Short & Long Answer Question: The Solid State - 2 | Chemistry Class 12 - NEET PDF Download

Q.51. Although pure silicon is an insulator, how does it behave as a semiconductor on heating?
Ans. Small gap between the valence band and conduction band allows electrons to jump from valence band to conduction band. This causes the conduction of electricity by semiconductors.

Q.52. What causes the conduction of electricity by semiconductors?Ans. Any element of group-15 a periodic table.
Example: P, As, Sb, etc. may be added to silicon to make electrons available for conduction of electric current.
Q.53. What other element may be added to silicon to make electrons available for conduction of an electric current?
Ans. Conductance of a semiconductor increases with rise in temperature.

Q.54. What is the effect of temperature on conduction of semi conductors?
Ans. 
(a) Ferromagnetism persists even in the absence of magnetic field whereas paramagnetism is lost in the absence of magnetic field.
(b) Paramagnetism arises due to random arrangement of magnetic domains whereas ferromagnetism arises due to alignment of magnetic domains in same direction.

Q.55. How is ferromagnetism different from paramagnetism?Ans. Ferrimagnetism is a special case of antiferromagnetism. Ferrimagnetism arises due to parallel and antiparallel alignment of magnetic domains in unequal numbers.

Q.56. Solid solution of group-13 or group-15 impurities with group-14 elements are found to exhibit unusual electric properties. Why?
Ans. 
(i) Compounds (semiconductors) formed by elements of group-12 with elements of group-16 are called 12-16 compounds
Example: ZnS, CdS etc.
(ii) Compounds (semiconductors) formed by elements of group-13 with elements of group-- 15 are called 13-15 compounds. 
Example: A1P, GaS, InSb etc.

Q.57. Why do solids have definite volume as well as shape?
Ans. In solids constituent particles have fixed positions and strong inter-particle force. Hence they have definite volume as well as shape.

Q.56. What are crystalline solids?
Ans. Solids having ordered arrangement of constituent particles are called crystalline solids.

Q.57. Define the term amorphous.
Ans. Solids having irregular arrangement of constituent particles are called amorphous solids.

Q.58. Why are amorphous solids called pseudo solids or super cooled liquids?
Ans. Amorphous solids have tendency to flow like liquids. Hence these are called pseudo solids or super cooled liquids.

Q.59. Crystalline solids are anisotropic. Why?
Ans. Crystalline solids have different arrangement of particles in different directions. Hence they show anisotropy.

Q.60. Amorphous solids are isotropic. Why?
Ans. Arrangement of constituent particles is irregular in all directions. Thus amorphous solids are isotropic.

Q.61. Metallic solids are conductors of heat and electricity. Why?Ans. Metallic solids are collections of positive ions embedded in a sea of electrons. These free and mobile electrons are responsible for electrical conductivity of metallic solids.
Q.62. How a given material can be made amorphous?
Ans. Amorphous solid is made by melting a given material and then cooling it slowly.
Q.63. Why is graphite used as lubricant?
Ans. In graphite carbon atoms are arranged in layers. These layers are held together by weak van der Waals force. Different layers can slide one over the other. This is why graphite is used as solid lubricant.
Q.64. Graphite is conductor of electricity. Given reasons.
Ans. In graphite 3 electrons of each carbon atom are covalently bonded to three other C-atoms of its own layer. The fourth valence electron of each atom forms a delocalised layer of mobile electrons. This is why graphite is a conductor of electricity.
Q.65. Why is diamond extremely hard and insulator?
Ans. In diamond each carbon atom is bonded to four carbon L atoms by strong directional covalent bonds. Atoms are  very strongly held at their positions. Therefore diamond  is hard. Carbon atom has no free electrons. Hence it is insulator.
Q.66. Ionic solids are insulator of heat and electricity. Why?
Ans. In ionic solids ions are held by strong electrostatic force of attractions. Ions are not free to move. They do not have free electrons. Thus ionic solids are insulator of heat and electricity.
Q.67. Molecular solids are soft and brittle. Explain.
Ans. In molecular solids molecules are held by weak dipole-dipole interaction, London dispersion force or H-bonding. Thus these are soft and brittle.
Q.68 Why glass panes fixed to windows or doors of old buildings are found to be thicker at the bottom?
Ans. Glass panes fixed to windows or doors of old buildings are found to become thicker at the bottom because the glass flows down very slowly and makes the bottom portion thicker.
Q.69. Some glass objects from ancient civilizations are found to become milky in appearance. Explain.
Ans. Some glass objects from ancient civilizations are found to become milky in appearance because of crystallization of glass.
Q.70. What is a void? Name any two voids.
Ans. Empty spaces present in between the lattice points of a solid are called voids.
(i) Tetrahedral
(ii) Octahedral voids.

