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Evaporation

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.

 

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB
 
 
 
© Keith Gibbs 2020