Page No. 4

1.1. Why are solids rigid?

Answer

The solids are rigid because of the strong intermolecular forces of attraction present in solids which makes the constituent particles of solids in fixed positions. However, they can oscillate about their mean positions.

1.2. Why do solids have a definite volume?

Answer

The solids have a definite volume because of the strong intermolecular forces of attraction that are present in solids which makes the constituent particles of solids in fixed positions. Hence, solids have a definite volume.

1.3. Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, teflon, potassium nitrate, cellophane, polyvinyl chloride, fibre glass, copper.

Answer

Amorphous solids: Polyurethane, Teflon, cellophane, polyvinyl chloride, fibre glass.

Crystalline solids: Naphthalene, benzoic acid, potassium nitrate, copper.

1.4. Why is glass considered a super cooled liquid?

Answer

Like liquids, amorphous solids have a tendency to flow, though very slowly. Therefore, sometimes these are called pseudo solids or super cooled liquids.

1.5. Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this solid. Would it show cleavage property?

Answer

Nature of the solid: When measured along different directions, an isotropic solid has the same value of physical properties. Since the give solid is observed to have the same value along all directions, it is said to be isotropic in nature. Hence, the solid is an amorphous solid. Cleavage Property: When an amorphous solid is cut with a sharp edged tool, it cuts into two pieces with irregular surfaces.

Page No. 6

1.6. Classify the following solids in different categories based on the nature of intermolecular forces operating in them: Potassium sulphate, tin, benzene, urea, ammonia, water, zinc sulphide, graphite, rubidium, argon, silicon carbide.

Answer

Ionic solids: Potassium sulphate, Zinc sulphide

Metallic solids: Tin, Rubidium

Molecular (non-polar) solid: Benzene, Argon

Polar molecular solids: Urea, Ammonia

Hydrogen bonded molecular solid: Water

Covalent or network solid: Graphite, Silicon carbide

1.7. Solid A is a very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. What type of solid is it?

Answer

The covalent solid or network solids are very hard electrical insulator in solid as well as in molten state and melts at extremely high temperature. Hence, the solid A must be covalent solid or network solid.

1.8. Ionic solids conduct electricity in molten state but not in solid state. Explain.

Answer

Ions are the constituent particles of ionic solids. In solid state, ions are held together by strong electrostatic forces and are not free to move about within the solid. Hence, ionic solids do not conduct electricity in solid state. However, in molten state or in solution form, the ions are free to move and can conduct electricity.

1.9. What type of solids are electrical conductors, malleable and ductile?

Answer

Metallic solids are electrical conductors, malleable and ductile.

Page No. 10

1.10. Give the significance of a 'lattice point'.

Answer

The significance of a lattice point is that they are joined by straight lines to bring out the geometry of the lattice.

1.11. Name the parameters that characterize a unit cell.

Answer

A unit cell is characterised by:

(i) Its dimensions along the three edges, a, b and c. These edges may or may not be mutually perpendicular.

(ii) Angles between the edges, α(between b and c), β(between a and c) and γ(between a and b). Thus, a unit cell is characterised by six parameters, a, b, c, α, β and γ.