Chemical changes Study Guide

In chemistry, a base is a substance that can neutralise an acid. Bases are often metal oxides or metal hydroxides. Not all bases dissolve in water. Those bases that do dissolve in water are specifically called alkalis. Therefore, all alkalis are bases, but not all bases are alkalis. Common examples of alkalis include sodium hydroxide (NaOH) and potassium hydroxide (KOH), which are both highly soluble in water and produce hydroxide ions (OH-) in solution, leading to their characteristic alkaline properties.

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.