Question

ZnO turns yellow on heating. Why?

Answer 1

When ZnO is heating, it loses oxygen according to the reaction:

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

On heating, due to presence of electrons in the interstitial voids, the colour is yellow as the remaining colours of white are absorbed by these electrons.