When two ionic solutions are mixed, positive and negative ions can form new pairs. If one new pair is insoluble in water, it separates as a solid called a precipitate. This is a double displacement reaction.
Sodium sulphate solution and barium chloride solution are both clear and colourless. Pour one into the other and a white solid forms at once, making the mixture cloudy.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
Both reactants are ionic compounds. When an ionic compound dissolves in water, its ions separate and move independently. Sodium sulphate solution contains Na⁺ ions and SO₄²⁻ ions. Barium chloride solution contains Ba²⁺ ions and Cl⁻ ions.
Mixing the solutions brings all four kinds of ion together. Ba²⁺ and SO₄²⁻ form barium sulphate. Na⁺ and Cl⁻ correspond to sodium chloride. Each product contains one ion from each reactant. This exchange of ions is a double displacement reaction.
Two products are written in the equation, but only one solid is seen. The difference is solubility: how much of a substance dissolves in a given amount of water.
Barium sulphate is almost insoluble. Less than 0.003 g dissolves in a litre of water. So when Ba²⁺ and SO₄²⁻ ions meet, nearly all of them bind into solid barium sulphate. This solid is called a precipitate.
Sodium chloride is very soluble: about 360 g dissolves in a litre. Its ions stay separate in the water, and nothing is seen.
A reaction that forms an insoluble solid from solutions is a precipitation reaction. It is a double displacement in which one of the new combinations is insoluble.
Exchange the ions, then check each new pair. Barium sulphate is insoluble and separates as a solid. Sodium chloride is soluble, so its ions stay dissolved.
In the first session of this unit, lead nitrate solution and potassium iodide solution gave a bright yellow solid. Explain it as a double displacement.
The solutions contain Pb²⁺, NO₃⁻, K⁺ and I⁻ ions. Exchanging partners gives lead iodide and potassium nitrate.
Potassium nitrate is soluble, so its ions stay in solution. Lead iodide is insoluble, so it separates as the yellow precipitate.
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
Two steps:
If both new combinations are soluble, all the ions stay dissolved. No new substance separates, and there is no reaction to see. The mixture is just a solution of four kinds of ion.
In displacement, from the previous session, an element replaces another element in a compound. Whether it happens depends on reactivity.
In double displacement, two compounds exchange ions. Whether a visible product forms depends on solubility.
1In a double displacement reaction…
2An insoluble solid that forms when two solutions are mixed is called a ____.
3When sodium sulphate and barium chloride solutions are mixed, why is only one solid seen?
4Lead nitrate and potassium iodide solutions are mixed. The yellow solid is…
5What is the first step in predicting the products of a double displacement?
6How does double displacement differ from displacement?
7Two ionic solutions are mixed and no precipitate forms. Both possible new combinations are soluble. What is in the beaker?
Next: oxidation and reduction, and their effects in everyday life.
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