Question

why do the colour and solution of copper sulphate change when we put the copper sulphate solution in test tube and put a iron nail in it​

Answer 1

The greenish colour of the solution in the test tube shows that Fe2+ ions are present in the solution. This shows that iron is more reactive than copper as Fe2+ ions have displaced Cu2+ ions from copper sulphate solution and form light greenish coloured ferrous sulphate solution.

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Answer 2

Due to displacement reaction copper is displaced by iron to form iron sulphate.