Study of Compounds — Hydrogen Chloride — Question 47

(i) Direct absorption of hydrogen chloride gas in water is not feasible because it causes back suction in the delivery tube.

1. Hydrogen chloride gas is extremely soluble in water.
2. If a delivery tube through which HCl gas is passed is directly immersed in water, the rate of absorption of HCl gas is high and hence a partial vacuum is created in the tube.
3. The pressure outside being higher causes the water to be pushed up into the delivery tube and damages the apparatus. This is called back-suction.

(ii) Special funnel arrangement is used for avoiding back suction. It (a) Prevents or minimizes back-suction of water (b) Provides a large surface area for absorption of HCl gas.

1. The rim of the funnel is placed so that it just touches the trough containing water. If back-suction occurs, the water rises up the funnel and the level outside the funnel falls, creating an air gap between the rim of the funnel and the surface of water.
2. The pressure outside and inside equalize and the water which had risen in the funnel falls down again. This process continues till the water in the trough is saturated with hydrogen chloride gas resulting in the formation of hydrochloric acid.
3. Hence, hydrochloric acid is not prepared in the laboratory by passing hydrogen chloride gas directly through water, but prepared using a special funnel arrangement.