Thermodynamics: The 2016 advent calendar
Day 14

Supercooled water

How to freeze water instantly

Water freezes at 0 degrees Celsius. Everyone knows that. But is it possible to get liquid water to a lower temperature?

This experiment, which you can try at home, brings water down to below its freezing point, and then sets off the freezing reaction all at once. As long as the water is very pure, and left undisturbed in a cold environment, it’s hard for ice crystals to form as they need a nucleation point to start arranging around.

Filming this experiment with a thermal camera revealed an unexpected quirk: as the ice forms, the external temperature rises. The bonds forming by the reaction release energy, which warms the surrounding area. This is a clear demonstration of the second law of thermodynamics at work: though the entropy of the contents inside the bottle has decreased as the molecules align into an ordered structure, the overall entropy increases as heat is given off and the order of the surrounding environment decreases.

Try it yourself

This always takes a few tries to get right. Here are our top tips for trying it at home:

We found that unopened bottles of ‘smart water’ worked best. Carefully put them upright in your freezer and try not to disrupt them while they freeze. You can put another bottle of tap water next to it in the freezer to use as a guide for timing: if the bottle of tap water has frozen, your water should be about ready to go. This is because tap water has impurities in it which help the ice crystals form, so it should freeze more easily when it gets to 0 degrees. It took about 2 hours in our freezer.

Then simply take the bottle very carefully out of the freezer, and give it a whack! And don’t be disheartened if it takes a few goes!

You can also try to create a flowing ice tower with the same method, but instead of hitting the bottle, pour the supercooled water over an ice cube. Watch CHRISTMAS LECTURES assistant Natasha demonstrate:

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