Question

Evaporation can take place at any temperature, but boiling takes place at a fixed temperature. Give appropriate reasons.

Answer 1

Evaporation can take place at any temperature because it is not a bulk process (doesn't take place in the whole mass of a liquid). It's a slow process as it only takes place at the surface of the liquid unlike vaporisation which takes place almost every part of the liquid. For this process to take place in a given liquid, only the water molecules which are on the top surface of the liquid require some heat energy to evaporate whereas in the process of vaporisation, the whole mass requires heat energy which is comparatively much higher than that required for evaporation. The heat energy required for Evaporation can be acquired by the molecules from their surroundings. And it should also be noted that the molecules at the top surface of a liquid always have more kinetic energy than the molecules at other parts of the liquid.
In short, evaporation is not a bulk process and also requires very little heat energy which can be acquired from its surroundings having any temperature (but not less than the liquid) whereas vaporisation (or boiling) is a bulk process and requires large amount of heat energy which can only be supplied by some other agents other than the surroundings.