Q.71. What is meant by co-ordination number in a solid?
Ans. Number of nearest neighbours of a lattice point in a crystalline solid is called coordination number.
Q.72. How will you differentiate between bcc and fee lattice?
Ans. In bcc lattice, lattice points are at the corners as well as at the body centre. Z =2.
Q.73. How will you differentiate between hcp and ccp lattice?
Ans. If fee lattice, lattice points are at the corners as well as centre of all the six face centres Z = 4.
Q.74. If the radius of anion in an ionic solids is 100 pm, what should be the radius of cation that fits in the (a) cubic hole, (b) octahedral hole, (c) tetrahedral hole?
Ans. In hcp lattice each third layer is parallel to first it is ABAB ...........type. In ccp lattice each fourth layer is parallel to the first layer. It is ABCABC......... type.
(a) For cubic hole r = 0.732 R
(b) For octahedral hole r = 0.414 R
(c) For tetrahedral hole r = 0.225 R.
Q.75. What is the effect of Frenkel defect on electrical conductivity of solid?
Ans. Frenkel defect increases electrical conductivity.
Q.76. Name the non-stoichiometric point defects responsible for colour in alkali halides.
Ans. Non-stoichiometric defect responsible for colour in alkali metal halide is metal excess (anion -vacancy) defect.
Q.77. Schottky defects generate an equal number of cation and anion vacancies while doping produces only cation vacancies and not anion vacancies. Why?
Ans. Schottky defects exist in pairs to maintain electrical neutrality. So it generates equal number of cation and anion vacancies. Ionic solids are doped with metal ions of higher valency. Therefore some cations of lower valency are displaced to maintain electrical neutrality. Hence only cation vacancies are produced not anion vacancies.
Q.78. Give two reasons of crystal imperfections.
Ans. Two reasons of crystal defects are :-
(i) Improper growth of crystals at fast or moderate rate. 
(ii) Presence of impurities.
Q.79. State the difference between Schottky and Frenkel defects.
Ans. In Schottky defects cations and anions are missing from their normal sites in the ratio of formula of compounds.
Q.80. Which of the two changes the density of solid?
Ans. In Frenkel defects some ions are dislocated from normal site to some interstitial sites. Schottky defect decreases density of solid.
Q.81. What makes alkali metal halides sometimes coloured which are otherwise colourless?
Ans. Metal excess defect (anion vacancy) sometimes makes alkali metal halides coloured which are otherwise colourless.
Q.82. What are the solids? What are their characteristics?
Ans. General characteristics of solid state Matter exists in three different states:(i) Solid
(ii) Liquid and
(iii) Gas.
In solids, constituent particles have fixed positions and they can only oscillate about their mean positions. This is why solids are rigid. The solids possess following characteristic properties :
(i) Solids have definite mass, volume and shape. A solid is compact.
(ii) They have very strong interparticle force.
(iii) The constituent particles of a solid have fixed positions in space and they can only oscillate about their mean positions. They have no translational or rotational motion.
(iv) Solids are rigid and incompressible. They can not flow.
(v) Solids have many free surfaces.
Q.83. What are amorphous and crystalline solids? Give example of each type.
Ans. Classification of solids On the basis of arrangement of constituent particles solids are classified in two classes :
Crystalline solids and Amorphous solids
1. Crystalline solids -A solid which has definite ordered arrangement of constituent particles (atoms, molecules and ions) is called crystalline solid. 
(a) A crystalline solid consists of a large number of small crystals having a definite geometrical shape. These are known as unit cells.
(b) It has long range ordered arrangement of constituent particles i.e. it has same pattern of arrangement throughout the crystal.
(c) Arrangement of particles repeats itself periodically over the entire crystal.
(d) Crystalline solids have sharp melting points.
(e) Crystalline solids are anisotropic in nature. Examples of crystalline solids are sodium chloride, quartz, etc.

