Give a balanced equation for the thermal dissociation of a solid [containing the chloride ion].
A. \[{\text{2N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\left( {\text{s}} \right) \rightleftharpoons {{\text{N}}_{\text{2}}}{{\text{H}}_{\text{6}}}\left( {\text{g}} \right){\text{ + 2HCl}}\left( {\text{g}} \right)\]
B. \[{\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\left( {\text{s}} \right) \rightleftharpoons {\text{NH3}}\,\left( {\text{g}} \right){\text{ + C}}{{\text{l}}_{\text{2}}}\left( {\text{g}} \right)\]
C. \[{\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\left( {\text{s}} \right) \rightleftharpoons {\text{NH3}}\,\left( {\text{g}} \right){\text{ + }}{{\text{H}}_{\text{2}}}\left( {\text{g}} \right)\]
D. \[{\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\left( {\text{s}} \right) \rightleftharpoons {\text{NH3}}\,\left( {\text{g}} \right){\text{ + HCl}}\left( {\text{g}} \right)\]

Hint: A chemical reaction in which a substance decomposes reversibly when the temperature is raised is called thermal Dissociation. The substance comes back to its original form when it is cooled. \[{\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\]thermally decomposes to give ammonia and hydrogen chloride.

Complete step by step answer:
The heating of \[{\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\] above a temperature of \[{\text{34}}{{\text{0}}^{\text{o}}}{\text{C}}\], it thermally decomposes into two colourless gases i.e., ammonia and hydrogen chloride. This reaction can be reversed on cooling and solid ammonium chloride forms again. It is also an example of sublimation, but it involves a chemical change also. Sublimation is the process when a substance changes directly from a solid into a gas without condensing to form a liquid.
Ammonium chloride + Heat \[ \rightleftharpoons \] Ammonia + Hydrogen chloride
\[{\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\left( {\text{s}} \right) \rightleftharpoons {\text{NH3}}\,\left( {\text{g}} \right){\text{ + HCl}}\left( {\text{g}} \right)\]
Here, because of heat, the reaction moves to the right and when cooled, it moves the reaction to the left. So, the decomposition of ammonium chloride is the forward reaction, and the formation of ammonium chloride from ammonia and hydrogen chloride is the backward reaction. This can be tested by putting a neutral litmus paper at the top of the test tube containing \[{\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\] and it is observed that litmus paper change blue to and red due to the acidic hydrogen chloride.

So, the correct answer is Option D .

Note: The reaction in which the products cannot react to reform the reactants again are called irreversible reactions. This type of the reaction can take place in only one direction.

The breaking up of a compound into its simpler constituents that can be capable of recombining under other conditions is called Dissociation. In electrolytic, or ionic, dissociation, the addition of a solvent the form of heat causes molecules to break up into ions. Chemical combinations with the solvent produce ions by dissociating substances. Electrical conductivity and many other properties of electrolytic solutions can be explained by the idea of ionic dissociation.