GCSE Chemistry · Edexcel 1CH0 · Topic 3A
Acids and bases
Key points
- Acids release H⁺ ions in water; alkalis release OH⁻ ions. A base is a substance that neutralises an acid; alkalis are soluble bases.
- pH: below 7 acidic, 7 neutral, above 7 alkaline. Indicators: litmus (red in acid, blue in alkali), methyl orange (red in acid, yellow in alkali), phenolphthalein (colourless in acid, pink in alkali).
- Higher tier: a strong acid is fully ionised in water; a weak acid is only partly ionised. Concentrated and dilute describe the amount of acid per volume, not strength.
- Higher tier: a 10× increase in H⁺ concentration lowers pH by 1.
- Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l).
- Core practical: investigate pH change when calcium hydroxide powder is added to dilute hydrochloric acid.
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Acids and alkalis are everywhere, from stomach acid to cleaning products. These notes explain what makes a substance acidic or alkaline, how pH and indicators work, neutralisation, the core practical on pH change and, for Higher tier, acid…
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