2. Amorphous solids- Amorphous is a Greek word meaning no form. A solid which has irregular arrangement of constituent particles is called amorphous solid.
(a)Amorphous solid has no ordered arrangement or only very short range order, i.e. there is no periodically repeating pattern.
(b) Constituent particles (atoms, molecules or ions) are scattered and disordered.
(c) The structure of amorphous solids is similar to that of liquids. Hence these are also called pseudo solids or super cooled liquids. Examples of amorphous solids are glass, rubber and plastics. 
(d) Amorphous solid melts over a range of temperature.
(e) Amorphous solid can be moulded and blown into various shapes.
(f) Amorphous solids are isotropic in nature. The value of same physical property is same along all the directions. Isotropy of amorphous solids is due to same irregular arrangement of constituent particles along all the directions.
(g) Amorphous solids on heating and then cooling become crystalline. Example: Some glass objects from ancient civilization are found to become milky in appearance because of crystallization.
Q.84. How are crystalline solids classified? Give example of each type.
Ans. On the basis of nature of intermolecular forces crystalline solids have been classified into four categories.
(i) Ionic  (ii) Metallic (iii) Covalent   (iv) Molecular.
(i)Ionic solids : Solids in which constituent particles are held by ionic bonds are .called ionic solids. Ionic solids are made of ions (cations and anions). In ionic solids cations and anions are bound by strong electrostatic forces of attraction (coulombic force). Ionic solids are formed by three dimensional arrangement of cations and anions. 
Example: NaCI, ZnS, etc. 

(ii) Metallic solids: Solids in which constituent particles are held by metallic bond are called metallic solids. 
Examples: Iron, copper, silver, sodium, magnesium, aluminium, etc. 
Metallic solids are made of metal ions and electrons. These are formed by three dimensional arrangement of metal ions surrounded by electrons. 

(iii) Covalent solids: Solids in which constituent particles are held together by a network of covalent bonds throughout the crystal are called covalent solids.
Examples: Diamond, graphite, silica (quartz), carborundum (silicon carbide). These are also called network solids. Covalent solids are made of non-metallic atoms. These atoms are bound by forming covalent bonds between adjacent atoms throughout the crystals. These solids have giant structures. 

(iv) Molecular solids: Solids in which constituent particles are held together by weak van der Waals forces are called molecular solids. Molecular solids are made of discrete covalent molecules. 
Examples: Iodine, naphthalene, phosphorus, sulphur, camphor, etc.

Q.85. What are metallic solids? What are their characteristics?
Ans. Metallic solids: Solids in which constituent particles are held by metallic bond are called metallic solids.
Characteristics of metallic solids: Metallic solids have high electrical and thermal conductivity due to the presence of free mobile electrons. When an electric field is applied, these mobile electrons flow through the network of positive ions. This is why metallic solids are good conductors of electricity.

Q.86 What are covalent solids and what are their characteristics
Answer: Covalent solids: Solids in which constituent particles are held together by a network of covalent bonds throughout the crystal are called covalent solids. 
Properties of covalent solids: 
(a) In covalent solids atoms are held very strongly by strong and directional covalent bonds therefore these solids are very hard. 
(b) These solid have extremely high melting points. 
(c) They have no free electrons hence these are insulators.
Q.87. What are the differences between crystalline and amorphous solids?
Ans. Differences between crystalline and amorphous solids are as follows:

Q.88. Energy is needed when a solid at its melting point is converted into a liquid. Why?
Ans. At the melting point of the solid, energy is needed to overcome the inter particle forces of attraction. Actually in the liquid state, the particles can move more freely than in the solid state.

Q.89. Urea has a sharp melting point but glass does not. Explain.
Ans. Urea is a crystalline molecular solid and it has a sharp melting point. On the other hand, glass is an amorphous solid and its melting point is not sharp.

