Which of the following pairs of compounds would form a precipitate when their aqueous solutions are mixed?

Step 1: Analyze the question A precipitate forms when two aqueous solutions are mixed and the ions combine to make an insoluble compound. To solve this, swap the partners (double displacement) and check the solubility of the new products.

Step 2: Concept: solubility rules

• All nitrates (NO₃⁻) are soluble.
• All sodium, potassium and ammonium salts are soluble.
• Chlorides are soluble, except AgCl, PbCl₂ and Hg₂Cl₂.
• Sulphates are soluble, except BaSO₄, PbSO₄ and CaSO₄ (slightly soluble).

Step 3: Check each option

• A. NaCl + KNO₃ → NaNO₃ + KCl. Both products are soluble, so there is no precipitate.
• B. KCl + NaNO₃ → KNO₃ + NaCl. Both products are soluble, so there is no precipitate. (This is the same ion mixture as A.)
• C. K₂SO₄ + BaCl₂ → BaSO₄ + 2KCl. • KCl is soluble. • BaSO₄ is insoluble, so it forms a white precipitate.

Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

• D. NH₄NO₃ and CO₃ is not a proper pair of compounds, since "CO₃" is only an ion fragment and not a compound. Even if it meant (NH₄)₂CO₃, ammonium salts and nitrates are all soluble, so there would be no precipitate.

Step 4: Answer The pair that forms a precipitate is K₂SO₄ and BaCl₂, which gives white BaSO₄.

Exam tip: Whenever you see Ba²⁺ with a sulphate, think "white precipitate". This is also the standard test for sulphate ions: add BaCl₂ solution with dilute HCl, and a white precipitate that stays in the acid confirms SO₄²⁻. Also remember that Na⁺, K⁺, NH₄⁺ and NO₃⁻ salts are always soluble, so any option with only these ions can be eliminated quickly.