Explain the following assertions:

(i) Composition of dissolved air in water is different from ordinary air.

(ii) Steam at 100°C burns more severely than water at 100°C.

(iii) It is necessary to compare gases at STP.

(i) Ordinary air contains 78% nitrogen, 21% oxygen and 0.01% carbon dioxide. But oxygen is more soluble in water as compared to nitrogen. Hence, the composition of air dissolved is different from ordinary air.

The composition of air dissolved in water is 33% oxygen, 66% nitrogen and 1% carbon dioxide.

(ii) Steam has a higher heat content on account of high specific latent heat of condensation that is 2268 J g-1. Hence, steam at 100°C carries more heat than water. Therefore, steam at 100°C burns more severely than water at 100°C.

(iii) Since volume of a gas changes remarkably with change in temperature and pressure, it becomes necessary to choose standard values of temperature and pressure to which gas volumes can be referred.