At a given temperature, a substance in the liquid phase has the tendency to change into the gaseous phase without reaching its boiling point through the process called evaporation. Liquid molecules have intermolecular bonds. With sufficient heat energy, these bonds tend to dissociate, releasing the molecules into the gaseous phase. This process happens at the surface of the liquid. This is because the surface is in close contact with the atmosphere and heat energy can be easily absorbed. Evaporation happens below the boiling point of the liquid. Only the liquid molecules which are at the surface, absorb energy from the atmosphere to break their intermolecular bonds and go into the gaseous phase. Molecules at the bulk of the liquid only get evaporated when they reach the surface and get exposed to the atmosphere. The rate of evaporation is directly associated with the strength of intermolecular bonds between the liquid molecules. When the strength of intermolecular bonds is high, the liquid is less volatile. Liquids with weak intermolecular bonds are highly volatile. Water molecules are less volatile due to the strong hydrogen bonds between the molecules. Non-polar organic compounds do not have such strong intermolecular attractions. They have Van Der Waals bonds which are comparatively weak. Therefore, the liquid molecules can easily go into vapour phase. Most non-polar organic liquids are highly volatile. Distillation

Modern separation technique adapted based on the different boiling points of liquids. This is due to the strengths of different intermolecular forces of substances. Different liquids boil at different temperatures because the heat energy needed for bond breaking varies. Distillation is used to separate mixtures of liquids. This involves boiling and condensing liquids. The liquid is heated and boiled at its boiling point. The temperature remains constant until the relevant liquid vaporizes completely. The vapour is then turned into liquid phase with the aid of a condenser. There are several distillation methods such as simple distillation, fractional distillation and steam distillation. Simple Distillation This is used to separate liquids with a significant boiling point gap. The components of the liquid mixture are separated when they boil at their respective boiling points and change into the vapour phase. The vapour is then condensed and collected. Fractional Distillation In this methods, a fractionating column is employed to separate two miscible liquids, which have close boiling points. Steam Distillation Steam is used to separate compounds which are immiscible with water. When such compounds are mixed with steam, they tend to vaporize at a lower temperature than their usual boiling point