Skip to content

Evaporation

Heat and matter · level 2, lesson 4 of 6

A question to start

After the rain, there are puddles everywhere. A day later they are gone. Nobody boiled them. Where did the water go?

The idea

Water can turn into a gas without boiling. This is called evaporation. It happens slowly, at the surface of the water, at any temperature. The water goes into the air as water vapour, a gas you cannot see.

Water evaporates faster when:

  • it is warm, such as on a sunny day;
  • it is windy, so the damp air is blown away;
  • it is spread out, so more of it touches the air.
shade, still airsun and wind

Worked examples

1. Washing on a line. It dries fastest on a warm, windy day.

2. Wet hair. A hairdryer blows warm air, so the water evaporates quickly.

3. A spilt drink. Spread out on the table, it dries faster than the same drink in a cup.

Try it

  1. What is evaporation?
  2. Do puddles need to boil to dry up?
  3. Give two things that make washing dry faster.
  4. Why does a towel spread out dry faster than one in a heap?
Answers
  1. A liquid slowly turning into a gas at its surface.
  2. No. They evaporate.
  3. Warmth, wind (also spreading it out).
  4. More of the water touches the air.

Common mistake

"The water is gone for ever." It is in the air as water vapour. Later it can come back as rain or dew.

Where it leads