Q.90. Why is latent heat of fusion of solid carbon dioxide less than that of silicon dioxide?
Ans. Silicon dioxide (SiO₂) is a covalent solid in which the atoms are linked by covalent bonds resulting in a giant crystal. On the other hand, solid carbon dioxide (or dry ice) is a molecular solid in which the molecules of CO₂ are held together by weak van der Waal's forces of attraction. Therefore, attractive forces in silicon dioxide are more than in solid carbon dioxide. As a result, the latent heat of fusion of the former is more.

Q.91. What is the difference between London dispersion forces and dipole-dipole forces?
Ans. Dipole-dipole forces arise between two polar molecules. London forces on the other hand result from the presence of temporary dipole moments caused by the unsymmetrical distribution of electrons.

Q.92. Distinguish between crystal lattice and unit cell.
Ans. Crystal lattice is the regular three dimensional arrangement of the points in a crystal. A unit cell is the smallest but complete unit of the crystal lattice which when repeated over and again in the three dimensions generates the crystal of the given substance. 

Q.93. Classify the following into n-type and p-type semiconductors and assign suitable explanation. 
(i) Ge doped with In 
(ii) B doped with Si.
Ans.
(i) Ge doped with In: When Ge (group 14) is doped with In (group 13), out of the four valence electrons of Ge, three are shared with three electrons of In. This will result in holes in the crystal lattice of germanium and the crystals are p-type semiconductors.
(ii) B doped with Si: When B (group 13) is doped with Si (group 14), only three of the valence electrons of silicon will be shared by the three valence electrons of boron. The presence of extra or free electrons not involved in bonding will result in n-type semiconductors.

Q.94. Gas lighter when pressed produces flame. Explain.
Ans. Certain gas lighters consist of piezoelectric crystals. When pressure is applied, the displacement of constituents in the crystal takes place resulting in electric spark as a result of which fuel gas present in the lighter catches fire and flame is produced.

Q.95. Ferromagnetic and ferrimagnetic substances become paramagnetic upon heating. Discuss.
Ans. When ferromagnetic and ferrimagnetic substances are heated to certain temperature called curie temperature they become paramagnetic. This is because of the realignment of the electron spins or their magnetic moments which are now oriented in one particular direction.

Q.96. The electrical conductivity of a metal decreases with rise in temperature while that of semi-conductor increases. Justify
Ans. The electrical conductivity of a metal decreases with the rise in temperature because the positively charged kernels resent in the electron sea also acquire mobility and obstruct the movement of valence electrons responsible for conductivity. In semi-conductors, the electrical conductivity increases because more electrons are in a position to move from valence band to the conduction band since there is only a small energy gap.

Q.97. CaCl₂ will introduce schottky defect if added to AgCl crystal. Explain.
Ans. Two Ag⁺ ions will be replaced by one Ca²⁺ ion to maintain electrical neutrality. A hole gets created in the lattice when every Ca²⁺ion is introduced. This leads to schottky defect.

Q.98. Why is common salt sometimes yellow instead of being white?Ans. The change in colour occurs due to the creation of anion vacancies. The Cl^- ions leave the lattice and their place is taken up by the electrons. These electrons are known as F-centres and are responsible for the colour. 

Q.99. Why are defects in the crystalline solids called thermodynamic defects?
Ans. There is a perfect arrangement of constituent particles in a crystalline solid at OK. As the temperature increases, the particles are likely to leave their lattice sites and may occupy positions elsewhere. In other words, the defects are likely to increase with the rise in temperature. Therefore, these are called thermodynamic defects.

Q.100. Addition of CdCl₂ to the crystals of AgCl will produce Schottky defects but the same is not produced when NaCI crystals are added. Discuss.
Ans. When CdCl₂ is added to the crystals of AgCl the replacement of one Ag⁺ ion with Cd²⁺ ion necessitates the removal of second Ag⁺ ion from the lattice to maintain the electrical neutrality of the crystal. However, the same does not take place in case of NaCI because both Ag⁺ and Na⁺ ions are monovalent. Thus, Schottky defects are caused by the addition of CdCl₂ and not by NaCI.

Q.101. Cesium chloride is more stable than sodium chloride. Assign reason.
Ans. It may be remembered that higher the value of the C.N., more will be forces of attraction in the closely packed cations and anions and, thus, greater will be the stability of the crystal lattice. Cesium chloride has higher C.N.(8 : 8) than in sodium chloride (6 : 6) and is therefore, more stable is nature.

