CBSE Class 9 Answered

(a) When a liquid is left exposed to air, its volume decreases gradually because of evaporation of some of the water from its surface.

(b) The four factors affecting the rate of evaporation are:

1. Surface area: Evaporation is a surface phenomena. If the surface area is increased, the rate of evaporation increases.

2. Temperature: With the increase of temperature, more number of particles gets enough kinetic energy to go into the vapour state and hence the rate of evaporation increases.

3. Humidity: If the humidity of air is high, then the rate of evaporation decreases.

4. Wind speed: With the increase in wind speed, the particles of water vapour move away with the wind, decreasing the amount of water vapour in the surroundings hence the rate of evaporation increases.