Acids can be prepared through several chemical methods. The most common include reactions involving acid anhydrides, direct combination, displacement, and double decomposition.
(a) From Acid Anhydrides
- Acid anhydrides are oxides of non‑metals.
- When dissolved in water, they produce acids.
- Example:
- SO₃ + H₂O → H₂SO₄ (Sulphur trioxide forms sulphuric acid)
(b) By Direct Combination
- Hydrogen combines directly with halogens (Br, I, Cl, F) in the presence of catalysts.
- This yields a gaseous product which becomes acidic when dissolved in water.
- Example:
- H₂ + Cl₂ → 2HCl (gas)
- Dissolved in water → Hydrochloric acid
(c) Displacement Method
- A strong, non‑volatile acid can displace a weaker, volatile acid from its salt.
- Example:
- H₂SO₄ + NaCl → HCl (gas) + NaHSO₄
(d) Double Decomposition / Precipitation Method
- Achieved by passing hydrogen sulphide gas (H₂S) into a metallic salt solution.
- This causes precipitation of an insoluble sulphide.
- Example:
- H₂S + Pb(NO₃)₂ → PbS (precipitate) + 2HNO₃
- Lead sulphide (PbS) is formed as an insoluble solid.
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