Chemical changes Study Guide

In chemistry, oxidation and reduction are fundamental processes that describe the transfer of electrons during a chemical reaction. Oxidation is defined as the loss of electrons by an atom, ion, or molecule. When a substance loses electrons, its oxidation state increases. Conversely, reduction is defined as the gain of electrons by an atom, ion, or molecule. When a substance gains electrons, its oxidation state decreases. These two processes always occur simultaneously; if one substance loses electrons (is oxidised), another substance must gain those electrons (is reduced). This type of reaction is known as a redox reaction. A helpful mnemonic is 'OIL RIG': Oxidation Is Loss, Reduction Is Gain (of electrons).

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.