Question

Why is the pH of distilled water usually <7? How can you prevent this from happening?

Short answer

The pH of distilled water usually <7 due to reaction of dissolved carbon dioxide with water.

Step-by-step solution

Concept Used

ph is a measurement of the level of acid or alkali in a substance or liquid.

pH is the negative logarithm of the concentration of${\mathrm{H}}^{+}$ ions in solution.

To calculate the pH of an aqueous solution we need to know the concentration of the Hydronium ion in moles per liter (morality).

Therefore, the pH of a substance is given by:

pH = - log [H₃O⁺].

Explanation for pH of distilled water being less than 7.

Carbon dioxide $\left({\mathrm{CO}}_2\right)$is present in the atmosphere. Thus, dissolved ${\mathrm{CO}}_2$lowers pH of the dissolved water by reacting with it to form carbonic acid. Hence, the concentration of hydronium ion decreases in a solution the pH of distilled water becomes less than 7.

Prevention of formation of carbon dioxide.

To prevent the formation of carbonic acid, water can be distilled under an inert atmosphere so that carbon dioxide is not dissolved in water. Most of the dissolvedcarbon dioxide can be eliminated by boiling the distilled water.