Question

From the list of following salts choose the salt that most appropriately fits the descriptions given in the following:
$[AgCl,MgC{{l}_{2}},NaHS{{O}_{4}},PbC{{O}_{3}},ZnC{{O}_{3}},KN{{O}_{3}},Ca{{(N{{O}_{3}})}_{2}}]$
(i) A deliquescent salt
(ii) An insoluble chloride.
(iii) On heating, this salt gives a yellow residue when hot and white when cold.
(iv) On heating this salt, a brown coloured gas evolved.

Answer 1

Write the decomposition reaction of each salt with heat and see which salt gives out a coloured gas. Change in colour of compound on heating can be because of ions occupying the interstitial sites which is a reversible reaction.

Complete step by step answer:
-Deliquescence, the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution.
-Deliquescence occurs when the vapour pressure of the solution that is formed is less than the partial pressure of water vapour in the air.
In the above-mentioned salts, the salt that is deliquescent is $MgC{{l}_{2}}$.
-Insoluble salts are the salts that do not dissolve in water and settle down at the bottom of the container as a solid.
From the list of salts given, the salt which is insoluble is $AgCl$.
We will now write down the thermal decomposition of $ZnC{{O}_{3}}$.
$ZnC{{O}_{3}}\xrightarrow{\Delta } ZnO+C{{O}_{2}}_{(\uparrow )}$
In $ZnO,Z{{n}^{2+}}$ ions occupy the interstitial sites and electrons get trapped in the interstitial sites for neutralization. These types of crystals show colour due to the defect. Zinc oxide is white in colour but on heating, it turns into yellow because it loses oxygen reversibly at high temperature.
Hence $ZnC{{O}_{3}}$ on heating gives a yellow residue which turns white on cooling.
The decomposition of $Ca{{(N{{O}_{3}})}_{2}}$ is as follows,
$2Ca{{(N{{O}_{3}})}_{2}}\xrightarrow{\Delta }.2CaO+4N{{O}_{2(Brown\\_gas)}}+{{O}_{2}}$
In the above reaction we see that decomposition of $Ca{{(N{{O}_{3}})}_{2}}$ leads to evolution of $N{{O}_{2}}$ which is brown.

Therefore, from the above reactions we can conclude the following:
(A) Deliquescent salt - $MgC{{l}_{2}}$
(B) An insoluble chloride - $AgCl$
(C) On heating, the salt gives a yellow residue when hot and white when cold - $ZnC{{O}_{3}}$
(D) On heating this salt, a brown coloured gas evolved - $Ca{{(N{{O}_{3}})}_{2}}$

Note: There is $PbC{{O}_{3}}$ in the list of salts which is also insoluble in water, however the option say insoluble chloride and not just insoluble, So the answer is $MgC{{l}_{2}}$.