Question

Give a chemical test to distinguish between the following pair of compounds

Ferrous sulphate and ferric sulphate solution using sodium hydroxide solution

Answer 1

Reagent	Ferrous sulphate solution	Ferric sulphate solution
When sodium hydroxide solution is added	When ferrous sulphate solution react with sodium hydroxide solution, a dirty green precipitate is formed. ${\mathrm{FeSO}}_4\left(\mathrm{aq}\right)+2\mathrm{NaOH}\left(\mathrm{aq}\right)\to \underset{\mathrm{dirty}\mathrm{green}\mathrm{precipitate}}{\mathrm{Fe}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)}+{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)$	When ferric sulphate solution react with sodium hydroxide solution, a reddish brown precipitate of Fe(OH)3 is formed. ${\mathrm{Fe}}_2{\left({\mathrm{SO}}_4\right)}_3\left(aq\right)+6\mathrm{NaOH}\left(\mathrm{aq}\right)\to \underset{\mathrm{a}\mathrm{dirty}\mathrm{brown}\mathrm{precipitate}}{2\mathrm{Fe}{\left(\mathrm{OH}\right)}_3\left(\mathrm{s}\right)}+3{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)$
When excess reagent is added	When excess reagent is added, it does not dissolve. Due to atmospheric oxidation, the precipitate slowly turns brown. $4\mathrm{Fe}{\left(\mathrm{OH}\right)}_2\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 4\mathrm{Fe}{\left(\mathrm{OH}\right)}_3\left(\mathrm{s}\right)$	When excess reagent is added, no reaction takes place.