Properties of Acids & Bases
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge IGCSE Chemistry (0620)
Acids and Bases
Acids release hydrogen ions (H⁺) in water and have a pH less than 7. Common acids: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), nitric acid (HNO₃), ethanoic acid (CH₃COOH).
Bases are oxides or hydroxides of metals that neutralise acids and have a pH greater than 7. An alkali is a base that is soluble in water.
- Soluble bases (alkalis): NaOH, KOH, Ca(OH)₂ (slightly soluble), NH₃(aq)
- Insoluble bases (not alkalis): CuO, ZnO, MgO
All alkalis are bases, but not all bases are alkalis - only soluble bases count as alkalis.
Worked Example: Metal Oxide and Alkali
- Question: which row correctly describes a metal oxide and an alkali? (A) acidic / insoluble base (B) acidic / soluble base (C) basic / insoluble base (D) basic / soluble base
- Answer: D. Metal oxides are basic oxides (react with acids, not bases). An alkali is, by definition, a soluble base.
Characteristic Reactions of Acids
| Reaction | Equation type | Gas/product |
|---|---|---|
| Acid + metal | acid + metal → salt + hydrogen | H₂ gas (squeaky pop with a lit splint) |
| Acid + carbonate | acid + carbonate → salt + water + carbon dioxide | CO₂ gas (turns limewater milky) |
| Acid + base (incl. metal oxide/hydroxide) | acid + base → salt + water | no gas - neutralisation |
| Acid + ammonium salt (base side) | base + NH₄⁺ salt → salt + water + ammonia | NH₃ gas (pungent, turns damp red litmus blue) |
Worked Example: Which Solids Release a Gas?
- Question: which solids react with dilute ethanoic acid to produce a gas? 1 - sodium, 2 - sodium hydroxide, 3 - sodium oxide, 4 - sodium carbonate
- Answer: 1 and 4. Sodium (a reactive metal) gives H₂ gas; sodium carbonate gives CO₂ gas. Sodium hydroxide and sodium oxide are both bases - they neutralise the acid to give only salt + water, with no gas.
Characteristic Reactions of Bases
- Base + acid → salt + water (neutralisation), e.g. CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
- Alkali + ammonium salt → salt + water + ammonia, e.g. NaOH(aq) + NH₄Cl(s) → NaCl(aq) + H₂O(l) + NH₃(g)
Effect on Indicators
| Indicator | In acid | In alkali |
|---|---|---|
| Litmus | red | blue |
| Thymolphthalein | colourless | blue |
| Methyl orange | red | yellow |
These are the three indicators named in the syllabus. Note: thymolphthalein is colourless in both acidic and neutral solutions, and only turns blue once a solution is clearly alkaline.
Worked Example: Titrating an Alkali with Acid
- Question: excess aqueous potassium hydroxide is added to dilute hydrochloric acid containing methyl orange. State the colour change observed. [2]
- Answer: dilute HCl is acidic, so methyl orange starts red; adding excess KOH neutralises the acid and makes the mixture alkaline, so methyl orange ends yellow - colour change: from red to yellow
Common Mistakes
- ❌ Confusing thymolphthalein with phenolphthalein
- ✓ the Cambridge 0620 syllabus uses thymolphthalein (colourless → blue), not phenolphthalein (colourless → pink) - they are different indicators
- ❌ Expecting every base to fizz or dissolve with an acid
- ✓ only carbonates and metals release a gas with acid; oxides and hydroxides just neutralise (salt + water, no gas)
Neutralisation
H⁺(aq) + OH⁻(aq) → H₂O(l)
Neutralisation is exothermic - temperature rises as the acid and alkali react, and the pH moves towards 7.
Interactive revision notes, videos and practice questions load below.