When CuSO$_{4(aq)}$ is added to Pb(NO$_{3}$)$_{2(aq)}$--------

Analyze the question: We are mixing two aqueous salt solutions, copper(II) sulphate and lead(II) nitrate. We must predict what is observed.

Concept: When two soluble salts are mixed, a double displacement (precipitation) reaction may occur if one of the possible products is insoluble. Recall the solubility rules:

• All nitrates are soluble.
• Most sulphates are soluble, except those of lead (PbSO₄), barium (BaSO₄) and calcium (CaSO₄, sparingly soluble).

Solution:

The ions swap partners:

CuSO₄(aq) + Pb(NO₃)₂(aq) → PbSO₄(s) + Cu(NO₃)₂(aq)

• Cu(NO₃)₂ is a nitrate, so it is soluble and stays in solution (it keeps the blue colour of Cu²⁺).
• PbSO₄ is insoluble and appears as a white precipitate.

Ionic equation:

Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s)

Checking the options:

• A. No visible change: wrong, a precipitate forms.
• B. Blue precipitate: wrong. Blue Cu(OH)₂ would need hydroxide ions (e.g., NaOH or NH₃), and the Cu²⁺ stays dissolved here.
• C. Colourless solution: wrong, Cu²⁺ still remains in solution, so the liquid stays blue.
• D. White precipitate: correct, this is PbSO₄.

Answer: D

Tip: Remember that sulphates of Pb, Ba and Ca are the exceptions among sulphates. Whenever Pb²⁺ meets SO₄²⁻ or Cl⁻ (PbCl₂, which dissolves in hot water), expect a white precipitate.