Fun and easy science experiments for kids and adults.

Water thermometer

Earth science
Make a thermometer from a plastic bottle, a straw and some modeling clay. This is an experiment about water, heat and density.
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  • 1 0.5 L (16.9 fl oz) plastic bottle
  • 1 pair of scissors
  • 1 transparent drinking straw
  • Modeling clay
  • 1 bowl
  • Electric kettle, or other way of heating water (optional)
  • Food coloring (optional)
  • Water


Boiling water may be used in this demonstration.

Step 1

Make a hole in the cap using the scissors. The hole should be as wide as the straw is thick.

Step 2

Push the straw through the hole in the cap. It's not important how far down in the bottle it goes. Seal the hole around the straw using modeling clay.

Step 3

Pour a few drops of food coloring into the bottle. Then fill the bottle to the brim with cold water. Screw the cap back on. Check that it's air tight around the straw by squeezing the bottle. When doing so, the water level in the straw should rise.

Step 4

Pour hot water into the bowl (the hotter the better the result will be). Place the bottle in the bowl. See how the water level in the straw rises!

Short explanation

The water in the bottle expands when it's heated. Since there is no space in the bottle for the water to expand in, it rises up into the straw.

Long explanation

Most liquids, i.e. liquid chemical substances or mixtures, undergo thermal expansion, which means that their volume increase when heated.

Temperature is a measure of how much the particles in a substance move, that is, how much kinetic energy they have. As water is heated, the water molecules begin to move more and more. They also end up further and further apart. This is because the bonds (hydrogen bonds) that hold them together begin to tear. In liquid water, however, the water molecules always stay together, but the bonds are "stretched" out and the molecules change place more often the hotter the water is. When water is heated to its boiling point, the bonds break and the water molecules detach completely from each other. Then the liquid water changes to a gaseous state (water vapor).

The opposite is of course also true; if hot water cools down, the water molecules get closer to each other - but only to a limit. At 4 °C (39 °F), the water molecules are as close as they can get and water has its highest density. When water then cools down even more, and the water molecules thus move less and less, they actually end up further apart again! This is when the hydrogen bonds between the water molecules begin to dominate completely, and arrange the molecules in a well-arranged crystal structure. Water molecules have electrically charged ends (are dipoles). In this crystal structure, the water molecules are rotated so that the positive end of one molecule faces the negative end of another molecule, and so on. For this to work for all water molecules, they cannot be too close to each other. At 0 °C (32 °F), they have completely stabilized in fixed positions a short distance apart. The water is then in solid state (water ice) and has increased 9 % in volume compared to 4 °C (39 °F) hot water.

It's very unusual for the solid state of a chemical substance to have a lower density than the liquid state. But for water, it's true. And this has major consequences for life on Earth. This is the reason ice floats and creates an insulating layer on top of frozen lakes or seas. Would ice instead sink, a lake or sea would begin to freeze from the bottom up, and without the insulating layer at the top, the probability that the entire water column would freeze to ice is much greater.

In this demonstration, it's very clear that hot water takes up more volume than cold water. This is because the water molecules are further apart, and this is the same as the water having a lower density. If hot and cold water were mixed, the hot water would float on top of the cold - until their temperatures were equalized by conduction. This is why it's usually warmest on the surface of a lake. The same applies in the atmosphere, where hot air floats on top of cold air. In the atmosphere, the air usually heats up at Earth's surface and then rises to settle in its place higher up (and then cooled down by space).

The fact that hot air or hot water rises is due to the fact that the gravitational force from Earth has a weaker effect the lower the density of a substance is. Almost the entire globe is actually layered according to density, with heavy iron in the core and lighter rocks further up. This layering can only take place when the substances are liquids or gases and can change places (the whole Earth was liquid early in Earth's history).

In this demonstration, the water in the bowl heats the water in the bottle. This is done by conduction. Conduction is heat transfer by direct contact, more specifically the transfer of kinetic energy of particles by the particles colliding with each other. In this demonstration, the water molecules in the bowl collide with the plastic molecules in the bottle, which in turn collide with the water molecules inside the bottle. The kinetic energy is transferred in the direction from the particles that move the most to those that move the least. In the end, all particles will move equally. Since temperature is a measure of the kinetic energy of particles, conduction leads to the temperature being spread between all particles.

The thermometer you made in this demonstration works just like a regular (liquid) thermometer. It contains a container with a liquid that expands when heated and contracts when cooled. This change in volume is made extra clear in a narrow channel that is part of the container.


You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.
  • What is the water level at 20 °C, 30 °C, 40 °C and so on? Draw lines on the straw - and you have created a graduated thermometer. Here you may need to thread another straw over the first, so that the water cannot overflow.
  • What happens if you change the amount of water in the bottle?
  • What happens if you change the width of the straw?
  • What happens if you change the size of the bottle?
  • What happens if you close the upper opening of the straw with clay?
  • What happens if you change the height of the bowl?


You can skip the cap completely and only use modeling clay to seal the opening. However, it's more difficult to get it tight, because the clay often comes loose when it gets affected by heat and water vapor.
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© The Experiment Archive. Fun and easy science experiments for kids and adults. In biology, chemistry, physics, earth science, astronomy, technology, fire, air and water. To do in preschool, school, after school and at home. Also science fair projects and a teacher's guide.

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© The Experiment Archive. Fun and easy science experiments for kids and adults. In biology, chemistry, physics, earth science, astronomy, technology, fire, air and water. To do in preschool, school, after school and at home. Also science fair projects and a teacher's guide.

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The Experiment Archive by Ludvig Wellander. Fun and easy science experiments for school or your home. Biology, chemistry, physics, earth science, astronomy, technology, fire, air och water. Photos and videos.