1. Acids and Bases
A substance that produces H⁺ ions (hydrogen ions) in aqueous solution. The more H⁺ ions, the stronger and more acidic the solution.
A substance that neutralises an acid. Bases that dissolve in water are called alkalis — they produce OH⁻ ions in solution.
| Common acids | Formula | Strong/Weak |
|---|---|---|
| Hydrochloric acid | HCl | Strong (fully ionised) |
| Sulfuric acid | H₂SO₄ | Strong |
| Nitric acid | HNO₃ | Strong |
| Ethanoic acid | CH₃COOH | Weak (partially ionised) |
| Carbonic acid | H₂CO₃ | Weak |
A strong acid is fully ionised in solution (all molecules split into ions). A weak acid is only partially ionised. At the same concentration, a strong acid has a lower pH and reacts faster — but both eventually react with the same amount of alkali (same number of H⁺ ions available from the same number of moles).
2. pH and Indicators
The pH scale runs from 0 to 14. pH 7 = neutral; pH below 7 = acidic; pH above 7 = alkaline. Each unit change in pH represents a tenfold change in H⁺ ion concentration.
| Indicator | Colour in acid | Colour at neutral | Colour in alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Universal indicator | Red/orange (strong) / yellow (weak) | Green | Blue/violet |
| Phenolphthalein | Colourless | Colourless | Pink/red |
| Methyl orange | Red | Orange | Yellow |
Litmus and universal indicator show whether a solution is acid or alkali, but not the exact pH. Universal indicator gives approximate pH by colour. Phenolphthalein and methyl orange are used in titrations — they give a sharp colour change at the endpoint.
3. Reactions of Acids
| Reactant | General equation | Products |
|---|---|---|
| Metal | acid + metal → salt + hydrogen | Salt + H₂ gas |
| Metal oxide | acid + metal oxide → salt + water | Salt + H₂O (no gas) |
| Metal hydroxide (alkali) | acid + alkali → salt + water | Salt + H₂O (no gas) |
| Metal carbonate | acid + carbonate → salt + water + CO₂ | Salt + H₂O + CO₂ gas |
| Ammonia solution | acid + NH₃(aq) → ammonium salt | Ammonium salt only |
Naming the salt produced
- Hydrochloric acid → chloride salt (e.g. zinc chloride)
- Sulfuric acid → sulfate salt (e.g. copper sulfate)
- Nitric acid → nitrate salt (e.g. sodium nitrate)
Name the salt produced when zinc reacts with sulfuric acid.
acid + metal → salt + hydrogen. Zinc + sulfuric acid → zinc sulfate + hydrogen.
4. Preparing Salts
Soluble salts — titration method
Use for salts made from a soluble acid and a soluble alkali (e.g. sodium chloride from HCl + NaOH). Add acid from burette to alkali in conical flask; use indicator to find the exact volume needed for neutralisation; repeat without indicator; evaporate to crystallise.
Soluble salts — excess solid method
Use for salts made from a soluble acid and an insoluble metal/oxide/carbonate (e.g. copper sulfate from H₂SO₄ + CuO). Add excess solid to warm acid until no more dissolves; filter off excess solid; evaporate filtrate to crystallise.
Insoluble salts — precipitation
Mix two solutions, each containing one of the ions of the desired salt. The insoluble salt precipitates. Filter, wash and dry.
Mix barium chloride solution + sodium sulfate solution.
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s) (white precipitate)
Filter the precipitate; wash with distilled water; dry in oven.
- Acid + Metal -> Salt + Hydrogen. Acid + Metal oxide -> Salt + Water. Acid + Carbonate -> Salt + Water + CO2.
- Acid + Alkali -> Salt + Water (neutralisation). H+ + OH- -> H2O is the ionic equation.
- Salt name: first word from base/metal, second from acid. HCl -> chloride. H2SO4 -> sulfate. HNO3 -> nitrate.
- Strong acid: fully dissociates (HCl, H2SO4, HNO3). Weak acid: partially dissociates (ethanoic acid, citric acid).
- Insoluble salts by precipitation: mix two solutions containing the required ions. Filter, wash, dry.
- Test for acidity: pH meter, universal indicator, or litmus paper.
5. Common Exam Traps
Students incorrectly write hydrogen as a product when acid reacts with a metal oxide or hydroxide. Hydrogen is only produced when acid reacts with a metal (not a metal oxide or hydroxide).
Strength refers to degree of ionisation (strong = fully ionised). Concentration refers to moles per dm³. A dilute solution of HCl (strong) can be less reactive than a concentrated solution of ethanoic acid (weak) if the concentrations are very different.
In the excess solid method, the unreacted solid must be removed by filtration before evaporating. If you evaporate first, the excess solid remains in the salt crystals, contaminating the product.
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Original study notes for Singapore students. Not affiliated with MOE, SEAB or Cambridge.