Topic 2 of 3
Recognise the reaction pattern
Predict the salt and the other products.
O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)
Recognise the reaction pattern
Predict the salt and the other products.
| Reactants | Products | Balanced example |
|---|---|---|
| Acid + suitable metal | Salt + hydrogen | Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g) |
| Acid + base | Salt + water | CuO(s) + H2SO4(aq) -> CuSO4(aq) + H2O(l) |
| Acid + carbonate | Salt + water + carbon dioxide | CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g) |
| Alkali + ammonium salt, warm | Salt + water + ammonia | NH4Cl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) + NH3(g) |
The acid supplies the salt anion: hydrochloric acid gives chlorides, sulfuric acid sulfates, and nitric acid nitrates. The metal or base supplies the cation. Not every metal reacts with dilute acid: copper, for example, does not release hydrogen from dilute hydrochloric acid.
For an acid reacting with an alkali, the reacting ions are H+(aq) + OH-(aq) -> H2O(l). This is neutralisation. The other dissolved ions are spectators. For an ammonium salt and alkali, NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l); warming helps ammonia escape.
Worked example
Predict and confirm the gas
Dilute acid is added to a carbonate and bubbles appear. How should the result be described?
- Predict carbon dioxide from the carbonate reaction pattern.
- Report effervescence as the observation.
- Pass the gas into limewater: a white precipitate supports carbon dioxide.
Effervescence alone does not identify a gas. Carbon dioxide turns limewater milky; hydrogen instead gives a pop with a lighted splint.