Water Thermometer

Water Thermometer

Watch the temperature rise with this homemade thermometer.

Is it possible to make a thermometer out of water? Absolutely! The best part about our Water Thermometer experiment is that you have all the materials you need in your own home. That’s right, you’ll be measuring temperature with this amazing homemade tool in no time.

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Experiment Materials

  • Glass bottle
  • Straw
  • Modeling clay
  • Food coloring
  • Container of hot water
  • Container of cold water
  • Heavy glove

Experiment Videos



Add a few drops of food coloring to the bottle. You pick the color!

Water Thermometer - Step 2


Fill the glass bottle up to the bottom of its neck with cold water.

Water Thermometer - Step 3


Place a straw (make sure it’s long enough) into the bottle. Pack clay around the straw and bottle neck so that the straw is centered in the bottle. Make as tight of a seal as possible.

Water Thermometer - Step 4


Now that you’ve made your thermometer, set the bottom of the bottle into the container of hot water.


Make sure you are wearing a heavy glove and remove your water thermometer from the hot water.


Put the water thermometer into the container of cold water.

How Does It Work

As water heats up, the space between the water molecules expands. So, hot water is less dense than cold water. As the water heats up, the less dense liquid rises to the surface of the container. The opposite is true as the water cools down. The space between the molecules contracts, the liquid becomes more dense and the liquid sinks. This cycle is called convection.

Here’s the exception to the rule… when it gets cold enough to freeze, the molecules line up in an open crystalline structure that is actually less dense than the liquid form. This is why ice floats.

When the water in the bottle thermometer heats up, it expands. Normally the water would overflow the container, but the bottle was sealed with clay surrounding the straw. As the water expands, it pushed its way up the open straw.

In comparison, the opposite happens when water is cooled. The space between the water molecules contract, the liquid becomes more dense and the water level in the straw goes down.

Take It Further

Real thermometers do not use water. Why is that? If you have a thermometer that has a silver-looking liquid, your thermometer contain mercury. Because mercury is dangerous if you spill it, most thermometers today use alcohol colored with a red dye. Alcohol has a lower freezing point than water.

Also, real thermometers don’t use water inside because water doesn’t respond to temperature change very quickly. Try this variation. Fill your bottle with 50% rubbing alcohol and 50% water. Does the liquid move up and down the straw faster?

If you have a real thermometer, you can use it to make a scale on your homemade thermometer. Let your bottle get to room temperature and then mark the straw with what the actual room temperature is. Place the bottle thermometer in the refrigerator and mark the temperature after an hour. Repeat this process by placing the bottle in the sun to heat up the water. Mark the level. Make observations over the next few days to see how your homemade thermometer compares to the real thermometer.

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