Question

Explain why the pH of deionized water changes so drastically when acid or base is added.

Answer 1

Addition of acid to deionized water :

If we add a strong acid or strong base to water, the pH will change dramatically.For example if we add a strong acid such as HCl to deionized water results in the following reaction

HCl + H₂O --------> H₃O⁺ + Cl^-

The proton (H⁺) from the acid binds to neutral water molecules to form H₃O⁺ raising the concentration of H⁺. The resulting large concentration of (H⁺)

We know that pH = - log[H⁺]

So the pH value decreases so the solution will be more acidic and leads to a dramatic drop in the pH.

Addition of base to deionized water :

Here also the pH will change dramatically.For example if we add a strong base such as NaOH to deionized water results in the following reaction

NaOH --------> OH^- + Na⁺

The resulting large concentration of OH^- makes the solution more basic and leads to a dramatic increase in the pH.