Question

Name the gas which turns moist red litmus paper blue.

Answer 1

A gas that turns moist red litmus paper blue is ammonia.

• Dry or liquid ammonia has no effect on litmus paper, but the aqueous solution of ammonia turns red litmus blue.
• This is because dry or liquid ammonia cannot dissociate into ions, thereby it will not show any acidic or basic nature.
• In an aqueous solution of ammonia, due to solvation ammonia forms Ammonium hydroxide.
• This Ammonium hydroxide is capable of dissociating into ions as follows:

$$\begin{gathered} {\mathrm{NH}}_4\mathrm{OH}\rightleftharpoons \mathrm{N}{{\mathrm{H}}_4}^{+}+{\mathrm{OH}}^{-} \\ \mathrm{Ammonium}\mathrm{hydroxide}\mathrm{Ammonia}\mathrm{Hydroxide}\mathrm{ions} \end{gathered}$$

Thus, the ${\mathrm{OH}}^{-}$ions impart basicity to the aqueous solution.