Question

How do you identify the following cations in their aqueous solutions by using caustic soda solution? (i) \(\mathrm{Fe}^{+2}\) (ii) \(\mathrm{Zn}^{+2}\) (iii) \(\mathrm{Al}^{+3}\)

Short answer

Answer: When caustic soda solution is added to an aqueous solution containing Iron(II) ions, a greenish-white precipitate of iron(II) hydroxide is formed. When added to Zinc(II) ions, a white precipitate of zinc(II) hydroxide is formed, which dissolves in excess caustic soda solution to form a colorless solution containing the complex ion [Zn(OH)4]²⁻. When added to Aluminum(III) ions, a white gelatinous precipitate of aluminum hydroxide is formed, which dissolves in excess caustic soda solution to form a colorless solution containing the complex ion [Al(OH)4]⁻.

Step-by-step solution

Identification of Iron(II) (\(\mathrm{Fe}^{+2}\)) ions using caustic soda solution

When caustic soda (NaOH) solution is added to an aqueous solution containing \(\mathrm{Fe}^{+2}\) ions, a greenish-white precipitate of iron(II) hydroxide, \(\mathrm{Fe(OH)_2}\), forms. The reaction is given as follows: $$\mathrm{Fe}^{+2} + 2 \mathrm{NaOH} \rightarrow \mathrm{Fe(OH)_2} + 2 \mathrm{Na}^{+}$$

Identification of Zinc(II) (\(\mathrm{Zn}^{+2}\)) ions using caustic soda solution

When caustic soda (NaOH) solution is added to an aqueous solution containing \(\mathrm{Zn}^{+2}\) ions, a white precipitate of zinc(II) hydroxide, \(\mathrm{Zn(OH)_2}\), forms. The reaction is given as follows: $$\mathrm{Zn}^{+2} + 2 \mathrm{NaOH} \rightarrow \mathrm{Zn(OH)_2} + 2 \mathrm{Na}^{+}$$ The precipitate of \(\mathrm{Zn(OH)_2}\) dissolves in excess caustic soda solution, forming a colorless solution containing the complex ion \(\mathrm{[Zn(OH)_4]}^{2-}\): $$\mathrm{Zn(OH)_2} + 2 \mathrm{NaOH} \rightarrow \mathrm{[Zn(OH)_4]}^{2-} + 2 \mathrm{Na}^{+}$$

Identification of Aluminum(III) (\(\mathrm{Al}^{+3}\)) ions using caustic soda solution

When caustic soda (NaOH) solution is added to an aqueous solution containing \(\mathrm{Al}^{+3}\) ions, a white gelatinous precipitate of aluminum hydroxide, \(\mathrm{Al(OH)_3}\), forms. The reaction is given as follows: $$\mathrm{Al}^{+3} + 3 \mathrm{NaOH} \rightarrow \mathrm{Al(OH)_3} + 3 \mathrm{Na}^{+}$$ The precipitate of \(\mathrm{Al(OH)_3}\) will dissolve in excess caustic soda solution, forming a colorless solution containing the complex ion \(\mathrm{[Al(OH)_4]}^{-}\): $$\mathrm{Al(OH)_3} + \mathrm{NaOH} \rightarrow \mathrm{[Al(OH)_4]}^{-} + \mathrm{Na}^{+}$$