Question

Identify the substance ‘Q’ based on the information given below:
The white crystalline solid ‘Q’ is soluble in water. It liberates a pungent-smelling gas when heated with sodium hydroxide solution.
(A) Ammonia
(B) HCl
(C) Ammonium chloride
(D) None of these

Answer 1

Hint: Compound also known by the name Sal ammoniac is a white crystalline solid and is highly soluble in water. It is formed as a by-product in the Solvay process which is used for the manufacture of sodium carbonate.

Complete step by step solution:
WE can solve this question if we focus on the fact that ‘Q’ is a crystalline solid. Ammonia (${ NH }_{ 3 }$) is a gas which when dissolved in water produces an ammonia solution which has a pungent smell.
HCl is also a gas which when dissolved in water gives a hydrochloric acid solution which has a low pH.
Ammonium Chloride is a white crystalline solid (inorganic compound) which is soluble in water. The solution of ammonium chloride is mildly acidic due to the presence of ammonium ions which have a tendency to undergo hydrolysis reaction. The reaction is shown below:
$ { NH }_{ 4 }^{ + }(aq)+{ H }_{ 2 }O(l)\rightleftharpoons { NH }_{ 4 }OH(aq)+{ H }^{ + }(aq)$
Its natural, mineralogical form is called Sal ammoniac. This mineral is commonly formed due to the condensation of coal-derived gases on burning coal dumps. It is also found in some volcanic vents.
Ammonium Chloride is formed as a by-product in the Solvay process which is used for the preparation of sodium carbonate (also called soda ash). The reaction is given below:
$ { CO }_{ 2 }(g)+2{ NH }_{ 3 }(aq)+2NaCl(aq)+{ H }_{ 2 }O(l)\rightarrow 2{ NH }_{ 4 }Cl(aq)+{ Na }_{ 2 }{ CO }_{ 3 }$
Ammonium chloride is prepared commercially by ammonia gas with either hydrogen chloride gas or with hydrochloric acid.
The reaction is given below:
$ { NH }_{ 3 }(g)+HCl(g)\rightarrow { NH }_{ 4 }Cl(s)$
When ammonium chloride is heated, it liberates Hydrogen chloride gas and ammonia gas. The reaction is given below:
$ { NH }_{ 4 }Cl(s)\xrightarrow { \Delta } { NH }_{ 3 }(g)+HCl(g)$
Ammonium chloride reacts with sodium hydroxide solution when heated to give sodium chloride solution and ammonia gas which is pungent smelling. The reaction is given below:
$ { NH }_{ 4 }Cl(s)+NaOH(aq)\xrightarrow { \Delta } { NH }_{ 3 }(g)+NaCl(aq)+{ H }_{ 2 }O(l)$

Hence the correct answer is (c) Ammonium chloride.

Note: Remember that ammonium chloride also reacts with alkali metal carbonates at high temperatures to give alkali metal chlorides, carbon dioxide and ammonia gas.