Question

Why does white $ZnO\left(s\right)$ become yellow upon heating?

Answer 1

On heating $ZnO$, it loses oxygen and electrons, as shown below:

$ZnO\left(s\right)\stackrel{heat}{\to }Z{n}^{2+}+1/2{\text{O}}_2+2e^{-}$

$Z{n}^{2+}$ ions and electrons occupy interstitial sites in the lattice. The sites occupied by the electrons are called colour centres or $F-$ centres. These electrons get excited by absorbing energy from visible light. When they return to their ground state, they impart a yellow colour to the crystal.