Question

A solution has a pH of 9. On dilution the pH value

• A. decreases
• B. increases
• C. remain same
• D. none of these

Answer 1

The correct option is A

decreases

An explanation of the correct option

Option A:

• pH is the measurement of the hydrogen ion concentration in a solution. It is represented as a negative logarithm of hydrogen ions in a solution as given below: $\mathrm{pH}=-\log \left[{\mathrm{H}}^{+}\right]$
• The pH works at room temperature ($25{}^{o}C$. )
• Dilution is the process by which water or some other solvent is added to the solution to reduce the concentration.
• A solution with a pH of 9 is basic in nature.
• The pH measures the hydrogen ion ( ${\mathrm{H}}^{+}$) concentration.
• On dilution (adding water), the dissociation of water molecules will give an equal number of hydrogen ions and hydroxide ions. So, the concentration of hydroxide ions will also increase simultaneously.
• Hence, The pH of a basic solution decreases on dilution.

An explanation of the incorrect options

Option B: As the hydrogen ion concentration increases hence pH does not decrease.

Option C: As the hydroxide ions concentration increases pH cannot remain the same.

Final Answer: Hence, the correct option is (A), a solution has a pH of 9. On dilution, the pH value decreases.