Chapter 1 · Question 8

What happens when an iron nail is dipped in copper sulphate solution? Write the balanced chemical equation and explain the displacement process. Why does a colour change occur?

Answer Revealed

Direct Answer:

When an iron nail is dipped in blue copper sulphate (

\text{CuSO}_4

) solution, iron displaces copper from copper sulphate because iron is more reactive than copper. The blue colour of the solution fades, and a brownish coating of copper metal deposits on the iron nail. The reaction is:

\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}

. The colour change happens because the blue

\text{Cu}^{2+}

ions are replaced by pale green

\text{Fe}^{2+}

ions in the solution.

Simple Explanation

Iron is more reactive than copper, so when an iron nail goes into blue copper sulphate solution, the iron pushes the copper out — like a stronger person taking a seat. The nail gets coated with reddish-brown copper metal, while the blue solution turns greenish because the copper ions are replaced by iron ions. The reaction:

\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}

Exam-Ready Structure

The iron nail and copper sulphate reaction is the classic demonstration of a displacement reaction in NCERT Class 10: 1. A displacement reaction occurs when a more reactive element displaces (pushes out) a less reactive element from its compound. It is a single-displacement:

A + BC \rightarrow AC + B

. 2. Initial observation: Copper sulphate solution is blue and the iron nail is silvery grey. 3. When the iron nail is immersed, iron (

\text{Fe}

) displaces copper (

\text{Cu}

) from copper sulphate because iron is more reactive than copper. 4. The balanced chemical equation:

\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}

. 5. Observable changes: (a) The iron nail develops a brownish-red coating of deposited copper metal. (b) The blue colour of the copper sulphate solution gradually fades as iron sulphate is formed in solution. 6. Other displacement examples from the chapter:

\text{Zn} + \text{CuSO}_4 \rightarrow \text{ZnSO}_4 + \text{Cu}

(zinc displaces copper) and

\text{Pb} + \text{CuCl}_2 \rightarrow \text{PbCl}_2 + \text{Cu}

(lead displaces copper). Zinc and lead are also more reactive than copper. 7. The relative reactivity of metals can be inferred from displacement reactions — a more reactive metal can displace a less reactive metal from its salt solution.

Key Points

Displacement: more reactive element displaces less reactive element from its compound
Reaction: $\text{Fe(s)} + \text{CuSO}_4\text{(aq)} \rightarrow \text{FeSO}_4\text{(aq)} + \text{Cu(s)}$
Observations: brownish-red copper coating on nail, blue colour fades to pale green
Colour change reason: blue $\text{Cu}^{2+}$ ions are replaced by pale green $\text{Fe}^{2+}$ ions
Other examples: Zn + CuSO₄ and Pb + CuCl₂ — zinc and lead also displace copper

Common Mistakes

Confusing the displaced copper coating with rust — the brownish coating on the nail is copper metal
Confusing this as a double displacement — it is a single displacement because only one element exchanges place

What happens when an iron nail is dipped in copper sulphate solution? Write the balanced chemical equation and explain the displacement process. Why does a colour change occur?