Question

A metal oxide is yellow when hot and white when cold. The metal oxide is:
a.) ZnO
b.) CuO
c.) PbO
d.) SiO

Answer 1

Hint: The metal oxide which is yellow when hot and white when cold is amphoteric in nature which means that it can react with both acid and base and form compound .It is also used in the rubber industry.

Complete step by step answer:
Zinc oxide acts as an amphoteric oxide and it reacts with acids to form zinc salt. Zinc oxide reacts with a strong base to form zincates, when this is strongly heated it assumes a yellow color which disappears when it is cooled down.
Zinc oxide is a white powder which is insoluble in water. It is found in the earth crust in the form of zincate. Hence zinc oxide is commercially synthetic. Zinc ore zincate contains magnesium and other impurities which give it a yellow color. Crystalline zinc oxide undergoes reversible change in color when heated this property is known as thermo-chronic. It changes from white to yellow when heated and reverts to white when it is cooled down. The hot zinc oxide absorbs the light of the blue wavelength making the reflected light more yellow. The loss of oxygen during this change of color makes it a reversible process.
So, the correct answer is “Option A”.

Note: Zinc oxide is used in the rubber industry. It is used along with stearic acid in the vulcanization of rubber to produce tires, shoe soles etc. Another use of zinc oxide is in concrete manufacturing. The addition of zinc oxide helps in processing concrete and it also improves the water resistance property. It also has antibacterial and deodorizing properties.