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The Challenge: Make Ice


As you might expect, we want our cooling liquid to be colder than 0°C (32°F) to turn our water into ice. As the cooling liquid absorbs energy from the water there's a danger it will heat up. We can stave off this rise in temperature by using evaporation which takes heat energy away by forming vapor. Have you ever wondered why you feel cold as you get out of the swimming pool, even on a hot day? That's evaporation: the water on your skin evaporates, taking heat energy from your skin, and using it to break the hydrogen bonds between water molecules as the liquid turns to vapor.

Magnified molecules escaping from liquidEvaporation occurs when there is sufficient energy to enable the fastest moving molecules to break the inter-molecular bonds in a liquid at its surface.

Since the temperature we measure depends on the average speed of the molecules, if the faster molecules escape from the liquid the average molecular speed is reduced and the temperature falls. The hotter the liquid, the more molecules there are moving rapidly so the faster it evaporates and the greater the cooling effect.

Obviously there is a top limit involved here — the boiling point. At this temperature we have the maximum possible rate of evaporation and hence the fastest cooling.