Q.102.  Why is diffusion in solids less as compared to the liquids?
Ans. In solids, the constituent particles are very closely packed and the number of vacant spaces are comparatively less as compared to the liquids where the particles are not so closely packed. Therefore, the diffusion in the solids is almost negligible while liquids can diffuse.

Q.103. How will you show that glass is a super cooled liquid?
Ans. If we examine carefully the window panes of an old building, we find that they are little thicker at the bottom. This is due to the reason that glass has flown under the influence of gravity over the years though very slowly. It has behaved liked liquid. Therefore, glass is a super cooled liquid.

Q.104. The melting point of sodium chloride is more than that of sodium. Justify.
Ans. Sodium chloride is an ionic solid in which the Na⁺ and Cl^- ions have strong columbic forces of attraction. But sodium is a metallic solid in which the attractive forces are comparatively weak. Therefore, melting point of sodium chloride is more than that of sodium.

Q.105. The ions of NaF and MgO have the same number of electrons and inter nuclear distances are about the same (235 pm and 215 pm). Why are then the melting points of NaF and MgO so different (992°C and 2642°C)?
Ans. In MgO, both the ions are divalent in nature (Mg²⁺ and O^2- while in NaF, these are of monovalent nature (Na⁺ and F^-).The electrostatic forces among the ions in the crystal lattice of MgO are stronger than in case of NaF. Therefore, the melting point of MgO is higher (2642°C) as compared to NaF (992°C).

Q.106. Agl crystallizes in a cubic close-packed ZnS structure. What fraction of tetrahedral sites is occupied by Ag⁺ ions?
Ans. In the face centered unit cell of Agl there are four Ag⁺ ions and four I^- ions. As there are four I^- ions in the packing, therefore there are eight tetrahedral voids. Half of these are occupied by Ag⁺ ions.

Q.107. Crystals possessing CsCl structure change to NaCI structure upon heating. Give a suitable explanation.
Ans. A crystalline solid AB having CsCl structure has 8 : 8 co-ordination of its constituents. When heated, its co-ordination changes to 6 : 6. Therefore, it acquires NaCI structure upon heating.

Q.108. Why does zinc oxide which is white in colour become yellow upon heating?
Ans. Zinc oxide (ZnO) crystals upon heating decompose as follows:  
The excess of Zn²⁺ ions formed are trapped in the interstitial spaces and the electrons are also trapped in the neighbourhood. These electrons absorb radiations corresponding to certain specific colour from the light and emit yellow light.

Q.109. Schottky defect lowers the density of ionic crystals while Frenkel defect does not. Discuss.
Ans. In Schottky defect, certain cations and anions are missing from the crystal lattice. Therefore, the density of the crystal decreases. However, in Frenkel defect, the ions do not leave the lattice but they simply change their positions from lattice points to the interstitial spaces. As a result, the density of the crystal remains unchanged.

Q.110. A metallic element crystallises into a lattice containing sequence of layers AB AB AB....... Any packing of spheres leaves out voids in the lattice. What percentage by volume of this lattice is empty space?
Ans. The AB AB AB ..... type arrangement is known as hcp packing. For hcp, the packing fraction is 0.74. This means that the empty space is 0.26 or 26%.

Q.111. What difference in behaviour between glass and sodium chloride would you expect to observe if you break off a piece of either cube?
Ans. If we break or cut a piece of cube of sodium chloride, we find a smooth surface on both sides. However, in case of glass, we get an irregular surface. This is on account of difference in the cleavage property of crystalline and amorphous solids. If we cut a crystalline solid, such gas sodium chloride, we get two smooth surfaces. On the other hand, the cleavage of an amorphous solid such as glass leads to irregular surfaces.

Q.112. Frenkel defect is not found in the halides of alkali metals. Assign reason.
Ans. Frenkel defect arises when certain ions (particularly cations) leave their normal sites and occupy positions in the interstitial space. The ions of the alkali metal halides are of large size and cannot be easily accommodated in the interstitial spaces. Therefore, the halides of alkali metals do not normally show Frenkel defect.

