Question

Why does white ZnO (s) becomes yellow upon heating ?

Answer 1

The excess Zn ions thus formed get entrapped in the interstitial site and electron in the neighborhood vacant interstitial sites. This electron is responsible for the colour and electrical conductivity in crystals.

On heating ZnO loses oxygen as follows

$ZnO\stackrel{Heat}{⟶}Z{n}^{2+}+\frac{1}{2}{O}_2+2e^{-}$

$Z{n}^{2+}$ ions and electrons move to interstitial sites and F-centres are formed which impart yellow colour to ZnO(s).