question_answer 52)

  Comment on the following statements. (a) Evaporation produces cooling. (b) Rate of evaporation of an aqueous solution decreases with increase in humidity. (c) Sponge though compressible is a solid.

Answer:

  (a) Evaporation produces cooling. This is based on the fact that when a liquid evaporates, it takes (or draws) the latent heat of vaporization from 'another matter' which it touches. This 'another matter' looses heat and gets cooled. Therefore, evaporation causes cooling. (b) If humidity is high, then air is already saturated with water vapours, i.e., it has a lot of water vapours. Therefore, it will not take more water vapours easily. Hence, rate of evaporation decreases. (c) Sponge has minute holes in which air is trapped. The material is also not so rigid. On pressing this, air is expelled out, that is why it can be compressed but it is solid as it has a definite shape and volume and does not change its shape unless compressed.