Question

$ZnO$ is white when cold and is yellow when hot. What is the reason behind this colour change?

Answer 1

$ZnO$ is normally a white, finely divided material with the wurtzite structure. On heating, the color changes to yellow, due to the evaporation of oxygen from the lattice to give a nonstoichiometric phase. The supernumerary $Zn$ atoms produce lattice defects which trap electrons, which can subsequently be excited by absorption of visible light.