JAMB Chemistry 2024 Paper

Concept:Sodium hydroxide ($\mathrm{NaOH}$) solution forms characteristic precipitates with certain metal cations, allowing their identification.Explanation:Add aqueous $\mathrm{NaOH}$ to the test solution and observe the precipitate formed and its solubility in excess $\mathrm{NaOH}$.For $\mathrm{Ca}^{2+}$: a white precipitate of $\mathrm{Ca(OH)_2}$ forms, and it does not dissolve in excess $\mathrm{NaOH}$.For $\mathrm{Zn}^{2+}$: a white precipitate of $\mathrm{Zn(OH)_2}$ forms, which dissolves in excess $\mathrm{NaOH}$ to give a colorless solution.For $\mathrm{Cu}^{2+}$: a blue precipitate of $\mathrm{Cu(OH)_2}$ forms, which does not dissolve in excess $\mathrm{NaOH}$.Therefore, $\mathrm{NaOH}$ can be used to test for all three ions: $\mathrm{Ca}^{2+}$, $\mathrm{Zn}^{2+}$, and $\mathrm{Cu}^{2+}$.Answer:C. I, II and III