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Question
Why does white ZnO (s) becomes yellow upon heating?
Solution
The ZnO crystal becomes yellow oh heating because of the metal excess defect which is caused due to the presence of extra cations at the interstitial sites and on heating this white crystal it loses oxygen and turns yellow. The reaction involved is given as-
\[\ce{ZnO ->[heating] Zn^{2+} {+} 1/2 O2 {+} 2e^-}\]
Here the excess of Zn2+ ions move to interstitial sites and electrons to neighbouring interstitial sites.
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