Question

Evaporation occurs only--
A. after boiling
B. after extreme cooling
C. at the surface of liquid
D. if boiling occurs at atmospheric pressure.

Answer 1

Hint:During evaporation, liquid changes into vapour without boiling. During evaporation, no bubbles form. It is a slow process.

Complete answer:
After boiling, the temperature of the complete liquid increases, so the complete liquid converts into gas directly. So, after boiling evaporation cannot take place.

During cooling the temperature of the substance decreases and the substance converts into solid form liquid. So, after extreme cooling liquid cannot convert into gas or vapour.

Evaporation is a type of vaporization. During evaporation liquid molecules from the surface of the liquid evaporates into the gas phase. So, evaporation takes place only from the surface, so evaporation is a surface phenomenon. Evaporation is a process in which on increasing temperature, the phase of the substance changes from liquid to gas. So, the statement that evaporation occurs only at the surface of the liquid is correct.

The boiling point is the temperature at which the vapour pressure of the liquid becomes equal to the vapour pressure of the atmosphere, so the liquid boils and converts into the gas from liquid. During boiling complete liquid boils and convert into the gas phase. So, evaporation cannot take place after boiling.

Therefore, option (C) at the surface of the liquid, is correct.

Note:
Evaporation and boiling are different. Evaporation does not require heating whereas boiling requires heating. At boiling point, vaporization occurs where the evaporation takes place at below boiling point. During evaporation vapour pressure of the liquid remains less than the vapour pressure of the atmosphere. During vaporization vapour pressure of the liquid becomes equal to the vapour pressure of the atmosphere.