# NCERT Solid State Questions and Answers

0

NCERT Solid State Questions and Answers

#### Q 1. Define the term ‘amorphous’. Give a few examples of amorphous solids.

Ans. Those solids is which there is no regular arrangement of atoms, ions or molecules are called amorphous solids, e.g. plastics, amorphous SiO2, amorphous Si, glass, etc.

### Q 2. What is meant by the term ‘coordination number’?

Ans. It is the number of atoms surrounding a given atom in a crystal lattice.

#### Q 3. What is the coordination number of atoms (a) in a cubic close-packed structure? (b) in a body-centred cubic structure?

Ans.  (a) 12 (b) 8

## Solid state questions and answers

#### Q 4. What makes a glass different from a solids such as quartz? Under what conditions could quartz be converted into glass?

Ans. Quartz is a three dimensional crystalline solid having regular arrangement of Si or O­2. The ordinary glass which is metal silicate is amorphous solids. When quartz is melted and the melt is cooled rapidly, it forms a glass or amorphous solids.

#### Q 5. How will you distinguish between the following pairs of terms:

(i) Hexagonal close-packing and cubic close-packing?

(ii) Crystal lattice and unit cell?

(iii) Tetrahedral void and octahedral voids?

(i) Hexagonal close-packing– In this type of packing, second row of spheres are               placed in depressions of first row and third row is vertically aligned with first,                     fourth  with second forming a pattern AB, AB … …. …..

Cubic close-packing– In this type of packing second row of spheres is placed in depressions of first row, third row of spheres is placed in depressions of second row, fourth row is vertically aligned with first, fifth with second, sixth with third forming a pattern ABC, ABC … ……

(ii) Crystal lattice– It is regular three dimensional arrangement of atoms, ions or molecules.

Unit cell-It is a portion of crystal lattice which on repetition leads to the formation of crystal structure.

(iii) Tetrahedral void– The void between three spheres in direct contact capped by fourth sphere is called tetrahedral void. The coordination number of tetrahedral void is 4.

Octahedral void– The void between three spheres in direct contact capped by three more spheres in direct contact is called octahedral void. The coordination number of octahedral void is 6.

#### Q 6. How many lattice points are there in one unit cell of each of the following lattice?

(i) face-centred cubic (ii) face-centred tetragonal (iii) body-centred cubic

Face-centred cubic- $8*\frac { 1 }{ 8 } +6*\frac { 1 }{ 2 } =1+3=4$

Face-centred tetragonal- $8*\frac { 1 }{ 8 } +6*\frac { 1 }{ 2 } =1+3=5$

Body-centred cubic-$8*\frac { 1 }{ 8 } +1=2$

Explain:

(i) The basis of similarities and difference between metallic and ionic crystals.

(ii) Ionic solids are hard and brittle.

(i) Similarities– Metallic solids consist of positively charged kernels and valence electrons could like ionic solids which have positively and negatively charged ions. Both have electrostatic forces of attraction.

Difference(a) Metallic solids conduct electricity in solid state because  electrons are free to move, whereas ionic solids conduct electricity in molten state or in aqueous solution because ions become free to move.

(b) The conductance of metal decreases with increase in temperature because     moment of kernels increases which obstruct the flow of electrons, whereas, the   conductance of ionic solids increases with increase in temperature because    movement of ions increases.

(ii) Ionic solids are hard due to strong forces of attraction between oppositely

charged ions. They are brittle because bonds break on applying forces.