Question

Give reasons for the following:

Blue colour of copper(II) sulphate solution is destroyed when iron filings are added to it.

Answer 1

1. When iron filings are dipped in ${\mathrm{CuSO}}_4$ solution, displacement reaction takes place and formation of ferrous sulphate takes place which is a green colour solution and on iron filings deposition of copper takes place which turns grey iron filings to brown in colour.

2. Reaction

$\mathrm{CuSO}{}_4(\mathrm{aq})+\mathrm{Fe}(\mathrm{s})\to \mathrm{FeSO}{}_4(\mathrm{aq})+\mathrm{Cu}\left(\mathrm{s}\right)$

Copper(II)sulphate Iron Iron sulphate Copper

(Blue ) (Grey) (Green) (Brown)

Since $\mathrm{Fe}$ is more reactive than $\mathrm{Cu}$ displacement takes place.

3. Displacement reaction: Reaction in which one of the higher reactive element displaces a less reactive element from its solution. $\mathrm{A}+\mathrm{BC}\to \mathrm{AC}+\mathrm{B}$

(Here A is more reactive than B)

4. Reactivity series

$\mathrm{K}>\mathrm{Na}>\mathrm{Ca}>\mathrm{Mg}>\mathrm{Al}>\mathrm{Zn}>\mathrm{Fe}>\mathrm{Ni}>\mathrm{Sn}>\mathrm{Pb}>\mathrm{H}>\mathrm{Cu}>\mathrm{Hg}>\mathrm{Ag}>\mathrm{Au}>\mathrm{Pt}$

Here potassium is the most reactive metal and platinum is the least reactive metal in this series.

So, from the reactivity series, we can see $\mathrm{Fe}$ is more reactive than $\mathrm{Cu}$ so it can displace copper from its solution and therefore the blue colour is destroyed.
This is a displacement reaction.