Question

ZnO is white when cold and yellow when hot because it ……
$\left( A \right)$ Sublimes
$\left( B \right)$ Melts
$\left( C \right)$ Form non stoichiometric compound at high temperature
$\left( D \right)$ All of the above

Answer 1

Hint : Zinc oxide is white powder at room temperature . It has a density of $561g/c{m^3}$ . The powder is used as an additive in numerous materials. Zinc oxide is insoluble in water . zinc oxide is amphoteric in that it reacts with both acid or base.

Complete step by step solution
Zinc oxide is used in the rubber industry . Zinc oxide is used in sunscreen in the range of $20 - 60nm$ .
Zinc oxide changes its colour when heated and again returns to white when it cools down . Naturally zinc is present in zincite ore . Zinc oxide is termed as thermo chromic , meaning changing of colour from white to yellow . This colour change occurs due to the small loss of oxygen while heating and forming non – stoichiometric $Z{n_{1 + x}}O$ .
So , the option $\left( C \right)$ is correct .

Additional Information
There are two types of zinc ore wurtzite and zinc blend . wurtzite ore is commonly used . wurtzite is hexagonal in shape whereas zinc blend is cubic. Wurtzite is stable in ambient conditions.
Zinc oxide is a soft . which has an elastic constant less than . zinc has the highest piezoelectric tensor among all other semiconductors , this makes it suitable in the piezoelectric applications. Zinc oxide has a large band gap which means it has the ability to sustain high electricity . its band gap can also be increased by alloying it with magnesium oxide or cadmium oxide .
Zinc oxide is n-type which is present even in the absence of doping .Non stoichiometry is a type of origin of n type character . n-type doping can occur by alloying it with group -3 which are aluminium , gallium etc.

Note:
Zinc oxide has UV protection ability, high refractive index , high thermal conductivity , binding and antibacterial properties. So, it is used in paints, cement, ceramics , ointments , adhesives , batteries , pigments , food and fire retardants .