On cooling hot zinc oxide, the yellow colour become white. Why ?

Explanation

The composition of `Zn^(2+) and O^(2-)` ions in stoichiometric `ZnO` is `n : n`.
On heating ZnO, oxygen is lost reversibly.
A non - stoichiometric compound with excess `Zn^(2+)` ions is formed possessing a composition of `Zn^(2+) and O^(2-)` ions `n:n-x`.
Due to excess metal in the interstities, it is yellow.
On cooling the non-stoichiometric zinc oxide, it gains oxygen. A stoichiometric zinc oxide is formed and hence it is white.
`Zn^(2+) + (1)/(2) O_(2) 2e^(-) to ZnO`