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Question
Why does table salt, NaCl, some times appear yellow in colour?
Solution
The yellow colour of sodium chloride is due to metal excess defect. In metal excess defect anionic vacanies are created due to diffusion of CI– to the surface of the crystal and there after unpaired electrons occupy anionic sites. These sites are called F-centres. The electrons at F-cetres then absorb energy from the visible region for the excitation which makes crystal appear yellow.
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