Question

The pH of a sample of pure water is 7 at room temperature. What will be its pH when a pinch of solid baking soda is dissolved in it?

• A. Very near to 7
• B. Less than 7
• C. More than 7
• D. Exactly 7

Answer 1

The correct option is C

More than 7

An explanation for correct answer

The pH values of a solution vary from 0 to 14 and neutral solutions have a pH of 7. The pH of the acidic solution is less than 7 and basic solutions have a pH greater than 7. Pure water $\left({\mathrm{H}}_2\mathrm{O}\right)$ is an example of a neutral solution. The addition of acid decreases the pH of the water and pH increases with the addition of a base.

Baking soda is chemically sodium bicarbonate $\left({\mathrm{NaHCO}}_3\right)$. It is a salt of a weak acid carbonic acid $\left({\mathrm{H}}_2{\mathrm{CO}}_3\right)$ and a strong base sodium hydroxide $\left(\mathrm{NaOH}\right)$. Therefore, a solution of sodium bicarbonate will be basic. This is because the bicarbonate ions formed from sodium bicarbonate react with water to form hydroxide ions which make the solution basic.

That is, when sodium bicarbonate is added to water, the pH of the solution increases to a basic pH. Therefore, the pH of the water will be more than 7.

An explanation for incorrect answers

Since the addition of baking soda increases the pH of the water, the pH of the solution cannot be 7, very near to 7 or less than 7. Therefore, options (A), (B), and (D) are incorrect.

Therefore, the correct option is (C) More than 7.