The Solid State

Question 105
CBSEENCH12005453

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 106
CBSEENCH12005454

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.



Property

Quartz

Glass

1. Structure

2. Melting point

Quartz is crystalline and it has long range order.

Quartz has a sharp melting point. It changes into viscous liquid at 1983 K.

Amorphous silica does not have a regular structure of long range order.

Glass does not have a sharp melting point. On heating it softens and melts over a wide range of temperature.

 

If SiO2 is melted and the melt is cooled very, rapidly it forms a glass.
Question 107
CBSEENCH12005455

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

 

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 108
CBSEENCH12005456

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
Co-ordination number : The total number of nearest neighbour atoms of a particular atom in a crystal lattice is known as its co-ordination number.


Example: In hexagonal close-packing (hcp) arrangement, each particle is surrounded by 12 equidistant spheres, i.e., 6 spheres in contact with in the same plane and 3 each in adjacent layers-one immediately above and the other immediately below. This, co-ordination number in hcp packing is 12.

(ii) The coordination number of atoms

(a) In a cubic close-packed structure is 12

(b) In a body-centered cubic structure is 8

 

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