• Stove or hot plate

1. Fill the saucepan about half full with water and place it on the stove. Heat it until the water is steaming but not boiling.

2. While waiting for the water to heat, take the pie pan and turn it upside down. Use the hammer and nail to gently put a small hole in the center of the pan.

3. Put the oven mitts on and place the pan right side up over the pot. What does the steam do?

4. Still wearing the oven mitts, take the pie pan off towards you so that it makes a shield between you and the steam. Never lift the pan off away from you, or the escaping steam may burn you!

5. Turn the pan over again and gently hammer another hole into the bottom of the pan, about an inch away from the first hole.

6. Place the pan on the pot again. What do you see happening to the steam?

7. Continue this cycle about 4 or 5 times, adding one more nail hole with each new cycle. What do you notice happening to the steam with each new hole in the pan?

In this experiment, you were demonstrating how fumaroles work. A fumarole is a vent (hole) that lets out steam from within the Earth. The holes in the tin pan are simulating how steam escapes the Earth. When there is just one hole or fumarole, steam only has one exit, causing it to exit quickly and forcefully. Sometimes, the amount of steam coming out of one fumarole becomes too much for it, and the steam will follow cracks in the Earth to a new place to vent out of the surface. This formation of a new fumarole causes the pressure of the steam to ease up a bit, and the escaping steam comes out less quickly and less forcefully from both fumaroles. The more fumaroles present, the less pressure the steam is under. Later on in the life of these fumaroles, the steam escaping may decrease due to not enough water and/or a decrease in the heat from the underground magma chamber, causing smaller steaming vents.

Geothermal Science Lesson

What causes geothermal areas.

In places such as Yellowstone National Park, New Zealand, and Iceland, the land is covered in spewing geysers, colorful hot springs, and bubbling mud pots. Even in winter, these areas are very steamy. These parts of the Earth are known as geothermal areas and form when an abundant source of water meets an intense source of heat. Since the Earth is covered in about 70% water, it’s the heat source that is crucial.

Beneath the Earth’s crust is a layer of magma (hot liquid rock). Geothermal areas exist where this magma is closer to the surface of the Earth than in other areas, causing these regions to have significantly higher surface temperatures. For instance, the average thickness of the Earth’s crust is about 12 to 50 miles thick, but in Yellowstone National Park, the magma chamber (magma housed by a layer of rock) is only 3 miles below the surface. Volcanoes are one of the main ways that magma gets pushed up so close to the surface. For this reason, geothermal areas often exist close to where volcanoes exist, though sometimes there is no apparent evidence of a volcano nearby. In these cases, it may be an isolated hotspot in the crust of the Earth where a new volcano may someday appear, or it is the remnants of an extinct volcano.

Features of Geothermal Areas

Hot springs, geysers, fumaroles, and mud pots are all geothermal features. They arise when cold groundwater seeps down and is heated by the rocks touching the underlying magma chamber. The hot water then rises to the surface in the form of a geothermal feature.

Hot springs occur when this heated water forms a pool on the surface of the Earth. Since that’s all it takes to form a hot spring, it the most common geothermal feature and can be found in places all over the Earth. Hot springs vary in temperature and can be calm, effervescent, or boiling depending on how hot the magma chamber below it is. When the hot water travels up, it dissolves material from the surrounding bedrock and brings this material up to the surface with it. For this reason, hot springs tend to be full of minerals, and people have used these hot pools for medicinal purposes for centuries. However, not all hot springs are safe to bathe in. Some are way too hot and/or way too acidic and can severely injure anyone stepping foot in them.

A geyser is a type of hot spring that periodically erupts, shooting columns of water and steam into the air. Like hot springs, geysers need an abundant supply of water and an intense heat source to exist. However, one more key ingredient is needed to keep them from being just a hot spring – the right plumbing. Unlike hot springs where the heated water has a simple path to travel upwards to reach the surface, geysers have a complex network of underground tunnels and reservoirs that trap the water and delay its arrival to the surface. While the water is trapped in the ground – sometimes as low as 10,000 feet – it gets heated far above the normal boiling point. However, due to the immense pressure that far down in the ground, the water cannot boil. The super heated water rises to the surface. As it rises, the pressure becomes less and less, and the water starts to boil and steam starts to escape. This release of steam allows some of the water to overflow out of the geyser’s mouth. This alleviates the pressure on the water below, causing a chain reaction. As the water at the top of the plumbing system starts to boil, it expands and is shot out of the geyser. This removes the pressure on the water below it, which suddenly boils and expands, causing the lower water to also be ejected out of the mouth of the geyser. This keeps happening to all the water within the chambers until there is no longer enough water left to continue the eruption. Groundwater then starts seeping back into this underground network, starting the cycle all over again.

Fumaroles are basically steam vents that allow water vapor and gases to escape the surface of the Earth. They can be found at the base of volcanoes or in geothermal fields, both on land and on the floor of the ocean. They are hotter than hot springs and geysers because any groundwater that enters a fumarole is instantly turned into steam – no liquid water is present in fumaroles. For this reason, they are sometimes called “dry geysers.”

