Question

$ZnO$ is white when cold but yellow when hot:

• A. As $ZnO$ is heated, its oxidation state changes
• B. Due to escape of oxide ions in the form of oxygen, the electrons left in the lattice causes colour to appear
• C. Due to metal deficiency on heating, holes transit, producing colours
• D. Due to polarisation of oxide ions

Answer 1

The correct option is B Due to escape of oxide ions in the form of oxygen, the electrons left in the lattice causes colour to appear

Zinc oxide is white in colour at room temperature. On heating it loses oxygen and turns yellow. Now there is excess of zinc in the crystal and its formula becomes $Z{n}_{1+x}O$. The excess $Z{n}^{2+}$ ions move to interstitial sites and the electrons to neighbouring interstitial sites.