Question

Zinc oxide is normally white but turns yellow on heating and becomes white again on cooling because of:
A.d-d transition spectra as well as crystal defect.
B.The two dimensional network structure of $\text{ZnO}$
C.Its high transition temperature $\left( {{\text{T}}_{\text{c}}} \right)$
D.Various types of lattice defects because of which the oxygen is lost during heating.

Answer 1

When a lattice is formed for an ionic compound, due to the attraction between the ions, there are certain defects that arise in the lattice which causes the colour of the compounds.

Complete step by step solution:
In the zinc oxide, there is an excess of metal ions in the lattice and this is called the metal excess deficiency. When heated, the zinc oxide lattice, loses electrons and also oxygen, thereby its formula becomes, $\text{Z}{{\text{n}}_{\text{1+x}}}\text{O}$.
The excess of zinc ions move to the interstitial sites while the released electrons are trapped in the neighbouring interstitial sites which gives rise to the colour as the transition requires energy of a certain wavelength.
Again when Zinc Oxide is cooled, oxygen is absorbed and the structure is restored thereby the colour becomes white again.

So, the correct answer is: D, various types of lattice defects because of which the oxygen is lost during heating.

Note:
The ZnO crystals can have three crystal structures, either the wurtzite structure, or the rock salt structure, or the zinc blende structure. Due to improper arrangement of the ions in the lattice, there arises some defects. These defects can be classified into three categories: stoichiometric, non-stoichiometric, and impurity defects. Zinc Oxide or$\text{ZnO}$ is a classic example of a non-stoichiometric defect compound. These defects do not disrupt the stoichiometry of the compounds.