The process of changing of a liquid into a gas phase is called evaporation.
Explanation: Evaporation is an endothermic process in which heat is absorbed (energy is consumed) to convert liquid into solid. In the liquid state molecules are in continuous motion. They possess kinetic energy but all the molecules do not have same kinetic energy.
Majority of the molecules have average kinetic energy and a few have more than average kinetic energy. The molecules having more than average kinetic energy overcome the attractive forces among the molecules and escape from the surface. It is called as evaporation. This process is reverse to condensation in which gas changes into liquid.
H2O(l) ——-→à H2O (g)
Properties of evaporation:
1) Evaporation is a continuous process: Evaporation is a continuous process taking place at all temperatures. The rate of evaporation is directly proportional to temperature. It increases with the increase in temperature because of increase in kinetic energy of the molecules.
Evaporation is a cooling process. When the high kinetic energy molecules vapourize, the temperature of remaining molecules falls down. To compensate this deficiency of energy, the molecules of liquid absorb energy from the surroundings. As a result the temperature of surroundings decreases and we feel cooling. For example, when we put a drop of alcohol on palm, the alcohol evaporates and we feel cooling effect.
Factors effecting evaporation:
- Surface area: Evaporation is a surface phenomenon. Greater is surface area, greater is evaporation and vice versa. For example, sometimes a saucer is used if tea is to be cooled quickly. This is because evaporation from the larger surface area of saucer is more than that from the smaller surface area of a tea cup.
- Temperature: At high temperature, rate of evaporation is high because at high temperature kinetic energy of the molecules increases so high that they overcome the intermolecular forces and evaporate rapidly. For example, water level in a container with hot water decreases earlier than that of a container with cold water. This is because the hot water evaporates earlier than the cold water.
- Intermolecular forces: If intermolecular forces are stronger, molecules face difficulty in evaporation. For example, water has stronger intermolecular forces than alcohol, therefore, alcohol evaporates faster than water.