1. Why are soild rigid ?
Solids are rigid because the intermolecular force of attraction in them are very strong.The constituent particles in soild have their fixed position. They can oscillate about their mean position. So, this is the reason soild are rigid.
2. Why do soilds have a definite volume?
Soilds have definite volume because of –
Strong intermolecular force of attraction
3. Classify the following as amorphous or crystalline soild – Polyurethane, naphthalene, Benzoic acid, Teflon, potassium nitrate, celloplane, polyvinyl chloride, fiber glass, copper.
Amorphous soild – Polyurethane, Teflon, cello plane, polyvinyl chloride, fibre glass
Crystalline solid – Naphthalene, benzoic acid, potassium nitrate, copper
4. Why glass is considered as super-cooled liquid?
Glass is considered as super-cooled liquid because it has tendency to flow similar to liquid, but slowly.
5. Refractive index of a solid is observed to have the same value along all directions. Comment on the nature of this soild. Would it show cleavage property.
An isotropic solid which has same physical properties along different directions. Therefore, they have same value of refractive index along all directions. That type of the solid is known as an amorphous solid.
When an amorphous solid is cut with a knife, it cuts into two pieces with irregular surfaces.
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.
Potassium sulphate = Ionic solid
Tin = Metallic solid
Benzene = Molecular (non-polar) solid
Urea = Polar molecular solid
Ammonia = Polar molecular solid
Water = Hydrogen bonded molecular solid
Zinc sulphide = Ionic solid
Graphite = Covalent or network solid
Rubidium = Metallic solid
Argon = Non-polar molecular solid
Silicon carbide = Covalent or network solid
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?
These properties are of a covalent or network solid. Hence, the given solid is a covalent or network solid.
8. Ionic solids conduct electricity in molten state but not in solid state. Explain.
In solid state, ions are held together by strong electrostatic forces of attraction. So, they are not free to move. Therefore, ionic solids do not conduct electricity in solid state. However, in molten state or in solution form, the ions are free to move and so, can conduct electricity.
9. What type of solids are electrical conductors, malleable and ductile? Answer –
10. Give the significance of a ‘lattice point’.
The significance of a lattice point is that each lattice point represents one constituent particle of a solid which may be an atom, a molecule (group of atom), or an ion.
11. Name the parameters that characterize a unit cell.
There are six parameters that characterise a unit cell –
(i) Dimensions along the three edges, a, b, and c.
(ii) Angles α, β, γ between the edges
α (between edges b and c), β (between edges a and c), and γ (between edges a and b).
12. Distinguish between
(i) Hexagonal and monoclinic unit cells
(ii) Face-centred and end-centred unit cells.
(i) Hexagonal unit cell
For hexagonal unit cell –
a = b != c and α = β = 90° , γ = 120°
Monoclinic unit cell
For monoclinic cell,
a != b != c α = γ = 90°, β! = 90°
(ii) Face-centered unit cell –
In a face-centered unit cell, the constituent particles are present at the corners and one at the centre of each face.
End-centered unit cell –
In end-centered unit cell contains particles at the corners and one at the centre of any two opposite faces.
13. Explain how much portion of an atom located at (i) corner and (ii) body-centre of a cubic unit cell is part of its neighbouring unit cell.
i. An atom located at the corner of a cubic unit cell is shared by eight adjacent unit cells. So, the atom is shared by one unit cell.
ii. An atom located at the body centre of a cubic unit cell is not shared by any neighbouring unit cell. So, its contribution to the unit cell is 1.
14. What is the two dimensional coordination number of a molecule in square close packed layer?
In square close-packed layer, four neighbours are in contact with a molecule. Hence, the twodimensional coordination number of a molecule in square close packed layer is 4.
15. A compound forms hexagonal close-packed structure. What is the total number of voids in 0.5 mol of it? How many of these are tetrahedral voids? Answer –
16. A compound is formed by two elements M and N. The element N forms CCP and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the compound?
The CCP lattice is formed by the atoms of the element N. The number of tetrahedral voids = 2 x twice the number of atoms of the element N
The atoms of element M occupy the tetrahedral voids.
The number of atoms M = number of atoms N
Ratio – M: N Hence, the formula of the compound is M2N3.
17. Which of the following lattices has the highest packing efficiency (i) simple cubic (ii) body-centred cubic and (iii) hexagonal close-packed lattice? Answer –
i. The packing efficiency of simple – cubic lattice is 52.4%.
ii. The packing efficiency of body – centred cubic lattice is 68%.
iii. The packing efficiency of hexagonal close-packed lattice is 74%.
18. An element with molar mass 2.7 × 10-2 kg mol-1 forms a cubic unit cell with edge length 405 pm. If its density is 2.7 × 103 kg m−3 , what is the nature of the cubic unit cell?
19. What type of defect can arise when a solid is heated? Which physical property is affected by it and in what way?
When a solid is heated, vacancy defect can arise. It occurs when some of the lattice sites are vacant. It decreases the density of the solid.
20. What type of stoichiometric defect is shown by: (i) ZnS (ii) AgBr
(i) ZnS shows Frenkel defect.
(ii) AgBr shows Frenkel defect as well as Schottky defect.
21. Explain how vacancies are introduced in an ionic solid when a cation of higher valence is added as an impurity in it.
The cation of higher valency is added to an ionic solid as an impurity, then the cation of higher valency replaces more than one cation of lower valency to keep the crystal electrically neutral. In that case, some sites remains vacant.
22. Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.
The colour is due to the presence of electrons in the anionic sites. These electrons absorb energy from the visible radiation and get excited. For example, when crystals of NaCl are heated in an atmosphere of sodium vapors, the sodium atoms get deposited on the surface of the crystal and the chloride ions from the crystal diffuse to the surface to form NaCl with the deposited Na atoms. During this process, the Na atoms on the surface lose electrons to form Na+ ions and the released electrons diffuse into the crystal to occupy the vacant anionic sites. These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.
23. A group 14 element is to be converted into n-type semiconductor by doping it with a suitable impurity. To which group should this impurity belong?
The n-type semiconductor conducts electricity due to the presence of extra electrons. So, the group 14 element can be converted to n-type semiconductor by doping it with a group 15 element.
24. What type of substances would make better permanent magnets, ferromagnetic or ferrimagnetic. Justify your answer.
Ferromagnetic substances would make better permanent magnets. The substances which are strongly attracted by magnetic field are called ferromagnetic substance for example – Fe, Co, CrO2, gadolinium etc. If once these substances have been magnetized, then they remain permanently magnetized even in the absence of magnetic field. In solid state, the metal ions grouped together into small regions, called domain. Each domain act as tiny magnet. In an un-magnetized ferromagnetic substance, domains are randomly oriented. So, their magnetic moments get cancelled. But when this substance is placed in magnetic field, then all domains get oriented themselves in the direction of magnetic field. This results a strong magnetic effect. This order of domain persists even in the absence of magnetic field. Hence, the ferromagnetic substance becomes a permanent magnet.
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