Solid states- very short and short answer type question

 Topic: Solids and their classification

1. What are fluids? Give examples. [1] 

2. Solids are rigid why? [1] 

3. How are solids classified? [1] 

4. Define the term amorphous with example. [1] 

5. The window panes of the old buildings are thick at the bottom. Why? [2] 

6. The stability of a crystal is reflected in the magnitude of its melting point. Explain [2] 

7. Graphite is soft and good conductor of electricity. Explain. [2] 

8. Ionic solids are good conductors in molten state and in aqueous solutions but not in solid state. Why [2] 

9. Differentiate between amorphous and crystalline solids with reference to (1) Melting point (2) Cleavage property (3) Nature [3] 

10. How are crystalline solids classified on the basis of nature of bonding? Explain with examples. [3] 

ANSWERS 

Topic: Solids and their classification. 

Ans 1: Substances which flow are fluids e.g. liquids and gases. 

Ans 2: Rigidity in solids is due to fixed positions of the constituent particles and their oscillations about their mean positions 

Ans 3: Solids may be classified into two categories – crystalline and amorphous. 

Ans 4: Amorphous means disordered and random arrangement of particles for example – silica, plastic, rubber 

Ans 5: Glass panes of old buildings are thicker at the bottom than at the top as from is an amorphous solid and flows down very slowly and makes the bottom portion thicker. 

Ans 6: Melting point of a solid gives an idea about the intermolecular forces acting between particles. When these forces are strong, the melting point is higher and when these forces are weak, low melting point is observed. Higher is the melting point, more stable the solid is. 

Ans 7: Graphite is soft and good conductor due to its typical structure here carbon atoms are arranged in different layers and each atom a covalently bonded to three of its neighbouring atoms in the same layer. The fourth electron of each atom is free to move about due to which it conducts electricity. Different layers can slide over the other which makes it a soft solid. 

Ans 8: In the solid state, the ions in the ionic solids are not free to move about due to their rigid structure & strong electrostatic forces. Therefore they cannot conduct electricity whereas in molten state and aqueous solution, the ions become free to move about and they conduct electricity. 

Ans 9:

Property	Crystalline solids	Amorphous solids
1. Melting point	They have sharp melting point	They have a range of melting point.
2. Cleavage property	They split into pieces of plain and smooth surfaces when cut with a sharp edged tool.	When cut with a sharp edged tool, pieces of irregular surfaces are obtained.
3. Nature	They are true solids.	They are pseudo solids or super cooled liquids.

 Ans 10:        Classification of crystalline solids.

 1.       Molecular solids: The forces operating between molecules are dispersion or London forces, dipole-dipole interactions, hydrogen bounding e.g. CCl₄, HCl, ice etc.

2.     Ionic solids:   The intermolecular forces are coulomibic or electrostatic forces, e.g. NaCl, MgO etc.

3.     Metallic solids: The forces operating is metallic bonding e.g. Fe, Cu, Ag etc.

4.       Covalent or network solids: The attractive forces are covalent bonding e.g Diamond, Quartz etc.

Topic: Structures of crystals 

1. Define the term: Crystal lattice [1] 

2. What is a unit cell? [1] 

3. What are the axial angles and edge length in a cubic crystal system? [1] 

4. Give one example of each – Tetragonal and hexagonal crystal system. [1] 

5. Name three types of cubic unit cells? [1] 

6. How many atoms are there in a unit cell of a metal crystallizing in a: [2] 

(a) FCC structure 

(b) BCC structure 

7. What is the contribution of an atom per unit cell if the atom is: [3] 

(a) At the corner of the cube. (b) On the face of the cube. (c) In the centre of the cube. 

8. A compound formed by A & B crystallizes in the cubic structure where ‘A’ are at the corners of the cube and B are at the face centre. What is the formula of the compound? [2] 

9. Calculate the no. of atoms in a cubic based unit – cell having one atom on each corner and two atoms on each body diagonal. [2] 

10. What is the no. of octahedral and tetrahedral voids present in a lattice? [2]  

ANSWERS 

Topic: Structures of crystals 

Ans 01: The three dimensional diagrammatic representation of constituent particles, in which each particle is depicted as a point is called crystal lattice. 

Ans 02: The smallest portion of a crystal lattice which, when, repeated in different directions, generates the entire lattice, is called its unit cell. 

Ans 03: Axial angles, 0 α β γ = 90 and edge lengths a = b = c. 

Ans 04: Tetragonal crystal system – white tin, SnO2. Hexagonal crystal system – Graphite, ZnO. 

Ans 05: (a) Simple cubic (b) Face – centred cubic (c) Body centred cubic 

Ans 06: (a) FCC = 4 (b) BCC = 2 

Ans 07: (a) When atom is at the corner of the cube, the contribution is 1/8 atom. 

     (b) When the atom is on the face of the cube, its contribution is ½ atom.     

    (c) If the atom is in the centre of the cube, its contribution is 1 atom. 

Ans 08: Contribution of atom A per unit cell = 1/8 ×8 = 1 atom 

        Contribution of atom B per unit cell =½ ×6 = 3atom 

        Ratio of A & B = 1:3 Formula = AB₃

Ans 09: No. of atoms contributed by 8 corners per unit cell = ¹/₈  x 8  = 1 atom. 

 No. of atoms contributed by one diagonal = 2 

 No. of diagonal = 4 

∴Total contribution by diagonal = 4× 2 = 8 

∴Total no. of atoms = 8 + 1 = 9 atoms 

Ans 10: No. of octahedral voids present in a lattice is equal to the no. of close packed particles and the number of tetrahedral voids is twice the no. of close packed particles.  

