I know it sounds crazy but the explanation for why hot water rises and cold water sinks is a theory! You would think we would know something like that right!

Actually, we do know. In short, hot liquid (like water) or hot gas (like air) rises because heat causes matter to expand, making it less dense, or lighter, than cold liquid, or cold air.

What is this about a theory?

In everyday language we use the word theory to mean an idea or a hunch about something. In science it has quite a different meaning. It still means an idea, but it is an idea that is supported by facts and observations. The idea or theory may change over time as we make more observations and discover more facts but at any given time it is the best interpretation of the known facts and observations.

What is the theory that explains why hot water rises?

The theory that explains why hot water rises and cold water sinks is call the Kinetic Theory of Matter. Kinetic comes from the Greek word “to move”. In other words it is the idea that all stuff is constantly moving. Or in more detail, the idea that everything is made up of tiny particles (atoms and molecules) that have space between them and are constantly moving. These particles can be moving slower or faster and can be closer together or further apart.

When something is heated this gives energy to the tiny particles and they start moving faster, which in turn makes them move further apart. When something is cooled, energy is removed from the particles and they slow down and get closer together. Density is the word we use to describe the amount of matter (made up of tiny particles) present in any given amount of space (volume). If something is denser than something else then it is also heavier – because there is more of it in any given space.

This is a very cool interactive animation where you can change the “temperature” of water and see what happens to the water molecules:

This interactive animation also shows hydrogen bonds, which is the force that attracts water molecules to each other. For a beautifully written explanation of hydrogen bonds (wink) please check out our post about walking water.

What is a cool way to demonstrate density?

Experiment One

This experiment gives a good visual demonstration of hot water rising and cool water moving downwards.

Warning! Adult supervision required – hot water

You will need:

  • A large, clear container – we used an old fish tank
  • Two bottles – glass will stay submerged better. We found ours in a charity shop
  • A bowl or dish – to use as a platform
  • Food coloring – two colors. Red and blue is most effective
  • Water – room temperature
  • Hot water
  • Fridge-cold water


  1. Fill the clear container with room temperature water
  2. Submerge the bowl and place upside-down at the bottom of the container, to act as a platform
  3. Put cold water into one of the bottles, together with blue food coloring
  4. Put hot water into the other bottle, together with the red food coloring. We used water from the kettle, but it wasn’t boiling. Keep in mind that the bottle will get hot – take care not to scald yourself.
  1. Put your thumbs over the opening of the bottles to stop the water coming out before you are ready. Carefully place both bottles on the platform in the container, trying to minimise disturbance to the water as you release your thumbs and remove your hands from the container.

What happens?

Observe! As the water settles, you should see a clear line of red from the hot water rising, and a line of cold water moving downwards

Experiment Two

This experiment gives a good visual demonstration of hot water rising.

Warning! Adult supervision required – hot water

You will need:

  • Two clear glasses
  • Old CD/DVD
  • Hot and cold water


  1. Put some food coloring into a glass.
  2. Fill the same glass with warm water. Make sure it is full to the brim.
  3. Put cold water (tap is fine) in the other glass
  4. Place the CD on the cold water glass, place your fingers over the hole, flip it over and place it on top of the hot water glass. TAKE CARE NOT TO SCALD YOURSELF!

What happens?

Watch as the warm water rises to the upper glass. What do you think is happening to the cold water in the upper glass?

What about gas, what’s a cool way to demonstrate density using gas?

There are so many ways to demonstrate density. One of my favorite demonstrations of density is party balloons filled with helium. When inflated with helium a party balloon will rise (it becomes less dense than the surrounding air) and then over time, as the helium comes out of the balloon, it sinks again (it becomes more dense than the surrounding air).

The balloon, which is much heavier than air before it is inflated, becomes much lighter than air when filled with helium. This is because helium is a very tiny particle (atom).

Helium is the second smallest atom. The smallest atom is hydrogen, but that can explode, so we stick with the much safer option – helium.

It is such a small atom that the amount of matter present in a helium-filled party balloon is less than the same volume of air. As an aside, you will also notice that party balloons filled with helium deflate much faster than party balloons filled with air. This is because particles of helium are much smaller than air particles, and they escape through the surface of the balloon much faster.

We don’t have any helium so what’s another way to demonstrate density?

Experiment Three

You will need:

  • Three balloons
  • Tape measure (one used for sewing works well)
  • Access to a fridge
  • Access to a car on a warm day


  1. Blow three balloons up to about the same size, not too big, not too small.
  2. Measure and write down the diameter of each balloon
  3. Place one of these balloons in the car on a hot day
  4. Place another balloon in the fridge
  5. Leave the other in the house somewhere sensible.
  6. After some time, collect them all them all together and re-measure.

What happens?

The one taken from the car contains hot air, so the balloon has expanded, the one from the fridge has cold air inside, so it has shrunk, and the one left inside the house is still about the same size it started out. Leave them inside and as they come back to room temperature, they will all return to about the same size once again.

Further experiments:

Experiment 2 – try the same experiment with clear warm water in the bottom glass and colored cold water in the top glass. Can you see the cold water moving downwards?

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