Question

Name a metallic oxide which is yellow when hot and white when cooled.

Answer 1

• Zinc oxide(ZnO), the oxide is initially white but when heated it changes its color to yellow.
• Zinc oxide (ZnO) upon heating losses oxygen and results in the formation of Zinc ions(Zn²⁺), Oxygen gas(O₂), and its losses of some electrons(e^-).
• This results in the production of excess zinc ions (Zn²⁺)and electrons. (e^-)
• Zinc goes to interstitial sites of ZnO and electrons go to neighboring interstitial sites.
• Interstitial sites: The space that exists between the atoms.
• Here, The defect occurs in the ZnO structure because of heating.
• The defect is known as a metal excess defect.
• Metal excess defect: The ions of metal will be in excess.
• Similar is the case with ZnO upon heating.
• $\underset{\mathrm{Zinc}\mathrm{Oxide}}{\mathrm{ZnO}\left(\mathrm{s}\right)}\stackrel{\mathrm{heat}}{\rightleftarrows }\underset{\mathrm{Zinc}}{{\mathrm{Zn}}_2}++\underset{\mathrm{Oxygen}}{1/2{\mathrm{O}}_2}+\underset{\mathrm{Electron}}{2{\mathrm{e}}^{-}}$
• Zinc oxide upon heating loses oxygen and becomes yellow.
• This is a reversible type of reaction.
• Therefore can be reversed back to ZnO.

Zinc oxide(ZnO) is a metallic oxide that is yellow when hot and white when cooled.