When sodium sulphate solution is mixed with barium chloride solution, a white precipitate forms and sodium chloride remains dissolved. Explain why one product forms a precipitate while the other stays in solution, and identify what type of reaction this is.
Generated by claude-sonnet-4-6 · 2026-06-26 00:55 · grounding rag
Model Answer
When Na₂SO₄ and BaCl₂ solutions are mixed, the ions exchange partners. The reaction is:
$$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq)$$
BaSO₄ forms a precipitate because it is insoluble in water. The SO₄²⁻ and Ba²⁺ ions combine to form this white insoluble solid.
NaCl remains dissolved because it is soluble in water; Na⁺ and Cl⁻ ions stay in solution.
This is a double displacement reaction (also called a precipitation reaction) because the ions of the two reactants exchange places to form two new compounds.
Source: Chapter 1, Section 1.2.4 – Double Displacement Reaction
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Explanation
- Examiners expect the balanced equation with state symbols — always include (s) for precipitate and (aq) for dissolved species.
- You must name both types: double displacement reaction AND precipitation reaction for full marks.
- The key reason for BaSO₄ precipitating is its insolubility in water — state this explicitly.
- NaCl staying in solution must be explained by its solubility, not just stated.
- Three marks typically split as: equation (1) + reason for precipitate/solution (1) + type of reaction (1).