An example of an alkaline gas is

An alkaline gas is a gas that dissolves in water to produce a solution with a pH greater than 7 (a basic solution).

NH₃ (ammonia) is the classic example. When ammonia dissolves in water, it reacts to form ammonium hydroxide:

\[\text{NH}_3(g) + \text{H}_2\text{O}(l) \rightleftharpoons \text{NH}_4^+(aq) + \text{OH}^-(aq)\]

The production of hydroxide ions (OH^-) makes the solution alkaline.

The other gases are not alkaline:

• HCl (hydrogen chloride) dissolves in water to form hydrochloric acid, which is strongly acidic.
• N₂ (nitrogen) is an inert, unreactive gas that does not dissolve appreciably in water and produces neither acidic nor basic solutions.
• NO₂ (nitrogen dioxide) dissolves in water to form a mixture of nitric acid and nitrous acid, both of which are acidic.

Exam tip: Ammonia is the only common alkaline gas encountered at this level. Its characteristic pungent smell and ability to turn moist red litmus paper blue are standard identification tests.