Boiling and Evaporation

Boiling and Evaporation

When water evaporates, it changes from liquid to gas. When water boils, it changes from liquid to gas. What is the difference?

Boiling takes place at the boiling point of 100^oC, with all the sound and fury. Evaporation takes place at room temperature, peacefully. At least, those are how we use the words boiling and evaporation in everyday life.

If both involves the same change in phase from liquid to gas, why do they behave so differently? The reason has to do with the vapour pressure.

Water at any temperature is able to change into vapour. This because water molecules are in random motion, and there would always be a small fraction that just happen to have large enough energy to overcome the attraction between molecules and escape from the surface. These escaped molecules form the vapour. The vapour molecules are in random motion, just like air molecules. So the vapour also has a pressure. However, the a water molecule that escapes from the surface is quite likely to hit some air molecules and bounces back into the water.

At high temperature, a larger fraction of the molecules can escape. This results in high vapour pressure. When temperature is high enough, the vapour pressure can become larger than the air pressure. An escaped water molecule has enough energy to knock aside most of the air molecules in the way, and continue its escape from the water surface. Because vapour pressure is now greater than air pressure, vapour can overcome the pressure inside the water as well. This results in the bubbles that we see in boiling water, with the accompanying sound and fury.

In contrast, below boiling point, bubbles cannot form because the vapour pressure is smaller than air pressure, and cannot overcome the pressure in the water. So it is silent and peaceful. We call this evaporation.

The Physics Notes