1.

On cooling hot zinc oxide, the yellow colour become white. Why ?

Answer»

Solution :The composition of `Zn^(2+) and O^(2-)` ions in STOICHIOMETRIC `ZnO` is `n : n`.
On heating ZnO, oxygen is lost reversibly.
A non - stoichiometric COMPOUND with excess `Zn^(2+)`ions is formed possessing a composition of `Zn^(2+) and O^(2-)` ions `n:n-x`.
Due to excess METAL in the interstities, it is YELLOW.
On cooling the non-stoichiometric zinc OXIDE, it gains oxygen. A stoichiometric zinc oxide is formed and hence it is white.
`Zn^(2+) + (1)/(2) O_(2) 2e^(-) to ZnO`


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