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Question

Why does white ZnO(s) become yellow upon heating?

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Solution

On heating ZnO, it loses oxygen and electrons, as shown below:

ZnO(s)heatZn2++1/2O2+2e

Zn2+ ions and electrons occupy interstitial sites in the lattice. The sites occupied by the electrons are called colour centres or F centres. These electrons get excited by absorbing energy from visible light. When they return to their ground state, they impart a yellow colour to the crystal.

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