Q.113. The ions of NaF and MgO have the same number of electrons and inter nuclear distances are about the same (235 pm and 215 pm). Why are then the melting points of NaF and MgO so different (992 C and 2642C)?
Ans. In MgO, both the ions are divalent in nature (Mg²⁺ and O^2- )while in NaF, these are of monovalent nature (Na⁺ and F^-).The electrostatic forces among the ions in the crystal lattice of MgO are stronger than in case of NaF. Therefore, the melting point of MgO is higher (2642°C) as compared to NaF (992°C).

Q.114. What difference in behaviour between glass and sodium chloride would you expect to observe if you break off a piece of either cube?
Ans. If we break or cut a piece of cube of sodium chloride, we find a smooth surface on both sides. However, in case of glass, we get an irregular surface. This is on account of difference in the cleavage property of crystalline and amorphous solids. If we cut a crystalline solid, such gas sodium chloride, we get two smooth surfaces. On the other hand, the cleavage of an amorphous solid such as glass leads to irregular surfaces.

Q.115. A metallic element crystallises into a lattice containing sequence of layers AB AB AB....... Any packing of spheres leaves out voids in the lattice. What percentage by volume of this lattice is empty space?
Ans. The AB AB AB ..... type arrangement is known as hcp packing. For hcp, the packing fraction is 0.74. This means that the empty space is 0.26 or 26%..

Q.116. Schottky defect lowers the density of ionic crystals while Frenkel defect does not. Discuss.
Ans. In Schottky defect, certain cations and anions are missing from the crystal lattice. Therefore, the density of the crystal decreases. However, in Frenkel defect, the ions do not leave the lattice but they simply change their positions from lattice points to the interstitial spaces. As a result, the density of the crystal remains unchanged.

Q.117. Crystals possessing CsCl structure change to NaCI structure upon heating. Give a suitable explanation.
Ans. A crystalline solid AB having CsCl structure has 8 : 8 co-ordination of its constituents. When heated, its co-ordination changes to 6 : 6. Therefore, it acquires NaCI structure upon heating.

Q.118. Agl crystallizes in a cubic close-packed ZnS structure. What fraction of tetrahedral sites is occupied by Ag⁺ ions? 
Ans. In the face centered unit cell of Agl there are four Ag⁺ ions and four I^- ions. As there are four I^- ions in the packing, therefore there are eight tetrahedral voids. Half of these are occupied by Ag⁺ions.

Q.119. The ions of NaF and MgO have the same number of electrons and inter nuclear distances are about the same (235 pm and 215 pm). Why are then the melting points of NaF and MgO so different (992°C and 2642°C)?
Ans. In MgO, both the ions are divalent in nature (Mg²⁺ and O^2- ) while in NaF, these are of monovalent nature (Na⁺ and F^-).The electrostatic forces among the ions in the crystal lattice of MgO are stronger than in case of NaF. Therefore, the melting point of MgO is higher (2642°C) as compared to NaF (992°C).

Q.120. The melting point of sodium chloride is more than that of sodium. Justify. 
Ans. Sodium chloride is an ionic solid in which the Na⁺ and  Cl^- ions have strong columbic forces of attraction. But sodium is a metallic solid in which the attractive forces are comparatively weak. Therefore, melting point of sodium chloride is more than that of sodium.

Q.121. How will you show that glass is a super cooled liquid?
Ans. If we examine carefully the window panes of an old building, we find that they are little thicker at the bottom. This is due to the reason that glass has flown under the influence of gravity over the years though very slowly. It has behaved liked liquid. Therefore, glass is a super cooled liquid.

Q.122. Why is diffusion in solids less as compared to the liquids?
Ans. In solids, the constituent particles are very closely packed and the number of vacant spaces are comparatively less as compared to the liquids where the particles are not so closely packed. Therefore, the diffusion in the solids is almost negligible while liquids can diffuse.

Q.123. Energy is needed when a solid at its melting point is converted into a liquid. Why?
Ans. At the melting point of the solid, energy is needed to overcome the inter particle forces of attraction. Actually in the liquid state, the particles can move more freely than in the solid state.

