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  • An empty, clear plastic bottle (500ml size is good)
  • Some water
  • Some food colouring
  • Some modelling clay or something similar (blutac etc)
  • A clear, straight straw
  • Some isopropyl alcohol … this is optional…

The thermometer will still work if you omit the isopropyl alcohol – I use it because it is more sensitive to temperature change than water alone.

If you do want to use the isopropyl alcohol you should be able to get a small bottle in your local pharmacy. You should NEVER drink this substance and if using it, make sure you have adult supervision.


1. Take equal amounts of the isopropyl alcohol and water and add them to the bottle, you want to fill the bottle about one third with this mixture.

2. Next add a few drops of food colouring.

3. Now you want to place your straw into the bottle so that the end of the straw sits under the level of the liquid, but doesn’t actually touch off the bottom of the bottle. You need to secure it in place and to do this you seal the top of the bottle with modelling clay.

4. You want to make sure you have an air tight seal so that the only way air can get in is through the straw.

And that is your basic thermometer!

Place the thermometer then into a bowl of hot water at first and then into a bowl of cold water.

Molecules move more rapidly when heated and they move apart from each other, so when you heat the liquid in your homemade thermometer, by placing it in the hot water, the liquid expands and moves up the straw.

When you place it in cold water it cools down, the molecules don’t move as much and move closer together, so the liquid contracts and moves back down the straw.

If you want to, you can even stick a strip of paper on the side of your bottle so you can mark in the level of the liquid in the straw at different temperatures, like room temperature, hot water or cold water.

What other things would you test with your thermometer?

Ag Teastáil:


Do it at home

Do it at Home