Chemistry NCERT Exemplar Solutions Class 12th Chapter One

18. Option (iv) is correct since in antiferromagnetic substances the domains  are oppositely oriented and hence they cancel out each other's magnetic moments. 

17. Option (ii) Quartz glass (SiO₂) is correct since quartz glass (SiO₂)is amorphous in nature as there is no long range ordered arrangement of the constituent particles being present in it and hence it is an amorphous solid.

16. Option (ii) Isotropic nature is correct since crystalline solids exhibit anisotropic properties like refractive index, electrical resistance etc. Since these are found to have different values when measured along different directions in the same crystal and hence they are not isotropic in nature. 

15. Option (ii) Low temperature is correct since at sufficiently low temperature, the thermal energy is low, so the intermolecular forces bring the molecules of a substance closer so that they cling to one another and occupy fixed positions. They keep on vibrating about their fixed positions. Such conditions favours the existence of the substance in solid state. 

14. On heating the amorphous substance it gets changed to the crystalline form at some temperature. This is due to the process called crystallization. As on heating at some temperature it may become crystalline since slow heating and cooling over a longer period of time makes these changes.

13. In ccp lattice the number atoms per unit cell = 4 

The number of tetrahedral voids is given as = 2 (n) = 2 x 4 = 8

Only one-third of tetrahedral voids are occupied by metal M so, the ratio of atoms of element M to that of element N = 1/3 (8) : (4) = 2 : 3 

or M : N = 2 : 3 

Hence, the formula of the compound is M₂N₃.

11. In semiconductors the gap between conduction band and valence band is small and hence some of the electrons from the valence band can easily jump to the conduction band and shows some conductivity but with rise in the temperature more of the electrons gets jump to the conduction band and thus, their electrical conductivity increases with rise in the temperature.

10. The ZnO crystal becomes yellow oh heating because of the metal excess defect  which is caused due to the presence of extra cations at the interstitial sites and on heating this white crystal it loses oxygen and turns yellow. The reaction involved is given as-

ZnO  Zn²⁺ + $\frac{1}{2}$ O₂ + 2^e-

Here the excess of Zn²⁺ ions move to the interstitial sites and electrons to neighbouring interstitial sites.

9. In FeO crystal, some of the Fe²⁺ ions are replaced by Fe³⁺ ions i.e., 3Fe²⁺ ions are replaced by 2Fe³⁺ ions to make up for the loss of positive charge. As a result of which it leads to lesser amount of metal as compared to the stoichiometric proportion.