Activity 1.10 Sodium sulphate and barium chloride

1. Take about 3 mL sodium sulphate solution in one test tube.
2. Take about 3 mL barium chloride solution in another test tube.
3. Mix the two solutions.
4. Record the substance formed.

1. 1
2. 2
3. 3
4. 4

1. 1Sodium sulphate solution
2. 2Barium chloride solution
3. 3Receiving test tube
4. 4Barium sulphate precipitate

Figure 1.9 Formation of barium sulphate and sodium chloride.

Observation: A white insoluble substance forms. An insoluble solid formed in a solution is a precipitate, and a reaction producing it is a precipitation reaction.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (1.27)

Ba²⁺ and SO₄²⁻ form insoluble BaSO₄; sodium chloride remains in solution. The reacting compounds exchange ions, so this is a double displacement reaction.

Recall Activity 1.2

Lead(II) nitrate solution was mixed with potassium iodide solution.

1. What was the colour of the precipitate? Name it.
2. Write the balanced chemical equation.
3. Is this also a double displacement reaction?

Answers:

1. The precipitate was yellow lead iodide, PbI₂.
2. Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
3. Yes. The two compounds exchange ions and form insoluble lead iodide.

Fast distinction: Displacement means one element replaces another. Double displacement means two compounds exchange ions.

Board extension: Double displacement without a precipitate

This extension connects Chapter 1 classification with a current CBSE mixed-chapter pattern. It is not an additional NCERT section.

2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

• Sodium hydroxide is a base and sulphuric acid is an acid, so this is a neutralisation reaction.
• The ions exchange partners, so it is also a double displacement reaction.
• It is not a precipitation reaction because sodium sulphate is soluble and no insoluble solid forms.

Do not use “double displacement” and “precipitation” as if they always mean the same thing. First identify ion exchange; then check whether an insoluble solid actually appears.

Board extension: Non-metal displacement

Displacement is not limited to metals. A more reactive non-metal can also displace a less reactive non-metal from its compound.

Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(s)

Chlorine displaces iodine from potassium iodide. In a case-based answer, state the displacement pattern and balance the iodine and potassium compounds with coefficient 2.