why does white Zn0(s) becomes yellow upon heating ?

Explanation

on heating ZnO loses oxygen as follows
`ZnOoverset("heat")to Zn^(2+)+(1)/(2)O_(2)+2e^(-)`
`Zn^(2+)` ions and electrons move to interstitial sites and F - centress are formed which imparat yellow colour to ZnO(S) .