Question

Zinc oxide is white when cold but yellow when hot because it develops

Answer 1

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In ZnO, Zn^2+ ions occupy the interstitial sites and electrons are trapped in the interstitial sites for neutralization. Crystals that show such type of defects are generally colored. Zinc oxide is white in color but on heating it turns into yellow because it loses oxygen reversibly at high temperature.

Answer 2

Answer:

This is due to metal excess defect  (non stoichiometric defect)

Explanation:

when zinc oxide is heated

ZnO ----> Zn2+ +2e-  +0.5 O2

some oxygen is lost

Zn2+ occupy interstitial sites and the electrons occupy the vacancy made by o2- atoms. These sites are called as f-centres and these electrons are responsible for colour