Question

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

Answer 1

Hint: Lets know about some important information that is In ammonia the presence of lone pairs of electrons on the nitrogen atom makes ammonia basic in nature in its aqueous solution . Aqueous solution of ammonia dissociates partially to produce hydroxyl ion. Aqueous solution of ammonia is alkaline in nature. We can understand this by the following reaction where ammonia is reacting with water and gives ammonium ion and hydroxyl ion which makes the solution basic in nature and we know that litmus indicators are used to find out acidic whether the solution is acidic or basic .

Complete answer:
\[\begin{gathered}
  N{H_3} + {H_2}O \rightleftharpoons N{H_4}OH \\
  N{H_4}OH \rightleftharpoons N{H^ + }_4 + O{H^ - } \\
\end{gathered} \]
The bases have property to cause colour change in indicators for red litmus paper it changes to blue as pH level of bases are from 8 to 14 .There are so many indicators which are used in indication of acidic and basic nature and litmus paper is a very famous indicator.
So now we conclude that the dry ammonia gas has no action on litmus paper ,but a solution of ammonia in water turns red litmus paper blue, because the solution of ammonia in water actually results in the better formation of ammonium hydroxide and we know that ammonium hydroxide ,it is actually basic in nature as well as turns red litmus blue.

Note: We have approached this problem when we know that dry ammonia does not give rise to $O{H^ - }$ ions .So there is no action on litmus paper but a solution of ammonia in water releases $O{H^ - }$ ions . Thus the litmus paper turns red litmus to blue .