Question

Why does the colour of the solution change when a piece of iron is added to copper sulphate solution?

Hint:

This experiment is used to demonstrate reactivity series of metals.

Answer 1

Step 1: Background
Copper sulphate solution is blue in colour due to the presence of $Cu^{2+}$ ions. When a piece of iron is dipped into it, a chemical reaction occurs which changes the colour of the solution.
Step 2: Reaction Explanation
Iron is more reactive than copper (higher in the reactivity series). Hence, iron displaces copper from copper sulphate solution: \[ Fe (s) + CuSO_{4} (aq) \longrightarrow FeSO_{4} (aq) + Cu (s) \]
Step 3: Observation
Blue colour of $CuSO_{4}$ solution fades gradually and turns green due to the formation of ferrous sulphate ($FeSO_{4}$).
Brownish-red copper metal deposits on the iron strip.
Step 4: Scientific Reason
This is a displacement reaction, where a more reactive metal (iron) replaces a less reactive metal (copper) from its compound.
Final Answer:
The colour changes from blue to green because iron displaces copper from $CuSO_{4}$ solution, forming green $FeSO_{4}$.