When an alkali (a base) is added to an acid, the pH value of the solution increases. This is because the alkali neutralises the acid, reducing the concentration of hydrogen ions (H+) in the solution. The pH scale measures the acidity or basicity of a solution, with a pH value of 7 indicating neutrality, values below 7 indicating acidity, and values above 7 indicating basicity. As the pH value increases, the solution becomes less acidic and more basic.

The reaction between an alkali and an acid can be represented by the following general equation:

Alkali + Acid → Salt + Water

For example, when sodium hydroxide (NaOH), an alkali, is added to hydrochloric acid (HCl), an acid, the following reaction takes place:

NaOH + HCl → NaCl + H2O

In this reaction, sodium hydroxide neutralises hydrochloric acid, producing sodium chloride (NaCl) and water (H2O). The sodium chloride is a salt, and the water is a neutral substance. As a result of the reaction, the concentration of hydrogen ions (H+) in the solution decreases, and the pH value increases.

The extent of the pH increase depends on the amount of alkali added and the concentration of the acid. Adding more alkali will neutralise more acid and cause a greater increase in pH. Similarly, adding alkali to a more concentrated acid will result in a smaller pH increase compared to adding the same amount of alkali to a less concentrated acid.

In summary, when an alkali is added to an acid, the pH value of the solution increases because the alkali neutralises the acid, reducing the concentration of hydrogen ions (H+) in the solution.