SOLID STATE
In Text Questions and Answer - 1
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
Answer:
(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.
NCERT Exercise SolutionS
Question – 1.1 - Define the term 'amorphous'. Give a few examples of amorphous
solids.
Solution:
Solids having constituent particles with irregular shapes and short range order are
called amorphous solids. Amorphous solids are isotropic in nature and melt over a
range of temperature. Thus, amorphous solids are also referred as pseudo solids or
super cooled liquids.
Amorphous solids do not have definite heat of fusion.
Amorphous solids give irregular surfaces, when cut with sharp tool.
Glass, rubber, plastic, etc. are some examples of amorphous solid.
Question – 1.2 - What makes a glass different from a solid such as quartz? Under what
conditions could quartz be converted into glass?
Solution:
It is the arrangement of constituent particles of glass which makes it different from
quartz. The constituent particles of glass have short range order while quartz has
constituent particles in long range order and short range order both.
By heating and cooling rapidly quartz can be converted into glass.
Question – 1.3 - Classify each of the following solids as ionic, metallic, molecular,
network (covalent) or amorphous.
(i) Tetra phosphorous decoxide (P4O10)
(ii) Ammonium phosphate (NH4)3PO4
(iii) SiC
(iv) I2
(v) P4
(vi) Plastic
(vii) Graphite
(viii) Brass
(ix) Rb
(x) LiBr
(xi) Si
Solution:
(i) Tetra phosphorous decoxide (P4O10) - Molecular
(ii) Ammonium phosphate (NH4)3PO4 – Ionic
(iii) SiC - Covalent (network)
(iv) I2 - Molecular
(v) P4 - Molecular
(vi) Plastic - Amorphous
(vii) Graphite – Covalent (network)
(viii) Brass - Metallic
(ix) Rb - Metallic
(x) LiBr - Ionic
(xi) Si – Covalent (network)
Question – 1.4 - (i) What is meant by the term 'coordination number'?
(ii) What is the coordination number of atoms:
(a) in a cubic close-packed structure?
(b) in a body-centred cubic structure?
Solution:
(i) Coordination number is the number of nearest neighbours of any constituent particle
present in the crystal lattice.
(ii) The coordination number of atoms
(a) In a cubic close-packed structure is 12
(b) In a body-centered cubic structure is 8
Question – 1.5 - How can you determine the atomic mass of an unknown metal if you
know its density and the dimension of its unit cell? Explain.
Solution:
The atomic mass of an unknown metal can be determined by knowing its density and
the dimension of unit cell.
Let ‘a’ be the edge length of a unit cell of a crystal.
‘d’ is the density of the metal
‘m’ is the atomic mass of the metal
‘z’ is the number of atoms in the unit cell
Now, the density of the unit cell
As we know that, mass of the unit cell = Number of atoms in the unit cell X Atomic mass
And Volume of the unit cell = (Edge length of the cubic unit cell) 3
Now, since mass of the metal (m)
If the edge lengths are different (say a, b and c), therefore, equation (iii) can be written
as
Thus, using equation (iii) and the atomic mass of an unknown metal can be determined.
Question – 1.6 - 'Stability of a crystal is reflected in the magnitude of its melting points'.
Comment. Collect melting points of solid water, ethyl alcohol, diethyl ether and methane
from a data book. What can you say about the intermolecular forces between these
molecules?
Solution:
Stability of a crystal is reflected in the magnitude of its melting points because higher
the melting point, greater is the intermolecular force and greater the intermolecular force
greater is the stability. And hence, a substance with higher melting point would be more
stable.
The melting points of the given substances are as follows:
Solid water - 273 K
Ethyl alcohol – 158.8 K
Diethyl ether – 156.85 K
Methane – 89.34 K
As we can see the melting point of solid water is highest and melting point of methane is
lowest among the given substance. This says that intermolecular force in solid water is
strongest and the intermolecular force in methane is weakest.