Give an example of a chemical reaction for the following situation: A change in colour is observed.

Displacement reaction:-

1. The type of reaction where one atom or a set of atoms is replaced with another atom within the same molecule is called displacement reaction.
2. For example when, Silver nitrate is reacted with Copper, the Copper displaces the Silver from the Silver nitrate solution and forms Copper nitrate solution.
3. This happens because Copper is more reactive than Silver, as Copper lies above Silver in the reactivity series.
4. The balanced reaction is :

$\underset{\mathrm{Copper}}{\mathrm{Cu}\left(\mathrm{s}\right)}+\underset{\mathrm{Silver}\mathrm{nitrate}}{2{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)}\to \underset{\mathrm{Copper}\mathrm{nitrate}}{\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Silver}}{2\mathrm{Ag}\left(\mathrm{s}\right)}$

The reaction between Iron$\left(\mathrm{Fe}\right)$ and Copper sulphate$\left({\mathrm{CuSO}}_4\right)$:-

1. When Copper sulphate is treated with Iron, the reaction undergoes displacement reaction.
2. The Iron here displaces the Copper from the Copper sulphate solution and forms Ferrous sulphate$\left({\mathrm{FeSO}}_4\right)$.
3. The Copper sulphate solution is blue in colour, but after this reaction the colour changes to pale green.
4. This is because Iron is more reactive than Copper, as it lies above Copper in the reactivity series.
5. The balanced reaction is:

$\underset{\mathrm{Iron}}{\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Copper}\mathrm{sulphate}}{{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\left(\mathrm{blue}\right)}\to \underset{\mathrm{Ferrous}\mathrm{sulphate}}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)(\mathrm{pale}\mathrm{green})}+\underset{\mathrm{Copper}}{\mathrm{Cu}\left(\mathrm{s}\right)}$