Topic – Packing in crystals 

1. What is square close packing? [1]

2. What is the coordination number in :- 

 (a) Square close packing 

(b) Hexagonal close packing. [1] 

3. Define – (a) void (b) coordination Number [2] 

4. What is the packing efficiency in 

(a) hcp structure (b) BCC structure (c) Simple cubic structure [3] 

5. Give the relationship between density and edge length of a cubic crystal. [2] 

6. Copper which crystallizes as a face – centred cubic lattice has a density of 8.93 g/cm3 at 20o C. calculate the length of the unit cell. [3] 

7. An element crystallizes in BCC structure. The edge of its unit cell is 288 pm. If the density is 7.2 g/cm3 . calculate the atomic mass of the element. [3] 

8. The compound CuCl has ZnS structure and the edge length of the unit cell in 500 pm. Calculate the density. (Atomic masses: Cu = 63, Cl = 35.5, Avogadro no = 6.02´ 1023 mol-1) [3] 

9. In a compound, B ions form a close – packed structure & A ions occupy all the tetrahedral voids. What is the formula of the compound? [2] 

10. In crystalline solid, anions C are arranged in cubic close – packing, cations A occupy 50% of tetrahedral voids & cations B occupy 50% of octahedral voids. What is the formula of solid? [3]

[ANSWERS] Topic – Packing in crystals 

Ans 01: The close packing of spheres – atoms or ions, in which each sphere is in contact with four of is neighbours, whose centres, if joined, form a square, is called square close packing. 

Ans 02: (a) in square close packing, the coordination no. is 4. (b) In hexagonal close packing, the coordination no. is 12

Ans 03 (a) void – the empty space left between close packed spheres are voids. (b) Coordination number – the number of spheres or atoms surrounding a single sphere or atom in a crystal is called coordination number. 

Ans 04: (a) hcp packing efficiency is 74%. (b) bcc packing efficiency is 68%. (c) packing efficiency in simple cubic structure is 52.4%. 

Ans 05: Density, d of a cubic cell is given by

d = ZM/a³N_a

Where Z = no. of atoms per unit cell M = molar mass NA = Avogadro number a = edge length 

Ans 06:  361.6 pm  (hint 4r = √2 a)  

Ans 07:      52 g/ mol  ( Hint-  Z= 2 ,d = ZM/a³N_a)

Ans 08:     5.22 g/cm3    (Hint d = ZM/a³N_a )

Ans 09:     Let the no. of B ions = 100

 No. of A ions = no. of tetrahedral voids

= 2 ´no. of ions in close – packed structure  = 2 ´ 100 = 200

Ratio of A & B = 200 : 100 = 2 : 1 Formula = A₂B.

Ans 10:     Suppose no. of anions, C = 100

      Suppose no. of cations, A =  ⁵⁰/₁₀₀  ´ no. of tetrahedral voids

                                 = ½ (2 ´ C)

No. of cations, B =   ⁵⁰/₁₀₀ ´ no. of octahedral voids

                           = ½ ´ C

  = ½ ´   100 = 50

Ratio of ions A : B : C = 100 : 50 : 100

                                        = 2 : 1 : 2

Formula = A₂BC₂ 

Topic – imperfections in crystals

1. What is the meaning of term ‘defect’ with reference to crystal? [1]

2. Define two main types of defects. [2]

3. Name the types of point defect. [1]

4. (a) Identify the defect in figure below : [2] 

(b) How does it affect the density of crystal? 

(c) Give an example of crystal where this defect can be found. 

(d) What is its effect on electrical neutrality of crystal?

5. Which defect is observed in a solid solution of CdCl2 and AgCl? Explain. [2]

6. Excess of lithium makes LiCl crystal pink. Explain. [2]

7. What are F centres? [1]

8. How does a crystal changes due to presence of F centre? [1]

9. Give an example which shows both frenkel and Schottky defect. [1]

10. Which type of ionic substances show? [2] (a) Schottky defect (b) Frenkel defect

[ANSWERS] Topic – imperfections in crystals

Ans 01: The defects are irregularities in the arrangement of constituent particles in a crystal.

Ans 02: Defects are of two types – Point defects: - Irregularities or deviations from ideal arrangement around a point or on atom in a crystal. Line defect: - Irregularities or deviation from ideal arrangement in entire row of lattice points.

Ans 03: point defects are of three types – stoichiometric defect, impurity defect & non – stoichiometric defect.

Ans 04: (a) Schottky defect. (b) It decreases the density of crystal. (c) NaCl, KCl (d) The crystal remains electrically neutral.

Ans 05: In a solid solution of CdCl2 and AgCl, impurity defect is observed. In the crystal of AgCl, some of the sites of Ag+ are occupied by Cd²⁺ ,  each Cd²⁺ replacing two Ag+. It replaces the site of one ion and other site remains vacant. 

Ans 06: When crystals of LiCl are heated in an atmosphere of Li vapour the Lithium atoms are deposited on the surface of crystal. The Cl- ions diffuse to the surface of crystal & combine with Li atoms to from LiCl which happens by loss of electrons by Li atoms to form Li+ ions. These released elements diffuse into the crystal & electrons get excited after absorbing light from visible region & emit pink colour.

Ans 07: F – centre is the position of an anion in an ionic crystal which is occupied by a trapped electron

Ans 08: The crystal becomes paramagnetic and coloured due to presence of f – centre

Ans 09: AgBr.

Ans 10: (a) Schottky defect – ionic substances in which the cation and anion are of almost similar sizes eg. NaCl, KCl, CrCl.

(b) Feenkel Defect – Ionic substances in which there is large difference in size of ions eg. ZnS, AgCl, AgBr.