Question

Zinc oxide, which in colourless at room temperature, acquires yellow colour on heating due to
A.Zinc being transition element
B.Paramagnetic nature of the compound
C.Trapping of electron at the site
D.Both (A) and (B)

Answer 1

We know that zinc oxide is an inorganic compound. It has a chemical formula of $ZnO$. It is white powder and is not soluble in water. Zinc oxide is an example of a wide-band gap semiconductor of the II-VI semiconductor gap. Zinc oxide is crystalline and is thermochromic. When heated in the presence of air, it changes its colour.

Complete step by step solution:
We have to remember that the colour of zinc oxide is colourless at room temperature. When it is heated, yellow colour is obtained. On heating, zinc oxide loses oxygen and so free $Z{n^{2 + }}$ ions are formed. The more $Z{n^{2 + }}$ ions enter interstitial sites and the electrons accommodate the interstitial positions.
In $ZnO$, the ions of $Z{n^{2 + }}$ occupy the interstitial sites and electrons are trapped in the interstitial sites because of neutralization. Crystal that exhibits these properties is generally colored. The colour of zinc oxide is white at room temperature whereas on heating it turns yellow due to loss of oxygen reversibly at temperatures which are very high. When zinc oxide is heated it forms $Z{n^{2 + }} + \dfrac{1}{2}{O_2} + 2{e^ - }$.
In oxygen deficient zinc oxide, the free electrons that hold the compound electrically neutral are less tightly linked and might be taken to higher level because of the absorption of photons in the violent region of the spectra and one could notice the yellow colour which is complementary colour of violet.
Zinc oxide is colourless at room temperature turns yellow colour on heating because of the trapping of electrons at the site.
Therefore, the option (B) is correct.

Note: We have to know that zinc oxide is an amphoteric oxide. It is soluble in several acids like hydrochloric acid but is insoluble in water. It forms soluble zincates when it is dissolved in alkalis such as sodium hydroxide. The two main forms of zinc oxide are wurtzite and zinc blender.