Question

Give one chemical test to distinguish between the following pairs of compounds:
(i) Zinc sulphate solution and zinc chloride solution.
(ii) Iron (II) chloride solution and iron (III) chloride solution.

Answer 1

(i) Zinc sulphate and zinc chloride solutions are distinguished by adding barium chloride solution to both.

Chemical test	Zinc sulphate solution	Zinc chloride solution
Addition of barium chloride solution	On reaction with barium chloride, it forms a white precipitate of barium sulphate, which is insoluble in mineral acids. $$\begin{gathered} {\mathrm{ZnSO}}_4+{\mathrm{BaCl}}_2\to {\mathrm{BaSO}}_4+{\mathrm{ZnCl}}_2 \\ \mathrm{White} \\ \mathrm{precipitate} \end{gathered}$$	It does not react with barium chloride solution.

(ii) Iron (II) chloride and Iron (III) chloride solutions are distinguished by adding sodium hydroxide solution to both.

Chemical test	Iron (II) chloride solution	Iron (III) chloride solution
Addition of sodium hydroxide solution	It reacts with sodium hydroxide solution to form a dirty green precipitate of iron (II) hydroxide, which is insoluble in excess of sodium hydroxide. $$\begin{gathered} {\mathrm{FeCl}}_2+2\mathrm{NaOH}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2+2\mathrm{NaCl} \\ \mathrm{Dirty}\mathrm{green} \\ \mathrm{precipitate} \end{gathered}$$	It reacts with sodium hydroxide solution to form a reddish brown precipitate of iron (III) hydroxide, which is insoluble in excess of sodium hydroxide. $$\begin{gathered} {\mathrm{FeCl}}_3+3\mathrm{NaOH}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_3+3\mathrm{NaCl} \\ \mathrm{Brown} \\ \mathrm{precipitate} \end{gathered}$$