Chemical changes Study Guide

A neutralisation reaction is a fundamental chemical process where an acid reacts with a base. The key characteristic of this reaction is that it typically produces a salt and water. Acids are substances that donate protons (H⁺ ions) in aqueous solution, while bases are substances that accept protons (H⁺ ions) or produce hydroxide ions (OH⁻ ions). When an acid and a base combine, the H⁺ ions from the acid react with the OH⁻ ions from the base to form water (H₂O), thereby 'neutralising' each other. The remaining ions from the acid and base then combine to form an ionic compound known as a salt. For example, hydrochloric acid (HCl) reacting with sodium hydroxide (NaOH) produces sodium chloride (NaCl, a salt) and water (H₂O).

When acids are dissolved in water, they release hydrogen ions (H⁺) into the solution. It is these hydrogen ions that are responsible for the characteristic acidic properties, such as a sour taste or reacting with metals to produce hydrogen gas. Conversely, alkalis are soluble bases that release hydroxide ions (OH⁻) when dissolved in water. These hydroxide ions are responsible for the characteristic alkaline properties, such as a slippery feel or reacting with acids in neutralisation reactions. The concentration of these ions determines the strength of the acid or alkali, and therefore its pH. Strong acids and alkalis fully dissociate in solution, releasing a high concentration of H⁺ or OH⁻ ions, respectively, while weak acids and alkalis only partially dissociate.