Question

Explain the process of evaporation.

Answer 1

1. The molecules of a liquid that kept in a dish, continuously move with random speeds in various directions. As a result, these molecules collide with other molecules.

2. During this collision they transfer energy to other molecules. Hence, the molecules at the surface acquire energy and may fly off from the surface. 

3. Some of these escaping molecules may be directed back into liquid when they collide with the particles of air. 

4. If the number of escaping molecules is greater than the number returned, then the number of molecules in the liquid decreases. 

5. Thus when a liquid is exposed to air, the molecules at the surface keep on escaping from the surface till the entire liquid disappears into air. This process is called evaporation.

Answer 2

•Evaporation causes cooling naturally. The underlying principle behind this is, in order to change its state, the matter must either gain or lose energy. In the case of change of phase from liquid to gas, molecules of matter require energy to overcome their potential energy by their kinetic energy. So, the liquid takes this energy from its surroundings.

•Generally, when energy transfer occurs, it results in an increase or decrease in temperature of the substance, depending on whether the energy is being transferred from the substance to the surroundings or vice versa. However, there are exceptions to this rule.

•Although there is an increase in temperature of the substance until the boiling point is attained during evaporation, phase change results in no observable heat transfer.

•The molecules of the substance absorb heat energy continuously from the surroundings and thus cool the surroundings until they reach the boiling point, after which they start to break free from the liquid and turn into vapour. Since there is no change in temperature till the evaporation process is complete i.e. the entire liquid gets converted into vapour, the amount of energy required for this phase change is called the latent heat of vaporization, where the word ‘latent’ means ‘hidden’, meaning this heat will not change the temperature reading on a thermometer.