Page 175 - Science Coursebook

11.7 Evaporation

Evaporation and energy
When water evaporates from your body, it carries
energy away. That is why you feel cooler.
Why does evaporation have a cooling effect? We need

to think about the particles of the water.
The particles in water are moving around. Some
have enough energy to escape from the surface. They
become water vapour in the air.
The particles with the most energy are the ones that Some particles in the water are moving around
escape. The ones with less energy are left behind, so the faster than others. The fastest ones (shown in
water is colder than before. Its temperature decreases. red) can escape into the air.

Questions
A+I 3 Explain why hot water evaporates more quickly than
cold water.
A+I 4 If you hang washing on a line to dry, it dries more
quickly on some days than others. What is the best sort
of weather for drying washing quickly? Explain your
answer.
A+I 5 The drawing shows two containers of water. Explain
why the water in the wide container will evaporate more
quickly than the water in the narrow container.

Activity 11.7
Observing and explaining evaporation

SE Your teacher will demonstrate two simple experiments which involve
evaporation.
Experiment 1: A drop of alcohol (ethanol) is placed on a cold metal plate, on
the back of someone’s hand, and on a warm metal plate.
Experiment 2: Two drops of water are placed on a warm metal plate. One is
spread out, the other is not.
For each experiment, note down what is done and what is observed. Then write
an explanation using the ideas you have learnt in this topic.

Summary
• When water evaporates, its temperature decreases.
• This is because faster particles escape from the water’s surface to
become water vapour in the air.

• When water evaporates, it takes energy and therefore has a cooling effect.

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