Acids, alkalis and pH · Year 10
The pH scale, from 0 to 14, measures how acidic or alkaline a solution is. Acids have a pH below 7, alkalis above 7, and neutral solutions exactly 7. Indicators change colour to show which it is: universal indicator shows the whole scale. An acid and an alkali cancel each other out in neutralisation, making a salt and water.
Key facts
- pH below 7: acidic (the lower, the more acidic). pH 7: neutral. pH above 7: alkaline (the higher, the more alkaline).
- Universal indicator: red (pH 0–2), orange (3–4), yellow (5–6), green (7), blue (8–10), purple (11–14).
- Litmus is red in acids and blue in alkalis. Phenolphthalein is colourless in acids and pink in alkalis. Methyl orange is red in acids and yellow in alkalis.
- Acids release hydrogen ions, H⁺, in water. Alkalis release hydroxide ions, OH⁻. A base neutralises an acid; an alkali is a base that dissolves in water.
- acid + alkali → salt + water. Hydrochloric acid makes chlorides, sulfuric acid sulfates and nitric acid nitrates. In every neutralisation: H⁺ + OH⁻ → H2O.
- Everyday acids: stomach acid (hydrochloric), vinegar (ethanoic), citrus fruit (citric). Everyday alkalis: soap, oven cleaner, bleach.
- A strong acid is fully ionised in water; a weak acid only partly. Each fall of 1 in pH means 10 times as many hydrogen ions.