One more unique feature found in these areas are mud pots . Mud pots are basically very acidic hot springs that dissolve the nearby rock. This rock turns into fine particles of clay and silica that becomes suspended in the water. Due to their sometimes close proximity to volcanoes, volcanic ash often gets mixed in the sediment in a mud pot. The hot water and steam rises from below, forming bubbles that burst when they reach the top. The bursting bubbles fling water and sediment to the edges and the ejected sediment builds a mound around the mud pot, making the opening look like a crater. A delicate balance of water and sediment is needed in order to keep a mud pot a mud pot. Too much water, and it becomes a hot spring. Too little water, and it becomes dry, cracked earth. Most mud pots go through cycles of overly wet to overly dry to just the right amount of water, depending on the season and the water table of the area.

Many of these geothermal features are very colorful. These colors are due to the substances found in the water, and the color is a very good indicator of what these substances are. If a spring has a red color to it, most likely it is caused by a large amount of iron. If it is yellow, it is probably due to the presence of sulfur (though the smell of rotten eggs pretty much guarantees it is sulfur). Pinks and whites are often caused by the presence of calcium.

Amazingly, not all of the colors are caused by minerals. Due to the extreme heat and high acidity of many hot springs, for a long time it was believed that life forms could not exist in them. Then it was discovered that microorganisms known as thermophiles (literally “heat loving”) can live and actually thrive in this very hot water. If the water is blue or green in color, that gives a very good indication that microorganisms, such as algae, protozoa, and bacteria, make their home here.

Flashback in History: The Pink and White Terraces

The beauty of geothermal areas often overshadows the ever present danger related with them. Seeing abundant wildlife and vegetation seeming to live harmoniously with bubbling hot springs and spewing geysers leads many to believe that these areas are unique but tame places. However, in many of these areas, there is a dormant giant that awakes with very little notice. One such place is on the North Island of New Zealand near a town called Rotorua.

During the mid to late 1800’s, this geothermal area was a popular tourist destination for many European travelers. Like Yellowstone National Park, the area is full of hot springs, geysers, mud pots, and fumaroles. But its main attraction was the famous Pink and White Terraces, known for their awesome beauty and use as warm mineral baths. They were formed by hot springs and geysers at the top of two hills. The hot water full of dissolved calcium bicarbonate would flow down the hills, leaving behind calcium carbonate precipitate that formed into limestone and travertine terraces, which were filled with water. The calcium carbonate and other minerals in the water colored the terraces so that they were named appropriately enough the White Terraces and the Pink Terraces.

However, in the early morning hours of June 10, 1886, the volcano Mount Tarawera erupted and spewed hot mud, huge boulders, and thick ash over an estimated area of 5,800 square miles. The eruption lasted for about four hours and was so violent that it completely destroyed the Pink and White Terraces. Two Maori villages (natives of New Zealand) that thrived on the tourism created by the terraces were also completely wiped out, being buried in the huge mudslide created by the erupting mountain. All that remains of the Pink and White Terraces are black and white photos .

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Colored Convection Currents Science Experiment

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We are recently tried out a fun and simple experiment called colored convection currents. Have you heard of a convection current before?

Learn all about it here today! Then go and teach it to someone else. 🙂

What Are Convection Currents?

Convection is the movement within a liquid (or air) when the hotter and less dense liquid rises, and the colder, denser liquid sinks due to gravity. These movements create the circulation patterns in our atmosphere through air and water. The warm water or air rises and allows cooler air (or water) to go underneath.

This results in a transfer of heat. Heat can be transferred by convection when there is a big difference in temperature between the two different liquids.

The same thing happens in the movement of air. If you have ever watched the weather channel you can hear them talking about cool air and warm air moving across the earth. I am sure you also have heard that hot air rises? Well you can watch that happen in this experiment.

How to Do the Convection Current Experiment

If you take  two bottles  of water colored with  food coloring , one with cold water and one with very warm water, you can create a current that is really fun to watch! The current will look completely different depending on whether the warm is on top or the cold is on top.

We tried both convection currents at the same time with four different bottles.   I made the cold water blue and the warm water red- so you could see the heat rising and mixing. Place a notecard or playing card on top of the bottle that you are going to flip over to hold in the water. There may be a few drops of spilled water, so be prepared for that!

To see both currents at once, leave warm on the bottom for one and cold on the bottom for the other. Once the bottles are on top, pull out the card and you will see the water with the warm on the bottom quickly rise and mix into the cold water. For the one with the cold water on the bottom, it barely mixes!

Try this convection current experiment sometime . It is SO fun & easy. Experiment with different colors, too!

Try More Fun Science Experiments:

The Science of Air

States of Matter: Solids, Liquids and Gasses

Density Experiment

Former school teacher turned homeschool mom of 4 kids. Loves creating awesome hands-on creative learning ideas to make learning engaging and memorable for all kids!

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This is very engaging and nice thanks for this

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Science project, heat convection in liquids.

