Question

Dry ammonia has no action on litmus paper, but a solution of ammonia in water turns red litmus paper blue. Why?

Answer 1

Dry ammonia does not liberate any ions like $O{H}^{-}$ as it is not in contact with water, thereby has no action on litmus paper.

Ammonia solution in water turns red litmus blue as it forms ammonium ion ($N{H}_4^{+}$) and hydroxyl ion ($O{H}^{-}$).

$N{H}_3+H_{2}O\to N{H}_4^{+}+O{H}^{-}$

Here ammonia act as a base by liberating $O{H}^{-}$ ion. Thus it turns red litmus paper blue. Acidic and basic nature of a substance is because of the formation of $H^{+}$ ions in solution and $O{H}^{–}$ ions in solution, respectively.