Why does the colour of Copper Sulphate change when an iron nail is kept in it?

Answer: The colour of copper sulphate when an iron nail is kept in it changes from blue to pale green because iron is more reactive than copper thus displacing copper from the solution resulting in colour change.

The metal activity series is an empirical method for predicting products in displacement reactions and metal reactivity with water and acids in replacement processes and ore extraction. The activity series is a table that lists metals in decreasing order of relative reactivity. The metals at the top are more reactive than those at the bottom having a less reactive nature. From most reactive to least reactive, the reactivity sequence is as follows:

To justify the colour change of copper sulphate, the concept of reactivity according to the Activity series of metals table is used. As we can observe from the above table iron is more reactive than copper, thus it displaces copper from the copper sulphate solution resulting in colour change of the solution.

When an iron nail is dipped in copper sulphate solution originally in blue colour, the surface of the iron develops a brown copper coating, and the colour of the copper sulphate solution changes from blue to pale green as the iron is dissolved in the solution as Fe (II), producing ferrous sulphate FeSO₄.

The reaction demonstrating the given chemical activity is:

CuSO₄(aq)+Fe(s)→FeSO₄(aq)+Cu(s) 

Here, a redox reaction is taking place,

(1) Reduction process Cu²⁺+2e → Cu⁰ and Cu²⁺is an oxidising agent.

(2) Oxidation process Fe⁰-2e → Fe²⁺ and Fe⁰is a reducing agent.

This shows that the reactivity series has several key applications aside from offering insight into the characteristics and reactivities of metals as it can be used to forecast the outcome of reactions between metals and water, metals and acids, single and double displacement processes between metals.