You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute $\mathrm{HNO}_3$, $\mathrm{ZnCl}_2$ and $\mathrm{H}_2\mathrm{O}$. In which of the containers can these solutions be kept?

[NCERT Exemplar]

Ans. Reactions of copper with:

(1) Dilute HCl: Copper exhibits no reaction with HCl, so it can be stored in the copper container.

(2) Dilute HNO₃: Nitric acid acts as a strong oxidising agent and reacts with copper vessel. With dilute nitric acid, copper metal forms copper nitrate with nitric oxide and water, so dilute nitric acid cannot be stored in the copper container.

$$3 \mathrm{Cu}_{(s)} + 8 \mathrm{HNO}_{3} \longrightarrow 3 \mathrm{Cu}(\mathrm{NO}_{3})_{2(aq)} \\ + 2 \mathrm{NO}_{(g)} + 4 \mathrm{H}_{2}\mathrm{O}_{(l)}$$

(3) $\mathrm{ZnCl}_2$: Since Cu metal is less reactive than Zn, it cannot displace Zn from its compounds and there will be no reaction. Hence, zinc chloride can be stored in the copper container.

(4) $\mathrm{H}_2\mathrm{O}$: There is no reaction between water and copper metal at room temperature, so water can be stored in a copper container.

Reactions of aluminium with:

(1) Dilute HCl: Al reacts with dilute HCl and forms aluminium chloride with hydrogen gas evolved, so it cannot be stored in the aluminium container.

$$2 \mathrm{Al}_{(s)} + 6 \mathrm{HCl}_{(aq)} \longrightarrow 2 \mathrm{AlCl}_{3(aq)} + 3 \mathrm{H}_{2(g)}$$

(2) Dilute HNO₃: Nitric acid is an oxidising agent and in its presence, aluminium metal gets oxidised to form a layer of $\mathrm{Al}_{2}\mathrm{O}_{3}$, and forms a protective layer of oxide, so it will not react further. Hence, dilute nitric acid can be stored in the aluminium container.

(3) $\mathrm{ZnCl}_2$: Aluminium, being more reactive than zinc, can displace zinc ion from the

solution. It readily reacts with zinc chloride and forms aluminium chloride with zinc metal.

$$2 \mathrm{Al}_{(x)} + 3 \mathrm{ZnCl}_{2(aq)} \longrightarrow 2 \mathrm{AlCl}_{3(aq)} + 3 \mathrm{Zn}_{(x)}$$

Hence, it cannot be stored in the aluminium container.

(4) $\mathsf{H}_2\mathsf{O}$: At room temperature, there will be no reaction between aluminium metal and hot or cold water, so it can be stored in the aluminium container. Aluminium is attacked by steam to form aluminium oxide and hydrogen.

$$2 \mathrm{Al}_{(x)} + 3 \mathrm{H}_2\mathrm{O}_{(g)} \longrightarrow \mathrm{Al}_2\mathrm{O}_{3(x)} + 3 \mathrm{H}_2(g)$$