Acids, Bases and Salts
📖Definition— Definitions
- Acid: a substance that produces H⁺ ions (protons) in aqueous solution. pH < 7.
- Base: a substance that neutralises an acid. Metal oxides, metal hydroxides, metal carbonates and ammonia are bases.
- Alkali: a base that dissolves in water to produce OH⁻ ions. pH > 7.
- Neutral: pH = 7 (e.g. pure water).
Common acids: HCl (hydrochloric), H₂SO₄ (sulfuric), HNO₃ (nitric), CH₃COOH (ethanoic — weak acid). Common alkalis: NaOH, KOH, Ca(OH)₂ (lime water), NH₃ solution.
The pH scale
pH measures acidity/alkalinity on a scale from 0 to 14 (and beyond):
- 0–6: acidic (lower = more acidic)
- 7: neutral
- 8–14: alkaline (higher = more alkaline)
pH can be measured with universal indicator (colour chart), a pH meter, or pH paper.
Neutralisation
Acid + base → salt + water Acid + alkali → salt + water
Ionic equation for any acid-alkali neutralisation: H⁺(aq) + OH⁻(aq) → H₂O(l)
Reactions of acids
| Reactant | Products | Example |
|---|---|---|
| Acid + metal oxide | Salt + water | H₂SO₄ + CuO → CuSO₄ + H₂O |
| Acid + metal hydroxide | Salt + water | HCl + NaOH → NaCl + H₂O |
| Acid + metal carbonate | Salt + water + CO₂ | HCl + CaCO₃ → CaCl₂ + H₂O + CO₂ |
| Acid + reactive metal | Salt + hydrogen | H₂SO₄ + Zn → ZnSO₄ + H₂ |
Naming salts
The salt name comes from: metal (or ammonium) + acid anion.
- HCl → chloride salts (e.g. NaCl, CaCl₂)
- H₂SO₄ → sulfate salts (e.g. CuSO₄, Na₂SO₄)
- HNO₃ → nitrate salts (e.g. KNO₃)
- H₃PO₄ → phosphate salts
Methods of salt preparation
1. Insoluble base + acid (titration not needed): Add excess insoluble base (e.g. CuO) to warm acid (e.g. H₂SO₄). Filter off excess, evaporate filtrate.
2. Acid + alkali (titration): Use an indicator or pH meter to find the exact volume of alkali to neutralise the acid. Repeat without indicator, then evaporate.
3. Precipitation: Mix two solutions; the insoluble salt precipitates. Example: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
Titration (CCEA practical)
- Pipette a known volume of alkali into a conical flask; add 2–3 drops of indicator (e.g. phenolphthalein — pink in alkali, colourless in acid).
- Fill burette with acid; record initial reading.
- Add acid slowly, swirling. At the end point, the indicator just turns colourless (one drop excess acid).
- Record final burette reading; volume added = titre.
- Repeat for concordant results (within 0.10 cm³ of each other).
Calculation: n(acid) = c × V (mol = mol/dm³ × dm³). Use molar ratio to find moles of alkali; then concentration of alkali = n ÷ V.
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