Thank you for reporting, we will resolve it shortly
Q.
The white $ ZnO $ turns yellow on heating because of
AMUAMU 2010The Solid State
Solution:
$ZnO$, on heating, loses oxygen and thus, the zinc metal becomes in excess in $ZnO$ crystal. Hence, the formula of $ZnO$ becomes $Zn_{1+x}O$ The excess $Zn^{2+}$ ions move to interstitial sites and the electrons to neighbouring interstitial sites. The electrons are responsible for the yellow colour of $ZnO$
$ZnO \xrightarrow{\Delta} Zn^{2+}+\frac{1}{2}O_{2}+2e^{-}$
Thus, yellow colour of $znO$ is due to metal excess defect