Question

Why NaCl become yellow upon heating ?

Answer 1

Due to metal excess defect

Answer 2

the yellow colour acquired by the NaCl crystal is due to defect called metal excess defect due to anionic vacancy.

When crystals of NaCl is heated in an atmosphere  the sodium ion are deposited on the surface of crystal.The Cl- ion diffuse to form NaCl.This happens by loss of electron by Na to form Na+.The released electron diffuse into the crystal and occupy anionic site.these are called F centre and it imparts yellow colour to NaCl