Chemical changes Study Guide

The pH of a solution is a measure of its acidity or alkalinity, directly linked to the concentrations of hydrogen ions (H⁺) and hydroxide ions (OH⁻). In acidic solutions, a higher concentration of hydrogen ions results in a lower pH—making the solution more acidic. In alkaline solutions, a higher concentration of hydroxide ions increases the pH—making the solution more alkaline. The relationship is logarithmic: each pH unit represents a tenfold change in the relevant ion concentration. Importantly, pH reflects the extent of acidity or alkalinity but does not indicate acid or alkali strength (degree of ionisation); it shows the concentration present in solution.

Neutralisation is a specific type of chemical reaction that occurs when an acid reacts with an alkali. At the heart of this reaction is the interaction between hydrogen ions (H+) from the acid and hydroxide ions (OH–) from the alkali. When these two oppositely charged ions encounter each other, they combine to form a neutral water molecule (H2O). The overarching effect is that the acidic properties (due to H+) and alkaline properties (due to OH–) are cancelled out, resulting in a solution that is typically neutral, with a pH of 7, provided the acid and alkali are mixed in stoichiometric proportions. This fundamental ionic reaction is common to all acid-alkali neutralisations.