TOPIC :- SOLID STATE CLASS : - 12
Q – 1.1 Why are solids rigid?
Ans. In a solid, the constituent particles are very closely packed. Hence, the forces of attraction among these particles are very strong. Moreover, the constituent particles in solid have fixed positions and can only oscillate about their mean positions. Hence, they are rigid.
Q – 1.2 Why do solids have a definite volume ?
Ans. The constituent particles of a solid have fixed position and are not free to move about, i.e they possess rigidity. That is why they have a definite volume.
Q – 1.3 Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, Teflon, potassium nitrate, cellophane, polyvinyl chloride, fiber glass, copper.
Ans. Amorphous solids. Polyurethane, Teflon, cellophone, polyvinyal chloride, fibre glass.
Crystalline solids. Benzoic acid, potassium nitrate, copper.
Q – 1.4 Why is glass considered a super – cooled liquid?
Ans. Glass is an amorphous solid. Like liquids, it has a tendency to flow, though very slowly. The proof of this fact is that the glass panes in the windows or door of old buildings are invariably found to be slightly thicker at the bottom that at the top. This is evidently due to the reason that glass flows down very slowly and makes the bottom portion slightly thicker.
Q – 1.5 Refractive index of a solid is observed to have the some value along all direction. Comment on the nature of the solid. Would it show cleavage property?
Ans. As the solid has same value of refractive index along all direction, this means that it is isotropic and hence amorphous. Being in amorphous solid, it would not show a clean cleavage when cut with a knife. Instead, it would break into pieces with irregular surfaces.
Q – 1.6 Classify the following solid 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.
Ans. Potassium sulphate = Ionic, Tin = Metallic, Benzene = Molecular (non-polar), Urea = Molecular (Polar), Ammonia = Molecular (Hydrogen bonded), Water = Molecular (Hydrogen bonded), Zinc sulphide = Ionic, Graphite = Covalent or Network, Rubidium = Metallic, Argon = Molecular (Non – polar), Silicon carbide = Covalent or Network.
Q – 1.7 Solid A is a very hard, electrical insulator in solid as well as molten state and melts at extremely high temperature. What type of solid is it?
Ans. Covalent Network solid like Sio2 (quartz) or Sic or C (diamond).
Q – 1.8 Ionic solid conducts electricity in the molten state but not in the solid state. Explain.
Ans. In the molten state, ionic solids dissociate to give free ions and hence can conduct electricity. However, in the solid state, as the ions are not free but remain held together by strong electrostatic forces of attraction, they cannot conduct electricity in the solid state.
Q – 1.9 What type of solid are electrical conductors, malleable and ductile?
Ans. Metallic solid.
Q – 1.10 Give the significance of a ‘lattice point’.
Ans. Each lattice point represents one constituent particle of the solid. This constituent particle may be an atom, a molecule (group of atoms) or an ion.
Q – 1.11 Name the parameters that characterize a unit cell.
Ans. A unit cell is characterized by the following six parameters:
· The dimension of the unit cell along three edges. These are represent by a, b and c. The edges may or may not be mutually perpendicular.
· The angles between the edges. These are represented by α, β and γ. The angle α is between b and c, β is between a and c and γ is between a and b.
Q – 1.12 Distinguish between:
(i). Hexagonal and monoclinic unit cells. (ii) Face- centered and-centered unit cells.
Ans. (i) For hexagonal unit cell, a = b ≠ c, α = β = 900 , γ = 1200
For monoclinic unit cell, a ≠ b ≠ c, α = γ = 900 , β ≠ 900
(ii) A face-centred unit sell has one constituent particle present at the center of
each face in addition to the particle present at the corners.An end – face centred has one constituent particle each at the center of any two
opposite faces in addition to the particle present at the corners.
Q – 1.13 Explain how much portion of an atom at (i) corner and (ii) body-center of a cubic unit cell is part of its neighboring unit cell.
