Reactions of acids
Acids react in three characteristic ways:
- Acid + reactive metal → salt + hydrogen
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) - Acid + base (metal oxide/hydroxide) → 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)
Key idea / 要点
Test for hydrogen gas: a lighted splint gives a "pop" sound. Test for carbon dioxide: it turns limewater cloudy/milky.
Reactions of alkalis
- Alkali + acid → salt + water (neutralisation)
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) - Alkali + ammonium salt → salt + water + ammonia
NaOH(aq) + NH4Cl(aq) → NaCl(aq) + H2O(l) + NH3(g) - Alkali + metal ion solution → insoluble metal hydroxide
2NaOH(aq) + CuSO4(aq) → Cu(OH)2(s) + Na2SO4(aq)
Worked example
Marble chips (CaCO3) are added to dilute nitric acid. What gas forms and how is it confirmed?
Answer: CaCO3 + 2HNO3 → Ca(NO3)2 + H2O + CO2. The gas is carbon dioxide; it turns limewater milky.
These reactions underpin many laboratory tests and everyday processes. For instance, the fizzing of a carbonate with acid is used to detect limestone, while the reaction of alkalis with metal-ion solutions produces coloured hydroxide precipitates that help identify the metal present. Knowing the general products lets you predict and balance equations for reactions you have not met before.
Remember
- Acid + metal ⇒ H2 (pop test).
- Acid + carbonate ⇒ CO2 (limewater milky).
- Warmed alkali + ammonium salt ⇒ NH3 (turns damp red litmus blue).