Why does evaporation cool a liquid?

Aha : Why does water evaporate?

The water in the swimming pool is warm, the air even warmer. Nevertheless, you get goose bumps when you step out of the pool and don't dry yourself off right away. As soon as water begins to evaporate on the skin, it gets cold.

At first glance, evaporation is just a volatilization of water. After a summer downpour, streets dry up quickly, puddles disappear, the water opens up and away. You don't see that it also takes heat with it. However, if you experience the evaporation process up close after bathing, you will feel how much your own body cools down.

A small water droplet consists of trillion water molecules, an unimaginable number of particles buzzing around in it. They attract each other and stay together in the liquid. The warmer the water, the more freely the molecules can move. If you heat them up vigorously in a saucepan, they gain so much energy that they escape as hot steam.

To a lesser extent, this also happens at lower temperatures. Then only the fastest and hottest particles can overcome the gravitational pull of their neighbors and leave the liquid. They take their energy with them. Therefore, the temperature of the remaining liquid drops, it cools down.

We feel this evaporation cold after bathing, when our body has to replenish the lost heat. When we sweat, our body even actively uses evaporation to regulate temperature. Sweat glands form a film of water on the skin that uses up excess heat. A fan can be used a little, the fresh wind accelerates the evaporation process.

"Energy is required for every evaporation," says Enrico Frahm, engineer for water management at the University of Rostock. The energy comes from solar radiation or the direct environment. For example, when plants release moisture through their leaves, they extract heat from the air. "That's why it's so pleasantly cool in the forest in summer."

In hot regions of the world, people cool their drinks in clay jugs, through whose fine pores some liquid can evaporate. This is how the fastest liquid molecules escape and the rest cools down. Even an ordinary beverage bottle can be chilled by wrapping it in a damp cloth. Thomas de Padova

A collection of the "Aha columns" has been published under the title "Wissenschaft im Strandkorb" (160 pages, 14.90 euros) by Piper-Verlag.

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