Question

By giving a reason, state why chemical change takes place in the following case: Heating of Zinc Oxide in a hard glass test tube.

Answer 1

Chemical change:

• The process in which the composition of the substance is completely altered and a new product is formed is called a chemical change.
• Zinc oxide $\left(\mathrm{ZnO}\right)$ is a white powder, when heated strongly (above $800°\mathrm{C}$) it turns yellow, but again becomes white again when cold.
• The hot zinc oxide absorbs some of the blue wavelengths of light making the reflected light more yellow.
• The loss of oxygen makes it a reversible chemical change.

$\mathrm{ZnO}\left(\mathrm{s}\right)\stackrel{}{\rightleftharpoons }{\mathrm{Zn}}^{2+}\left(\mathrm{aq}\right)+\frac{1}{2}{\mathrm{O}}_2\left(\mathrm{aq}\right)+2{\mathrm{e}}^{-}$

• Thus, the change of color to yellow on heating, and becomes white on cooling depicts reversible chemical change.

Therefore, in the end, we can conclude that this is a chemical change.