Understanding Evaporation

Solid, liquid and gas are called Phase, and they characterize an arrangement of atoms/molecules. Besides arrangement, all phases of a substance are made of the exact same atoms/molecules. Phase and their change depends on the weak attractive force between atoms/molecules and the distance between them.

Evaporation is the change of arrangement of molecules which causes liquid to become gas. In liquid, molecules are quite close to each other, and they have high inter-molecular forces. Because of this, liquids posses this fluidity – Molecules move with respect to each other. On the other hand, the molecules of gases have large gaps between them and they posses no or very small intermolecular forces. Because of this, molecules of gas are not bound to each other, so they are in constant haphazard motion. The molecules of gas posses much more kinetic energy than liquid molecules because they can move at high velocities independently.

So, seeing the characteristics of both phases, evaporation can be seen as the process in which liquid molecules gain energy, gain freedom from other molecules and lose intermolecular forces.

Liquid can be thought as uniform layers of atoms/molecules. These layers are stacked one upon each other. When one or more molecules gain energy, they start agitating. This agitation brings them to the surface of liquid. Because of this phenomenon, the high energy molecules of liquid are on its surface. When these already agitated molecules collide with other agitated molecules, or high energy molecules rising from bottom, they bounce and break away from the bonds of nearby molecules. Once it loses intermolecular forces, the molecule becomes independent, and being in gas phase, it has a lot of kinetic energy and starts constant motion.

Of course, breaking away from surface requires only a fixed energy. If additional energy is given, like when the liquid is heated, the extra energy increases the kinetic energy of the gas formed. Condensation requires absorption of that energy, and so this is why vapours from glass of water kept at room temperature condense easily as compared to steam formed in tea pot.

When heat is given to liquid, this process occurs very fast. This is because more molecules are agitated, and the frequency of molecules leaving the surface increases.

This also explains why evaporation causes cooling. The molecules on the surface poses the highest energy. When they leave the surface, the total energy of the whole liquid decreases drastically, causing the temperature to drop.  This is why splashing face with water or even washing hands feels slightly cooler even with warm water. This is the principal used in industries to cool down water, which in turn helps in cooling process equipments.