Chemistry Class Twelve

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Solid State

NCERT Exercise Solution (part 1)

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 cellclass 12 solid state ncert exercise solution1

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

class 12 solid state ncert exercise solution2

Now, since mass of the metal (m)

class 12 solid state ncert exercise solution3

If the edge lengths are different (say a, b and c), therefore, equation (iii) can be written as

class 12 solid state ncert exercise solution4

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.


Previous            Next
Solid State - Introduction
Solid State - Crystal Lattice
Solid State - Close Packing Structure
Solid State - Efficiency of Close Packing Structure - 1
Solid State - Efficiency of Close Packing Structure - 2
Solid State - Imperfections in Solids - Crystal Defects
Solid State - Electrical and Magnetic Properties
Solid State - NCERT In Text Solution 1
Solid State - NCERT In Text Solution 2
Solid State - NCERT In Text Solution 3
Solid State - NCERT Exercise Solution (Part 2)
Solid State - NCERT Exercise Solution (Part 3)
Solid State - NCERT Exercise Solution (Part 4)
Solid State - NCERT Exercise Solution (Part 5)

 
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