(i) Yellow colour of ZnO is due to metal excess defect. Zinc oxide which is white in colour on heating loses oxygen and turns yellow.
\(ZnO\overset { Heating }{ \longrightarrow } { Zn }^{ + }+\frac { 1 }{ 2 } { O }_{ 2 }+{ 2e }^{ -1 }\)
(ii) The excess Zn²⁺ ion thus, formed get trapped into vacant interstitial sites and the electrons in the neighbouring interstitial sites.

i) Crystals have long range repeated pattern of arrangement of constituent particles but in the process of crystallisation.
ii) Some deviations from the ideal arrangement may be introduced because the constituent particles may not get sufficient time to arrange themselves in a perfect order. Therefore, crystals are usually not perfect.

Number of tetrahedral voids formed = 2 x No. of atoms of element Y
No. of atoms of element Y in the ccp unit cell = 4
No. of tetrahedral voids by atoms of Y = 2 x 4 = 8
∴ No. of tetrahedral voids occupied by atoms of 
X = \(\frac{2}{3}\times 8=\frac{16}{3}\)
Ratio of the no. of atoms of X and Y is = \(\frac{16}{3}:4\)
= 16:12
= 4:3
Hence, formula would be X₄Y₃.

(i) Compressibility is the ability of the substance to change its shape by applying external pressure.
(ii) Solids form closed packed structure with negligible intermolecular space and possesses very strong intermolecular forces of attraction.
(iii) Thus, they do not change their shape in the presence of external pressure.

\(M=\frac { d\times { a }^{ 3 }\times NA }{ g } \)
d = Density of the material
a = Length of the edge of the cell.
N_A = Avogadro number
Z = No. of atoms
\(M=\frac { 10.5{ gcm }^{ -3 }{ (4.07\times { 10 }^{ -6 }cm) }^{ 3 }\times \left( 6.023\times { 10 }^{ 23 }{ mol }^{ -1 } \right) }{ 4 } \)
Atomic mass of silver M = 107.08 g mol^-1.