Question

Mention with reasons the colour changes observed when:

(i) silver chloride is exposed to sunlight.

(ii) copper powder is strongly heated in the presence of oxygen.

(iii) a piece of zinc is dropped in Copper Sulphate solution.

Answer 1

Explaining the reasons:

• (i) When Silver Chloride ($\mathrm{AgCl}$) is exposed to sunlight it changes from White to Gray.
• Reason: When Silver Chloride is exposed to sunlight, it undergoes photochemical decomposition to produce Silver and Chlorine.
• $$\begin{gathered} 2\mathrm{AgCl}\to 2\mathrm{Ag}+{\mathrm{Cl}}_2 \\ \mathrm{Silver}\mathrm{Chloride}\mathrm{Silver}\mathrm{Chlorine}\mathrm{gas} \\ \left(\mathrm{White}\right)\left(\mathrm{Gray}\right)(\mathrm{Greenish}-\mathrm{Yellow}\mathrm{Gas}) \end{gathered}$$
• (ii) Upon strongly heating Copper powder in the presence of oxygen, it changes its colour from brown to black.
• Reason: When Copper powder is strongly heated in the presence of oxygen, then Copper reacts with oxygen present in the air to form copper oxide.
• $$\begin{gathered} {\mathrm{Cu}}_{\left(\mathrm{powder}\right)}+{\mathrm{O}}_{2\left(\mathrm{air}\right)}\to 2\mathrm{CuO} \\ \mathrm{Copper}\mathrm{Oxygen}\mathrm{Copper}\mathrm{Oxide} \\ \left(\mathrm{Brown}\right)\left(\mathrm{Colourless}\right)\left(\mathrm{Black}\right) \end{gathered}$$
• (iii) When a piece of zinc is dropped in a copper sulphate solution, the colour of the solution fades from blue to colourless.
• Reason: The oxidation potential of Zinc is much higher than Copper, so Zinc replaces the Copper from the solution.
• And, the colour of the solution fades from blue to colourless.
• $$\begin{gathered} \mathrm{Zn}+{\mathrm{CuSO}}_4\to {\mathrm{ZnSO}}_4+\mathrm{Cu} \\ \mathrm{Zinc}\mathrm{Copper}\mathrm{Sulphate}\mathrm{Zinc}\mathrm{Sulphate}\mathrm{Copper} \\ \left(\mathrm{Blue}\right)\left(\mathrm{Colourless}\right) \end{gathered}$$