Q.When ZnO is heated it turns yellow and returns back to original white colour on cooling. What could be the reason?

No real crystal is perfect. Even a carefully grown crystal has occasional irregularities called point defects — a break in the otherwise regular pattern at an isolated point in the lattice. NCERT focuses on four kinds, and the key to every question here is telling them apart by what actually happens and what it does to the crystal's properties.

Schottky defect — a pair of oppositely charged ions (one cation, one anion) is missing entirely from their lattice positions, leaving two vacant sites. This shows up in ionic solids where the cation and anion are similar in size and the crystal has a high coordination number — the classic examples are NaCl, KCl, and CsCl. Because real mass is genuinely lost from the crystal while its overall volume barely changes, a Schottky defect decreases the density of the solid.

Frenkel defect — a smaller ion (almost always the cation, since cations are usually the smaller ion) is dislodged from its normal lattice site and relocated to an interstitial site — a gap between the regular lattice positions — rather than leaving the crystal. This happens where there's a large size difference between the cation and anion, so the small cation has room to squeeze into an interstitial gap: AgCl, AgBr, and ZnS are the standard examples (also called a dislocation defect, since the ion is displaced rather than removed). Because no mass actually leaves the crystal — the ion just moves — a Frenkel defect leaves the density unchanged.

This density difference is the single most tested fact in the whole concept: Schottky defect decreases density (mass genuinely lost); Frenkel defect leaves density unchanged (mass only relocated, never lost). Both defects, though, keep the crystal's overall chemical formula intact — they're called stoichiometric defects for exactly that reason.

Non-stoichiometric defects, by contrast, genuinely shift the ratio of atoms away from the ideal formula: …