The conversion of liquid into a vapour state is called the Vaporisation of liquid.
The phenomenon of the liquid gradually turning into vapour only from the surface at any temperature is called the evaporation of liquid.
Factors Affecting the Rate of Evaporation
The rate of evaporation depends on the following factors:
(i) Surface area of the liquid: The greater the surface area of the liquid, the faster the evaporation.
(ii) Nature of liquid: Different liquids have different rates of evaporation. The lower the boiling point of the liquid, the higher the rate of evaporation. The evaporation rate of a volatile liquid is maximum.
(iii) Pressure on the liquid: As the atmospheric pressure on the liquid increases, the rate of evaporation decreases. Evaporation rate increases as pressure decreases. The evaporation rate is highest in a vacuum.
(iv) Air movement: Evaporation is accelerated by increasing air movement over the liquid.
(v) Elevation of liquid and liquid-adjacent air: Evaporation is accelerated when the volume of liquid and liquid-adjacent air increases.
(vi) Dryness of air: The drier the air above the liquid, i.e. the less water vapour in the air, the faster the evaporation.
Difference Between Evaporation and Boiling
| Evaporation | Boiling |
|---|---|
| (i) It can occur at any temperature. | (i) It occurs at a given pressure at a given temperature. |
| (ii) It takes place under any pressure. | (ii) During boiling the vapour pressure above the liquid is equal to the pressure of the outside atmosphere. |
| (iii) It takes place from the surface of the liquid. | (iii) It occurs from all parts of the liquid. |
| (iv) Liquid collects the heat required for evaporation from the body. As a result, the temperature of the liquid decreases. | (iv) The heat required for boiling is supplied from outside. So the boiling point of the liquid is constant. |
| (v) Increasing the temperature increases the rate of evaporation. | (v) During Boiling temperature of the liquid remains constant. |
| (vi) It is a very slow process. | (vi) It takes place very quickly. |
| (vii) It is silent. | (vii) It is noisy. |