Table salt sometimes appear yellow in colour because:

The correct answer is Option (1) → It has metal excess defect

When crystals of NaCl are heated in sodium vapour, sodium atoms deposit on the surface. \( \ce{Cl^{-}} \) ions diffuse to the surface and combine with them to form NaCl; each Na atom loses an electron to become \( \ce{Na^{+}} \), and the released electrons occupy the anionic sites left behind. The crystal now has an excess of sodium: a metal excess defect due to anionic vacancies.

The anionic sites occupied by unpaired electrons are called F-centres (colour centres). These electrons absorb visible light and are excited, which gives NaCl its yellow colour. (Excess lithium makes LiCl pink and excess potassium makes KCl violet.)

Why the other options are incorrect

• Option (2) It has metal deficiency defect — Incorrect. This defect occurs in compounds of metals with variable oxidation states, such as FeO (Fe_0.95O); sodium shows only +1, and the defect creates no F-centres.

• Option (3) It gets oxidized — Incorrect. \( \ce{Na^{+}} \) and \( \ce{Cl^{-}} \) are not oxidised; the colour comes from trapped electrons in the lattice, not from a new oxidised compound.

• Option (4) It gets reduced — Incorrect. No reduced compound such as sodium metal is formed in the crystal; the yellow colour is due to electrons in anion vacancies, a lattice defect.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Metal Excess Defect – “… The anionic sites occupied by unpaired electrons are called F-centres (from the German word Farbenzenter for colour centre). They impart yellow colour to the crystals of NaCl. The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.”