Page 174 - Science Coursebook

11.7 Evaporation

The sea is a bit of a puzzle. Rivers flow into the

sea, but it never seems to get any fuller. Why not?
The answer can be found by looking at puddles.
When the rain stops, puddles gradually disappear.
The water has evaporated.

Evaporation is a bit different from boiling. The

temperature of the water doesn’t have to reach
100 °C. Even when the temperature is quite low,
the water gradually turns into water vapour in
the air.

Questions The cyclist gets wet but his clothes will soon dry

A+I 1 Use these ideas to explain why the sea does out, thanks to evaporation.
not get any fuller.

Evaporation cools you down
On a very sunny day, you may get too hot. One way to cool down is
to get wet – to have a swim in a river or the sea. When you get out,

water evaporates off your skin, and this cools you down. If there is
a breeze blowing, the water will evaporate more quickly and you
will cool down more rapidly.

Now think about getting wet on a cold, windy day. The wind makes
the water evaporate from your body and you get very cold. This
is the ‘wind chill factor’ which is sometimes mentioned on the
weather forecast.

Desert foxes cool themselves by panting. They breathe over their
tongues, and water evaporates, cooling them down. This boy got wet at the beach
and now he is feeling cold.

Questions
A+I 2 Look at the photograph of the cyclist at the
top of the page. Explain why he may start
to feel cold as he continues cycling.

A cape fox from southern Africa, panting to keep
itself cool.
172 11 Energy

A+E