Why evaporation cools

A cool feeling

Put a little acetone (nail polish remover) or a drop of perfume on your palm. You feel cold. When you sweat on a hot day, you feel cooler as the sweat dries. Why does evaporation cool?

The explanation

During evaporation, the particles with the most energy leave the liquid. So the particles that are left have, on average, less energy. The liquid becomes cooler.

There is a second way to say the same thing. For the liquid to change to a gas, each kilogram needs the latent heat of vaporisation. The liquid takes this heat from its surroundings. The surroundings lose heat, and they become cooler. For water, $2.26 \times 10^{6}\ \text{J}$ is taken from the surroundings for each kilogram that evaporates.

You should find that at rest a little sweat is enough, but in a hard game more than half a litre of sweat must evaporate every hour to keep up.

Examples

• Sweat. The sweat on the skin takes heat from the body as it evaporates. This keeps the body cool. A person who sits under a fan feels cooler, because the moving air makes the sweat evaporate faster.
• Cotton clothes in summer. Cotton takes in a lot of sweat and has a large surface, so the sweat evaporates fast and cools us. Cotton clothes are better than other cloth in the heat.
• An earthen pot. Water in a clay pot stays cool. Very small amounts of water seep through the walls of the pot and evaporate from the outer surface. This takes heat from the water inside.
• Acetone and perfume. They evaporate very fast, so they feel cold on the skin.
• Water on the roof. On a hot day, people sprinkle water on the roof or the courtyard. As the water evaporates, the roof cools.

A glass of cold water

Fill a glass with ice-cold water. After some time, small drops of water appear on the outside of the glass. Did the water leak through the glass? No. The cold glass cools the air next to it, and the water vapour in this air condenses into drops on the cold surface. This is the opposite of evaporation.

Practise with new numbers

Try it in Python

This program finds the heat that is taken from the surroundings when $10\ \text{g}$ of water or acetone evaporates.

Remember these

• Evaporation cools the liquid that is left, and its surroundings.
• It takes the latent heat of vaporisation from the surroundings.
• Examples: sweat, an earthen pot, acetone on the skin, cotton clothes in summer.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why do we feel cool when a drop of perfume is put on the wrist?

2. How much heat does $30\ \text{g}$ of sweat take from the body as it evaporates? Use $2400\ \text{J}$ for each gram.

3. Why do drops of water appear on the outside of a bottle taken out of the fridge?

4. Why does a fan cool us better on a dry day than on a humid one?