Question

A student tests a sample of drinking water and reports its pH value as 6 at room temperature. Which one of the following might have been added in water?

• A. Washing soda
• B. Common salt
• C. Baking soda
• D. Bleaching powder

Answer 1

The correct option is D

Bleaching powder

An explanation for correct answer

The pH scale varies from 0 to 14, and a neutral solution will have a pH of 7. Acidic solutions have a pH of less than 7, and the pH value for bases will be greater than 7. Pure water $\left({\mathrm{H}}_2\mathrm{O}\right)$ is a neutral solution with a pH equal to 7. The addition of acid decreases the pH of water, and the addition of a base increases its pH.

(D) Bleaching powder is chemically Calcium hypochlorite $\left({\mathrm{CaOCl}}_2\right)$. It is slightly acidic, which helps in killing the germs during cleaning. The addition of bleaching powder to water decreases the pH of the water to less than 7. Therefore, the acidity of the water can be 6 due to the presence of bleaching powder.

An explanation for incorrect options

(A) Washing soda is chemically Sodium carbonate decahydrate $\left({\mathrm{Na}}_2{\mathrm{CO}}_3·10{\mathrm{H}}_2\mathrm{O}\right)$. It is a slightly basic salt, and therefore, the addition of washing soda to water increase the pH of water. Option (A) is incorrect.

(B) Common salt is chemically Sodium chloride $\left(\mathrm{NaCl}\right)$. It is a neutral salt, and therefore, the addition of common salt will not change the pH of water. Option (B) is incorrect.

(C) Baking soda is chemically Sodium bicarbonate $\left({\mathrm{NaHCO}}_3\right)$. It is a slightly basic salt, and therefore, the addition of baking soda increases the pH of water. Option (C) is incorrect.

Therefore, the correct option is (D) bleaching powder.