| Word | Meaning |
| Acid | A substance that gives hydrogen ions when it dissolves in water |
| Hydrogen ion | The positive particle an acid releases in water, written \( \mathrm{H}^+ \) |
| Ionisation | The breaking of a substance into ions as it dissolves in water |
| Organic acid | An acid found naturally |
| Inorganic acid | An acid made in a laboratory from mineral substances |
| Strong acid | An acid that gives many hydrogen ions in water |
| Weak acid | An acid that gives few hydrogen ions in water |
| Litmus | A colour changing paper used to tell an acid from a base |
| Methyl orange | An indicator that turns red in acid |
| Phenolphthalein | An indicator whose colour does not change in acid |
| Lime water | The solution used to identify carbon dioxide; it turns milky |
| Neutralisation reaction | The reaction in which acid and base give salt and water |