The molecules in
a liquid are in a state of continuous motion, some moving faster than others. In the middle of
the liquid they collide with each other but at the surface of the liquid some of them are going
fast enough to escape from the liquid altogether.
We call this
EVAPORATION.
Of course the molecules need energy to escape and so
the hotter the liquid becomes the more evaporation there is. Notice that it is only the surface
molecules that evaporate and there are no bubbles of gas formed in the body of liquid
Evaporation is very important in the cooling of surfaces. If we put a drop
of methylated spirit on our hand the methylated spirit will soon evaporate and our hand feels
cold. The molecules of methylated spirit get the energy they need from our hand, leaving the
molecules of the hand with less energy and so colder. The same thing happens when you
sweat, only in this case you are losing water from your body by evaporation. The evaporation
of perfume is why you can smell it – no evaporation means no perfume molecules in the air
and so no smell.
You can show the cooling effect of evaporation by the following simple
experiment. Air is pumped through some ether in a boiling tube as shown in the diagram. As the ether evaporates pump into the air
bubbles it takes away heat energy from the tube, lowering its temperature. Ice will start to
form on the outside of the test tube. Ether is a highly volatile liquid, that means that it
evaporates easily and so it is ideal for this experiment.
Evaporation is vital
for drying washing. The washing will dry better on a warm, dry day than on a cold day and it
will also help if it is windy. The warm dry air can hold a lot of moisture and the wind will move
the damp air from near the washing, replacing it with more dry air that can then absorb more
moisture.