Chemical changes Study Guide

Electrolysis uses electrical energy to split compounds into their component elements or ions at two electrodes—cathode (negative) and anode (positive)—placed in an electrolyte. In the electrolyte, ions move: cations (positive) to the cathode, anions (negative) to the anode. At the cathode, metal ions of metals less reactive than hydrogen (such as copper) are reduced (gain electrons) to solid metal; if only ions from more-reactive metals or water/hydrogen ions are present, hydrogen is formed instead. At the anode, non-metal anions are usually oxidised (lose electrons): if the solution contains a high concentration of halide ions (like Cl⁻ or Br⁻), the halogen gas forms; otherwise, hydroxide ions from water are oxidised to oxygen gas. The specific products depend on ion types, concentrations, and the relative reactivity of any present metals compared to hydrogen.

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.