Chemistry NCERT Exemplar Solutions Class 12th Chapter One

28. Option (iv) CrO₂ is correct since it shows electrical conductivity similar to the metals. 

27. Option (iii) TiO₃  is correct since certain metal oxides such as VO, VO₂, VO₃and TiO₃  show either metallic or insulating properties depending upon the temperature. 

26.  (ii) Free valence electrons 

25. Option (i) Very low value of electrical conductivity in the molten state is correct since in molten state, the ionic solids possess free ions which help in the conduction of electricity. Due to the availability of these free ions the electrical conductivity of ionic compounds in molten state is found to be very high. 

24. Option (iii) (C) and (D) is correct since Iodine is a non-polar molecular solid which acts as an insulator while H₂O is a hydrogen bonded molecular solid which also acts as an insulator. Whereas Mg is a metal and acts as a conductor in solid and in its molten state and TiO is also an ionic solid which acts as an insulator in solid state but acts as a conductor in molten and in aqueous solutions. 

23. Option (iii) Diamond  is correct since in diamond, the carbon atoms are held together by strong covalent bonds. It is a giant molecule. Thus, it is a solid network.

22. Option (i) London forces is correct since iodine molecules are nonpolar  and covalent in nature. These molecules are found to be electrically symmetrical and have no dipole moment. The molecules in a crystal lattice of iodine are thus attracted together by weak London forces.

21. Option (ii) a regular arrangement of constituent particles observed over a long distance in the crystal lattice is correct since the regularity of the crystalline lattice creates local environments that are the same and hence crystals exhibit sharp melting point.

20.  (iv) They are anisotropic in nature since amorphous solid shows isotropic properties as they exhibit same values of the properties like refractive index, electrical resistance when measured along different directions.

19. Option (i) Same in all directions is correct since quartz glass is an amorphous solid showing isotropic properties and hence exhibits the same values of refractive index when measured along different directions.