Question

$A$ is an aqueous solution of acid and $B$ is an aqueous solution of a base. These are diluted separately. Then:

• A. pH of $A$ increases while that of $B$ decreases till neutral.
• B. pH of $B$ deceases while that of $A$ increases till neutral.
• C. pH of $A$ and $B$ decreases
• D. pH of $A$ and $B$ increases

Answer 1

The correct option is A pH of $A$ increases while that of $B$ decreases till neutral.

Acids release hydrogen ion in the solution medium whereas base release hydroxide ion in the solution medium.

When we dilute these solutions, the number of hydrogen ions (in case of acids) and the number of hydroxide ions (in case of bases) per unit volume decreases. This happens because we are increasing the volume by dilution but the number of dissociated ions remains the same.

Therefore, the total number of ions per unit volume decreases with dilution.

This also leads to increase in the pH of acids (because they become less acidic) and decrease in the pH of bases (because they become less basic).

At maximum dilution, the free ions won't have any effect on the pH and hence the solution will become neutral and its pH will become 7.

Hence, the correct answer is the option (A).