Question

Explain the effect of pressure on the solubility of the gases. 

OR 

State and explain Henry’s law.

Answer 1

(1) Since the gases are compressible, their solubility in the liquids is influenced by external pressure of the gas. The solubility of gases increases with the increase in pressure.

(2) Henry’s law : It states that the solubility of a gas. in a liquid at constant temperature is proportional to the pressure of the gas above the solution.

(i) If S is the solubility of a gas in mol dm^-3 at a pressure P and constant temperature then by Henry’s law, 

S ∝ P or S = K_H × P 

where K_H is called Henry’s law constant.

(ii) If P = 1 atm, then S = K_H.

(iii) If several gases are present, then the solubility of any gas in the mixture is proportional to its partial pressure at given temperature.

(3) Illustration of Henry’s law : In case of aerated or carbonated drink beverage, the bottle is filled by dissolving CO₂ gas at high pressure and then sealed. Above the liquid surface there is air and undissolved CO₂. Due to high pressure, the amount of dissolved CO₂ is large.

When the cap of the aerated bottle is removed, the pressure on the solution is lowered, hence excess of CO₂ and air escape out in the form of effervescence. Thus by decreasing the pressure, solubility of CO₂ is decreased.