Question

Which gas cannot be collected over water?
A. ${\text{N}}{{\text{H}}_3}$
B. ${\text{CO}}$
C. ${{\text{N}}_2}$
D. ${{\text{H}}_2}$

Answer 1

Gases show different solubilities in water. Some gases dissolve in water and form new products by undergoing a chemical reaction. Whereas, some gases do not show this behaviour.

Complete step by step answer:
While producing gases in a laboratory, they are often collected by a technique called water displacement. To do so, the gas to be collected should not dissolve in water.
Ammonia solubilizes easily in water to give ammonium hydroxide. Reaction for this in simple form is
${\text{N}}{{\text{H}}_3}{\text{ + }}{{\text{H}}_2}{\text{O }} \to {\text{ N}}{{\text{H}}_4}{\text{OH}}$

${\text{CO}}$, ${{\text{N}}_2}$, and ${{\text{H}}_2}$do not dissolve in water and hence can be easily collected by water displacement method.

Hence, option A is correct.

Additional information: In water displacement method, a beaker filled with water is kept upside down in a trough of water. A rubber or glass tube, on one end is attached to a reaction flask while the other end is kept under water in the trough.

The gas formed in the reaction flask, travels through the tube and displaces the water in the bottle. When the gas completely fills the flask, the flask is fitted with a rubber cork. Since the gas is collected over water, it is mixed with water vapours.
To collect ammonia, a gas jar is connected by a tube to a flask which is giving off gas. The air in the jar is displaced until the gas jar is filled with collected gas. Ammonia solubilised easily and hence collection apparatus should be dry with a drying agent such as calcium.

Note: Depending on whether a gas is soluble in water or not, its collection method is decided. Even though collection over water is most common, for gases which dissolve in water, other methods are also available.