8 min readConcept sheet1-mark MCQ; 2 or 3-mark questions on the iron nail and barium chloride activities

Displacement, double displacement and precipitation reactions

In displacement, a more reactive element pushes a less reactive one out of its compound. In double displacement, two compounds swap ions, often forming a precipitate.

DisplacementA + BC → AC + B, when A is more reactive than B

Double displacementAB + CD → AD + CB (ions are exchanged)

PrecipitateAn insoluble solid that separates out of a solution

Activity: iron nails in copper sulphate solution

1
Clean two iron nails with sandpaper.
2
Tie one nail with a thread and dip it in copper sulphate solution for about 20 minutes. Keep the other nail aside to compare.
3
Observe: the nail gets a brown coating of copper, and the blue solution turns pale green.
4★
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Iron is more reactive than copper, so it displaces copper

★ key step

More displacement reactions

Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)
Zinc and lead are more reactive than copper, so they push copper out.
Copper cannot displace iron from ferrous sulphate, because copper is less reactive.

Activity: sodium sulphate with barium chloride

1
Take about 3 mL of sodium sulphate solution in one test tube and 3 mL of barium chloride solution in another.
2
Mix the two solutions.
3
Observe: a white precipitate of barium sulphate forms at once.
4★
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
Ions are exchanged: Ba²⁺ pairs with SO₄²⁻, and Na⁺ pairs with Cl⁻

★ key step

Displacement vs double displacement

DisplacementOne element replaces another element in a compound. It needs a more reactive element. Example: iron + copper sulphate.

Double displacementTwo compounds exchange their ions. When an insoluble solid (a precipitate) forms, it is also called a precipitation reaction. Example: lead nitrate + potassium iodide gives yellow lead iodide.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). Any reaction that forms a precipitate is a precipitation reaction.

How to draw the iron nail activity

Draw a test tube of blue copper sulphate solution held in a stand.
Draw an iron nail tied with a thread, hanging inside the solution; tie the top of the thread to the stand.
Beside it, draw a second test tube of the same solution with no nail, to compare.
To show the result after 20 minutes, shade the nail brown and colour the solution pale green.
Label: test tube, stand, thread, iron nail, copper sulphate solution, brown coating of copper.

In real life

Blue copper sulphate (sold as "neela thotha") is used in sprays that protect crops from fungus. Farmers are told not to mix it in iron buckets, because iron would displace the copper and the bucket would corrode.

Exam angle

1 marks: “Name the precipitate formed when lead nitrate solution reacts with potassium iodide solution, and give its colour.” Lead iodide; it is yellow.

2 marks: “Mira leaves a clean iron nail in blue copper sulphate solution for 20 minutes. Explain the two changes she sees, with an equation.” Iron is more reactive than copper, so it displaces copper: Fe + CuSO₄ → FeSO₄ + Cu. A brown coat of copper forms on the nail, and the blue solution turns pale green (FeSO₄).

3 marks: “What is a double displacement reaction? Explain with the reaction of sodium sulphate and barium chloride. Why is it also a precipitation reaction?” Two compounds exchange ions. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl. White insoluble BaSO₄ separates out as a precipitate.

Remember it

Single swap: one element. Double swap: two compounds. The more reactive metal always wins the partner.