Class 12 Chemistry

Solid State

NCERT InText Solution

Question: 1.1 - Why are solids rigid?

Answer: The particles of solids are close packed and can only oscillate about their fixed positions. These properties make solids rigid.

Question: 1.2 - Why do solids have a definite volume?

Answer: The intermolecular force of attraction make the particles of solid closely packed and force them to only oscillate at their fixed positions. These give solids a definite volume.

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

Answer: Polyurethane, Teflon, cellophane, polyvinyl chloride, fibre glass – Amorphous solids

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

Question: 1.4 - Why is glass considered a super cooled liquid?

Answer: - Glass is an amorphous solids, it has tendency to flow but very slowly. This is the cause that glass is considered as super cooled liquid.

Question: 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: Amorphous solids are isotropic in nature, i.e. they have short range order of arrangement of particles. Because of this amorphous solids have same value of refractive index along all directions.

Amorphous solids do not show cleavage property, i.e. when cut into two pieces with a sharp knife, they give pieces with irregular surface.

Question: 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: Potassium sulphate, Zinc sulphate – Ionic solid

Benzene, urea, water, argon, ammonia – Molecular solid

Tin, rubidium – Metallic solid

Graphite, silicon carbide – Covalent solids or network solids

Question: 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: Given solid ‘A’ is a covalent solids, such as diamond.

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

Answer: Ionic solids conduct electricity because of movement of their ions. In solid state ions present in ionic solids do not move hence do not conduct electricity while in molten state ions can move and thus conduct electricity.

Question: 1.9 - What type of solids are electrical conductors, malleable and ductile?

Answer: Metallic solids are conductor of electricity, malleable and ductile.

Question: 1.10 - Give the significance of a ‘lattice point’.

Answer: Lattice point denotea the position of constituent particles (molecule, atom or ion) in space. When lattice points are joined together by straight line they give the geometry of lattice.

Question: 1.11 - Name the parameters that characterise a unit cell.

Answer: Unit cells are characterize on six parameters – dimensions along three edges and three angles between their edges, i.e. a, b, c which are edges and α, β and γ which are angles between the edges.

Question: 1.12 - Distinguish between

(i) Hexagonal and monoclinic unit cells


Hexagonal and monoclinic unit cell
Difference Between Hexagonal and Monoclinic Unit Cell
ParametersHexagonal Unit CellMonoclinic Unit Cell
Possible variationOne: PrimitiveTwo: Primitive and End centred
Axial Distancea = b ≠ ca ≠ b ≠ c
Axial angleα = β = 90° γ = 120°α = γ = 90° β ≠ 90°
ExampleGraphite, ZnO, CdSMonoclinic sulphur, Na2SO4.10H2

(ii) Face-centred and end-centred unit cells.

Answer: There are four atoms present in face centered unit cell while there are only 2 atoms present in end centered unit cell.

In face centered unit cell one constituent particles are present at the center of each of the faces besides one at each corner.

In end centered unit cell two constituent particles are present at the center of any of the two faces besides one at each corner of the unit cell.