Q.124. Urea has a sharp melting point but glass does not. Explain.
Ans. Urea is a crystalline molecular solid and it has a sharp melting point. On the other hand, glass is an amorphous solid and its melting point is not sharp.

Q.125 Why is latent heat of fusion of solid carbon dioxide less than that of silicon dioxide? 
Ans. Silicon dioxide (SiO₂) is a covalent solid in which the atoms are linked by covalent bonds resulting in a giant crystal. On the other hand, solid carbon dioxide (or dry ice) is a molecular solid in which the molecules of CO₂ are held together by weak van der Waa's forces of attraction. Therefore, attractive forces in silicon dioxide are more than in solid carbon dioxide. As a result, the latent heat of fusion of the former is more.

Q.126 Cesium chloride is more stable than sodium chloride. Assign reason.
Ans. It may be remembered that higher the value of the C.N., more will be forces of attraction in the closely packed cations and anions and, thus, greater will be the stability of the crystal lattice. Cesium chloride has higher C.N.(8 : 8) than in sodium chloride (6 : 6) and is therefore, more stable is nature.

Q.127. produce Schottky defects but the same is not produced when NaCI crystals are added. Discuss.
Ans. When CdCl₂ is added to the crystals of AgCl, the replacement of one Ag⁺ ion with Cd²⁺ ion necessitates the removal of second Ag⁺ ion from the lattice to maintain the electrical neutrality of the crystal. However, the same does not take place in case of NaCI because both Ag⁺ and Na⁺ ions are monovalent. Thus, Schottky defects are caused by the addition of CdCl₂ and not by NaCI.

Q.128. Why are defects in the crystalline solids called thermodynamic defects?
Ans. There is a perfect arrangement of constituent particles in a crystalline solid at OK. As the temperature increases, the particles are likely to leave their lattice sites and may occupy positions elsewhere. In other words, the defects are likely to increase with the rise in temperature. Therefore, these are called thermodynamic defects.

Q.129. Why is common salt sometimes yellow instead of being white?Ans. The change in colour occurs due to the creation of anion vacancies. The Cl^- ions leave the lattice and their place is taken up by the electrons. These electrons are known as F-centres and are responsible for the colour. 

Q.130. CaCl₂ will introduce  schottky defect if added to AgCl crystal. Explain.
Ans. Two Ag⁺ ions will be replaced by one Ca²⁺ ion to maintain electrical neutrality. A hole gets created in the lattice when every Ca²⁺ ion is introduced. This leads to schottky defect.

Q.131. The electrical conductivity of a metal decreases with rise in temperature while that of semi-conductor increases. Justify
Ans. The electrical conductivity of a metal decreases with the rise in temperature because the positively charged kernels resent in the electron sea also acquire mobility and obstruct the movement of valence electrons responsible for conductivity. In semi-conductors, the electrical conductivity increases because more electrons are in a position to move from valence band to the conduction band since there is only a small energy gap.

Q.132. Ferromagnetic and ferrimagnetic substances become paramagnetic upon heating. Discuss. 
Ans. When ferromagnetic and ferrimagnetic substances are heated to certain temperature called curie temperature they become paramagnetic. This is because of the realignment of the electron spins or their magnetic moments which are now oriented in one particular direction.

Q.133. Gas lighter when pressed produces flame. Explain.
Ans. Certain gas lighters consist of piezoelectric crystals. When pressure is applied, the displacement of constituents in the crystal takes place resulting in electric spark as a result of which fuel gas present in the lighter catches fire and flame is produced.

Q.134. Distinguish between crystal lattice and unit cell.
Ans. Crystal lattice is the regular three dimensional arrangement of the points in a crystal. A unit cell is the smallest but complete unit of the crystal lattice which when repeated over and again in the three dimensions generates the crystal of the given substance.

Q.135. What is the difference between London dispersion forces and dipole-dipole forces?
Ans. Dipole-dipole forces arise between two polar molecules. London forces on the other hand result from the presence of temporary dipole moments caused by the unsymmetrical distribution of electrons.

Q.136. What is the difference between London dispersion forces and dipole-dipole forces?
Ans. Dipole-dipole forces arise between two polar molecules. London forces on the other hand result from the presence of temporary dipole moments caused by the unsymmetrical distribution of electrons.