Question

The aqueous solution of which of the following turns phenolphthalein pink?

• A. $\mathrm{NaCl}$
• B. $\mathrm{KCl}$
• C. ${\mathrm{Na}}_2{\mathrm{CO}}_3$
• D. ${\mathrm{NH}}_4\mathrm{Cl}$

Answer 1

The correct option is D

${\mathrm{NH}}_4\mathrm{Cl}$

Explanation for the correct option:

(D) ${\mathrm{NH}}_4\mathrm{Cl}$

1. The phenolphthalein is a colorless indicator used to determine the acidic or basic nature of the solution.
2. In a basic medium, it turns into pink color.
3. In an acidic medium, it is colorless.
4. Hydrolysis of ammonium chloride $\left({\mathrm{NH}}_4\mathrm{Cl}\right)$ produces hydrochloric acid (HCl) which is strong acid.
5. Due to the acidic nature of the product formed, phenolphthalein turns pink.

Reaction involved:

$$\begin{gathered} {\mathrm{NH}}_4\mathrm{Cl}\left(\mathrm{aq}\right)\stackrel{}{\to }{{\mathrm{NH}}_4}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right) \\ {{\mathrm{NH}}_4}^{+}\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\stackrel{}{\to }{\mathrm{NH}}_3\left(\mathrm{aq}\right)+{\mathrm{H}}_3{\mathrm{O}}^{+}\left(\mathrm{aq}\right) \end{gathered}$$

Explanation for the incorrect options:

(A) $\mathrm{NaCl}$

1. Hydrolysis of $\mathrm{NaCl}$ produces $\mathrm{NaOH}$ and $\mathrm{HCl}$.
2. They both neutralize each other and hence there is no color change.

(B) $\mathrm{KCl}$

1. $\mathrm{KCl}$ on hydrolysis gives $\mathrm{KOH}\mathrm{and}\mathrm{HCl}$.
2. They both neutralize each other and hence there is no colour change.

(C) ${\mathrm{Na}}_2{\mathrm{CO}}_3$

1. ${\mathrm{Na}}_2{\mathrm{CO}}_3$ on hydrolysis gives $\mathrm{NaOH}\mathrm{and}{\mathrm{H}}_2{\mathrm{CO}}_3$.
2. They both neutralize each other and hence there is no colour change.

Therefore option (D) is correct.