Ans. An atom at the corner is shared by eight adjacent unit cells. Hence, portion of the atom at the corner that belongs to one unit cell = . (ii) The atom at the body center of a cubic unit cell is not shared by any other unit cell. hence, it belongs fully to the unit cell.
Q – 1.14 What is the two-dimensional coordination number of a molecule in square close packed layer?
Ans. In the two- dimensional square close packed layer, the atom touches 4 nearest neighboring atoms. Hence its coordination number = 4.
Q – 1.15 A compound is 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?
Ans. No. of atoms in the close packing = 0.5 mol = 0.5 × 6.022 × 1023 =3.011 × 1023
No. of octahedral voids = No. of atoms in the packing = 3.011 × 1023
No. of tetrahedral voids = 2 × No. of atoms in the packing = 2 × 3.011 × 1023
=6.022 × 1023
Total No. of voids = 3.011 × 10 + 6.022 × 1023 = 9.033 × 1023
Q – 1.16 A compound is formed by two elements M and N. The elements N forms ccp and atoms of M occupy 1/3rd of tetrahedral voids. What is the formula of the compound?
Q – 1.17 Which of the following lattices has the highest packing efficiency (i) simple cubic (ii) body-centred cubic (iii) hexagonal close-packed lattice.
Ans. Packing efficiencies are: Simple cubic = 52.4% Body-centred cubic = 68%, Hexagonal close packed = 74% Thus, hexagonal close packed lattice has the highest packing efficiency.
Q – 1.18 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 × 102 kg m-3, what is the nature of the cubic unit sell?
Q – 1.19 What type of defect can arise when a solid is heated ? which physical property is affected by it and in what way ?
Ans. When a solid is heated, vacancy defect is product in the crystal. This is because on hating, some atoms or ions leave the lattice site completely, i.e., some lattice sites become vacant. As a result of this defect, the density of the substance decreases because some atoms/ions leave the crystal completely.
Q – 1.20 What type of stoichiometric defect is shown by: (i) ZnS (ii) AgBr ?
Ans. (i) ZnS shows Frenkle defect because its ions have a large difference in size.
(ii) AgBr shows both Frenkle and schottky defects.
Q – 1.21 Explain how vacancies are introduced in an ionic solid when a caption of higher valence is added as an impurity in it.
Ans. Two or more cations of lower valency are replaced by a cation of higher valency to maintain electrical neutrality. Hence, come cation vacancies are created. For example, if is the ionic solid, Na+Cl– , impurity of Sr2+ is added (as SrCl2), two Na+ lattice sites will become vacant and one of these well be occupied by Sr2+ ion and the other will remain vacant.
Q – 1.22 Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.
Ans. Talking the example of NaCl, when its crystals are heated in presence of sodium vapor, some chloride ions leave their lattice sites to combine with sodium to from NaCl. For this reaction to accur, Na atoms lose electrons to from Na+ ions. The electrons thus released diffuse into the crystal to occupy the anion vacancies created by Cl– ions. The crystal now has excess of sodium. The sites occupied by unpaired electrons are called F-centres. They impart yellow colour to the crystal because they absorb energy from the visible light and get excited.
Q – 1.23 A group 14 element is to be converted into n- type semiconductor by doping it whit a suitable impurity to which group should this impurity belong?
Ans. n – type semiconductor means conduction due to presence of excess of negatively charged electrons. Hence, to convert group 14 element into n – type semiconductor, it should be doped with group 15 element.
Q – 1.24 What type of substances would make better permanent magnet, ferromagnetic or ferromagnetic. Justify your answer.
Ans. Ferromagnetic substances make better permanent magnets. This is because the metal ions of a ferromagnetic substance are grouped into small regions called ‘domains’ Each domain acts as a tiny magnet. These domains are randomly oriented. When the substance is placed on a magnetic field, all the domains get oriented in the direction of magnetic field and a strong magnetic field is produced. This ordering of domains persists even when the external magnetic field is produced. This ordering of domains persists even when the external magnetic field is removed. Hence, the ferromagnetic substance becomes a permanent magnet