Question

How does adding water to an acid or base solution change the pH? What is this type of reaction called?

Answer 1

pH of acid and bases depends on the H⁺ and OH^- ion concentration respectively. The higher the concentration of H⁺ is, the lower is the pH value of acid solution, so higher is the acidity (pH = -log[H+]). Similarly he higher the concentration of OH^- is, the higher is the pH value, so higher is the basicity (pOH = -log[OH^-], and pH = 14 - pOH). All the acids have pH less than 7, and all the bases has pH more than7.

Adding water to an acid solution makes the solution dilute. So H⁺ concentration decreases in the solution. So pH value increases towards 7 from lower pH value. Therefore acidity decreases. Similarly adding water to an base solution makes the solution dilute. So OH^- concentration decreases in the solution. So pH value decreases towards 7 from higher pH value. Therefore basicity decreases.

Therefore adding water to acid solution increases it's pH, whereas adding water to base decreases it's pH.

Adding water to acid and bases released lots of heat. So this reaction is called exothermic reaction.