Precipitation of insoluble barium sulfate

Steps:

• Identify Group 2 elements: barium (Ba) and magnesium (Mg) in the compounds.
• Apply solubility rules: Group 2 chlorides and nitrates are soluble; most sulfates soluble except BaSO4; carbonates insoluble except with ammonium.
• Test each pair: L and M must both be soluble, but mixing yields an insoluble product.
• Select pair where cations/anions form BaSO4 precipitate.

Why A is correct:

• BaCl2 and MgSO4 are both soluble; mixing gives Ba²⁺ + SO₄²⁻ → BaSO4 (s), insoluble per solubility rules.

Why the others are wrong:

• B: Ba(NO3)2 and MgCl2 both soluble; products BaCl2 and Mg(NO3)2 also soluble—no precipitate.
• C: BaSO4 insoluble, violates "both soluble in water."
• D: BaCO3 insoluble, violates "both soluble in water."

Final answer: A