Energy is all about action! Thermal energy is transferred in many ways. The thermal energy of a substance can be determined by adding up all the kinetic and potential energy of its molecules. Convection is one form of energy transfer where heat energy is transferred by large scale movement in a gas or liquid. Convection currents form, which are streams of gas or liquid powered by convection. Some of this movement is caused by differences in density . You might remember that density how much matter there is in a given amount of space. In this convection current experiment for kids, you are going to make convection currents in water, which you will be able to observe with the help of food coloring.

How does the convection of water work?

• Clear quart container or jar
• Coffee mug or other container that can withstand heat
• Blue food coloring
• Fill the clear jar halfway with cold water.
• Place the jar freezer for 15 minutes.  You don’t want the water to freeze.
• Fill the coffee mug about ¼ full with hot water.
• Add 10 drops of blue food coloring to the hot water and stir.
• Remove the jar from the freezer and set it on table.  Wait until all the sloshing around from moving it has stopped.
• Fill the dropper with hot blue water.
• Lower the tip of the dropper until it is near the bottom of the large jar.
• Carefully release two drops of hot blue water onto the cold water.  Observe what happens, looking at the side and top of the jar.
• Add ten more drops, two drops at a time, observing what happens between each.
• Once you have added all the hot blue liquid drops, observe the jar for an additional five minutes.

When you squeeze of the drops of water with blue dye near the bottom of jar, most of it rises through the cold water and then continues to travel across the water’s surface.   Ripples of blue color move through the water.  A blue layer forms at the top of water in the jar.  As time goes by, some of the blue water begins to sink, and after five to ten minutes, all of the water turns a lighter shade of blue.

The hot blue water molecules had more kinetic energy than the cold water molecules. That means the blue water molecules were colliding more, and pushing each other part. This lowered the density of the blue water because fewer molecules could fit in the given volume. The less dense blue water therefore rose through the cold water and floated at the top. Those streams of blue fluid you saw were convection currents. Over time, thanks to the convection currents, the hot water mixed with the cold water, evening out the temperature overall. The blue food coloring also diffused throughout the liquid. Diffusion happens constantly. The blue food coloring molecules moved from higher concentration in the hot water and zero concentration in the clear water to create a more uniform distribution throughout the liquid, giving it an even, light blue appearance.   

Going Further

Do some research on warm and cold ocean currents. Water of different temperatures can move hundreds of miles!

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Inverted Bottles

Investigate convection by using food coloring and water at different temperatures.

• Four identical wide-mouth glass bottles
• Two index cards
• Food coloring, two colors
• Hot and cold water
• Two plastic plates or trays (to hold any spilled water)
• A partner (optional)

• Completely fill two bottles with hot water. Keep filling until a meniscus (an upward bulge) forms on the surface of the water.
• Completely fill two bottles with cold water. Keep filling until a meniscus (an upward bulge) forms on the surface of the water.

• Cut a piece of index card so it’s slightly bigger than the opening of a bottle, and then place the card on the mouth of the second hot-water bottle. Gently tap the index card. This will help to make sure that the card is in contact with the entire rim of the bottle.

Try to do this next step at the same time to both sets of bottles: Carefully slide the card out from between each set of bottles without spilling the water. (You might need a helping hand to do this.) Watch what happens to the fluid in each set of bottles.

When you removed the cards from one set of bottles, the hot water stayed on top and the cold water stayed on the bottom, with the colors staying pretty much the same. In the other set, however, something very different happened. The hot water rose, and the cold water sank. As this motion occurred, the colors mixed. This happened because of differences in density, which is defined by the amount of material in a given volume.

Everything is made of molecules. Hot molecules move more than cold molecules, and things that are hot typically take up more space than the same things when they are cold. This means it takes fewer hot-water molecules to fill a bottle than cold-water molecules. Hot water is therefore less dense than cold water.

Gravity can separate fluids by their density. Because the cold water has more mass per unit volume than hot water, the force of gravity on a given amount of cold water is larger than that on the same amount of hot water. This forces the cold water downward and causes the hot water to be pushed or lifted upward. This motion of fluids is called convection. In the set of bottles where the hot water was above the cold water, the cold water was already on the bottom, so there was no convection.

Have you ever climbed on a stepstool or ladder to change a lightbulb? If so, you might have noticed that the air higher up in the room is warmer. This is due to convection.

The next time you go swimming in a pool, try noticing the temperature difference between the surface water and the deeper water. Again, convection may have separated fluids by density, and the water below will be cooler.

Compare two bowls of hot soup. Leave one alone and blow across the surface of the other. Compare them, and you’ll find that the bowl you blow on will cool faster than the one you leave alone. When you blow on hot soup, you help drive the process of convection. The top surface cools and sinks, and the hot soup below rises and also gets cooled.

Convection affects fluid movement on small scales, as in this Snack, but it affects fluid movement on very large scales, too. As a result, this investigation can also be used to teach earth and space science phenomena. Convection is an important part of the weather cycle. It drives ocean currents, as well as the motion of semi-solid rock within the earth. Convection even moves material in stars.

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