Short & Long Answer Question: The Solid State - 3 | Physical Chemistry for NEET PDF Download

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.