Question

An aqueous solution turns red litmus solution blue. Excess addition of which of the following solution would reverse the change?

• A. Baking powder
• B. Lime
• C. Ammonium hydroxide solution
• D. Hydrochloric acid

Answer 1

The correct option is D

Hydrochloric acid

Explanation for correct option:

(D) Hydrochloric acid

• The solution should be basic if it causes the red litmus test to turn blue. By adding an acid, its effects can be reversed.
• From the above option the acid is hydrochloric acid others are basic in nature.
• A base and an acid combine to generate salt and water in this acid-base reaction. The acidity of the reactants and their concentrations determine the $\mathrm{pH}$of the neutralized solution. So, this is the correct option.
• The neutralization reaction is best represented as:
• $\mathrm{Acid}+\mathrm{Base}\to \mathrm{Salt}+\mathrm{Water}$

Explanation for incorrect option:

(A) Baking powder $\left({\mathrm{NaHCO}}_3\right)$

• Baking powder is a base which does not bring the reversed effect. So, this is the incorrect option.

(B) Lime $\left(\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right)$

• Lime is a base which does not bring the reversed effect. So, this is the incorrect option.

(C) Ammonium hydroxide $\left({\mathrm{NH}}_4\mathrm{OH}\right)$

• Ammonium hydroxide is a base which does not bring the reversed effect. So, this is the incorrect option.

Hence, the correct option is (D) Hydrochloric acid.