Question

Reaction of a salt when heated gives a residue which is red when hot and yellow when cold

Answer 1

In ZnO, Zn2+ ions occupy the interstitial sites and electrons are trapped in the interstitial sites for the neutralization. Crystals that shows such type of defects are generally colored. Zinc oxide is white in color but on heating it turns into yellow because on heating it loses oxygen reversibly at high temperature.