ZIMSEC O Level · Chemistry · Form 3
Definitions
- An acid releases hydrogen ions (H⁺) in water; pH below 7.
- A base reacts with an acid to form a salt and water. An alkali is a soluble base and releases hydroxide ions (OH⁻); pH above 7.
- Strong acids (HCl, H₂SO₄, HNO₃) ionise completely; weak acids (ethanoic acid) only partly.
Neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)
Typical reactions
- acid + metal → salt + hydrogen
- acid + base (metal oxide or hydroxide) → salt + water
- acid + carbonate → salt + water + carbon dioxide
Oxides
Metal oxides are basic; non-metal oxides (CO₂, SO₂) are acidic; some (Al₂O₃, ZnO) are amphoteric.
Preparing salts
- Soluble salt from an insoluble base: add excess base to acid, filter off the excess, evaporate and crystallise (e.g. copper(II) sulfate from CuO and H₂SO₄).
- Soluble salt of sodium or potassium: titration with an indicator.
- Insoluble salt: precipitation by mixing two solutions, then filter, wash and dry (e.g. barium sulfate).
Tests for ions
Carbonate: add acid – fizzes, gas turns lime water milky. Sulfate: add barium nitrate/chloride – white precipitate. Chloride: add silver nitrate – white precipitate. Ammonium: warm with NaOH – ammonia turns damp red litmus blue.
Exam tip: Use “excess” solid base so that all the acid is used up – then filtering removes the leftover solid.
Check yourself
- Write the ionic equation for neutralisation.
- Name the salt formed from zinc oxide and nitric acid.
- Which method makes barium sulfate?
Show answers
- H⁺(aq) + OH⁻(aq) → H₂O(l)
- Zinc nitrate.
- Precipitation (mix solutions, filter, wash, dry).
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