Question

A white substance having alkaline nature in solution is
(A) $\rm{NaNO_3}$
(B) $\rm{NH_4Cl}$
(C) $\rm{Na_2CO_3}$
(D) $\rm{Fe_2O_3}$

Answer 1

Hint:Identify the type of each salt. The type means that it is salt of strong acid with strong base or salt of strong acid with weak base and so on.

Complete answer:
The colour of $\rm{Fe_2O_3}$ is brown to reddish brown. Hence, the option (D) is ruled out. $\rm{NaNO_3}$, $\rm{NH_4Cl}$ and $\rm{Na_2CO_3}$ are white substances.
The chemical formula $\rm{NaNO_3}$ represents the compound sodium nitrate. It is the salt of strong base sodium hydroxide with strong acid nitric acid. Salts of strong acid and strong base do not undergo hydrolysis in aqueous solution. Such salts form a neutral solution with pH being equal to 7. In such solutions, the hydronium ion concentration is equal to the hydroxide ion concentration.
The chemical formula $\rm{NH_4Cl}$ represents the compound ammonium chloride. It is the salt weak base ammonia with strong acid hydrochloric acid. For the salts of strong acid and weak base, the cation (ammonium ion) undergoes hydrolysis in aqueous solution. Such salts form acidic solutions with pH less than 7. In such solutions, the hydronium ion concentration is greater than the hydroxide ion concentration.
 $\rm{{NH_4}^+ +H_2O\;\to\;NH_3+{H_3O}^+ }$
The chemical formula $\rm{Na_2CO_3}$ represents the compound sodium carbonate. It is the salt strong base sodium hydroxide with weak acid carbonic acid. For the salts of weak acid and strong base, the anion (carbonate ion) undergoes hydrolysis in aqueous solution. Such salts form an alkaline solution with pH greater than 7. In such solutions, the hydronium ion concentration is less than the hydroxide ion concentration.
$\rm{ {CO_3}^{2-}+H_2O\;\to\;{HCO_3}^- +{OH}^- }$
Thus, a white substance having alkaline nature in solution is $\rm{Na_2CO_3}$.

Hence, the correct option is the option (C).

Note:
The pH of the aqueous solutions of the salts of weak acid with weak base depends on the relative strengths of acid and base. If the weak acid is stronger than the weak base, then the solution is slightly acidic with pH being slightly less than 7. If the weak acid is weaker than the weak base, then the solution is slightly basic with pH being slightly greater than 7.