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Questions
Why does table salt, NaCl, sometimes appear yellow in colour?
Why is common salt sometimes yellow instead of being pure white?
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Solution
- The yellow colour of sodium chloride is due to metal excess defect.
- In metal excess defects, anionic vacancies are created due to the diffusion of Cl– to the surface of the crystal and thereafter 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 the crystal appear yellow.
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