38. Evaporation of liquids

A well-closed ink jar can stay in the storage cabinet for an extremely long time and the amount of ink in it does not change. If the jar is left open, we can see that there is no liquid in it after a long time. The water in which the ink dye is dissolved has evaporated. A puddle on the asphalt dries much faster, especially if the air temperature is high and the wind blows.

This phenomenon is explained by the molecular-kinetic theory. Liquid molecules are involved in chaotic movement. At the same time, the higher the temperature of the liquid, the more intensive the molecules move, the greater their kinetic energy. But only the average value of the kinetic energy molecule has a certain quantity at a given temperature. At the moment each molecule may have either less or more energy than the average. Kinetic energy of some molecules may be so large that they are able to fly out of the liquid, overcoming the attraction forces of other molecules. This is the process of evaporation.

A flying molecule takes part in the chaotic heat movement of gas. By moving randomly, it can move away from the liquid surface in an open container forever. But it can also return to the liquid again.

If the air flow above the container carries away the liquid vapor, the liquid evaporates faster because the ability a vapour molecule to return to the liquid decreases.

The higher the temperature of the liquid, the more molecules have enough kinetic energy to escape from the liquid, the faster the evaporation takes place.

At evaporation the liquid escapes faster molecules. Therefore, the average kinetic energy of liquid molecules decreases. This means that the temperature of the liquid drops. Wetting your hand with some rapidly evaporating liquid (ether or acetone), you will immediately feel a strong cooling in the wetted area. Cooling will intensify if you blow on the wetted area. The same can be observed if you wet your finger with water, but the effect is less noticeable. However, you can use your finger to more precisely determine the direction of a very weak wind when it is difficult to immediately understand where it is blowing from. To do this, the finger wetted with water should be raised upwards. Cooling will be greatest from the side from which the wind blows.

If the liquid cannot evaporate, it will cool down much more slowly. Remember how long the fat soup cools down. A layer of fat on the surface of the soup prevents fast water molecules escaping. The liquid does not evaporate much, and its temperature drops slowly (the fat itself evaporates very slowly, as its larger molecules are more firmly bound to each other than water molecules).