Science (086) — CBSE Class 10 board question

Precipitation Reaction: Any reaction that produces an insoluble solid (called a precipitate) when two solutions are mixed is called a precipitation reaction.

Example: When sodium sulphate solution is mixed with barium chloride solution, a white insoluble precipitate of barium sulphate is formed.

Balanced Chemical Equation:

$$\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s) + 2\text{NaCl}(aq)$$

The white precipitate of BaSO₄ is formed by the reaction of SO₄²⁻ and Ba²⁺ ions. Sodium chloride remains dissolved in the solution.

Source: Chapter 1, Section 1.2.4 Double Displacement Reaction

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• 1 mark for the definition (mention "insoluble substance/precipitate").
• 1 mark for the example (naming both reactants and the precipitate formed).
• 1 mark for the correctly balanced equation with state symbols — (aq), (s) are important; don't forget the coefficient 2 before NaCl.
• Examiners expect state symbols in the equation; missing them can cost half a mark.