Question

Why does a crystal of KCl appear violet in colour in spite of the fact that both K⁺ and Cl⁻ ions are colourless?

Answer 1

1. A crystal of KCl appears violet in colour despite both K⁺ and Cl⁻ ions being colourless because of the presence of F-centres, which result from a metal excess defect.
2. When some Cl⁻ ions are missing from their lattice sites, electrons occupy these vacant anion positions to maintain electrical neutrality.
3. These electron-filled vacancies are called F-centres. The electrons in F-centres can absorb energy from visible light and get excited.
4. When they return to their original state, they emit energy, often in the violet region of the visible spectrum.
5. Thus, the violet colour of KCl crystal arises due to the presence of F-centres, even though the individual K⁺ and Cl⁻ ions are themselves colourless.