Chapter 1: The Solid State Class 12 Chemistry NCERT Solutions

Q1.1. Why are solids rigid?

Answer: In solids, the constituent particles are held in fixed positions due to strong intermolecular forces. They can only oscillate about their mean positions, which gives solids their rigidity.

Q1.2. Why do solids have a definite volume?

Answer: Solids have strong intermolecular forces and minimal intermolecular space. This structural rigidity resists compression, giving solids a fixed and definite volume.

Q1.3. Classify the following as amorphous or crystalline solids: Polyurethane, naphthalene, benzoic acid, Teflon, potassium nitrate, cellophane, polyvinyl chloride, fibreglass, copper

Answer: Crystalline solids: Benzoic acid, potassium nitrate, copper Amorphous solids: Polyurethane, Teflon, cellophane, polyvinyl chloride, fibreglass

Q1.4. Why is glass considered a supercooled liquid?

Answer: Glass behaves like a supercooled liquid because it lacks a sharp melting point and flows very slowly over time. This is evident from its slightly thicker base in old windowpanes.

Q1.5. A solid has the same refractive index in all directions. What type of solid is it? Would it show cleavage?

Answer: It is an amorphous solid (isotropic in nature). Amorphous solids do not exhibit clean cleavage; they break into irregular pieces.

Q1.6. Classify the following solids by intermolecular forces: Sodium sulphate, copper, benzene, urea, ammonia, water, zinc sulphide, diamond, rubidium, argon, silicon carbide

Answer: Ionic: Sodium sulphate, zinc sulphide Metallic: Copper, rubidium Molecular: Benzene, urea, ammonia, water, argon Covalent (network): Diamond, silicon carbide

Q1.7. A solid is very hard, an electrical insulator in both solid and molten states, and has a high melting point. What type is it?

Answer: It is a covalent or network solid.

Q1.8. Why do ionic solids conduct electricity in the molten state but not in the solid state?

Answer: In the solid state, ions are fixed and cannot move. In the molten state, ions become mobile and can conduct electricity.

Q1.9. What type of solids are good conductors and malleable?

Answer: Metallic solids are electrical conductors, malleable, and ductile.

Q1.10. What is the significance of a lattice point?

Answer: A lattice point represents the position of an atom, ion, or molecule in the crystal lattice. It defines the structure and shape of the crystal.

Q1.11. Parameters that characterize a unit cell:

Answer: Edge lengths: a, b, c Angles: α (between b & c), β (between a & c), γ (between a & b)

Q1.12. Distinguish between:

Answer: (i) Hexagonal vs Monoclinic unit cells: Hexagonal: a = b ≠ c; α = β = 90°, γ = 120°. Monoclinic: a ≠ b ≠ c; α = γ = 90°, β ≠ 90°. (ii) Face-centered vs End-centered unit cells: Face-centered: particles at all corners and face centers. End-centered: particles at all corners and centers of two opposite faces.

Q1.13. Atom contributions in a cubic unit cell:

Answer: Corner atom = 1/8 Body-centered atom = 1 (not shared)

Q1.14. 2D coordination number in a square close-packed layer:

Answer: 4

Q1.15. Total number of voids in 0.5 mol of hcp structure and number of tetrahedral voids:

Answer: Atoms = 3.011 × 10²³ Octahedral voids = 3.011 × 10²³ Tetrahedral voids = 6.022 × 10²³ Total = 9.033 × 10²³

Q1.16. Compound formed by M and N (M occupies 1/3 tetrahedral voids in ccp):

Answer: Ratio M:N = 2:3 Formula = M₂N₃

Q1.17. Lattice with highest packing efficiency:

Answer: Hexagonal close-packed (hcp) = 74%

Q1.18. Identify the unit cell from density and edge length data:

Answer: Face-centered cubic (fcc) or cubic close-packed (ccp).

Q1.19. What defect arises when a solid is heated?

Answer: Vacancy defect, which decreases the density of the solid.

Q1.20. Stoichiometric defects in (i) ZnS (ii) AgBr:

Answer: (i) ZnS: Frenkel defect (ii) AgBr: Both Frenkel and Schottky defects

Q1.21. How vacancies form when doping with a higher-valent cation?

Answer: To maintain electrical neutrality, two lower-valent cations are replaced by one higher-valent cation, creating cation vacancies.

Q1.22. Why do anionic vacancies cause color in some solids?

Answer: In NaCl, Cl⁻ ions leave vacancies that are occupied by electrons (F-centres), which absorb visible light and impart yellow color.

Q1.23. Group to use for n-type doping of a Group 14 element:

Answer: Group 15

Q1.24. Which makes better permanent magnets – ferromagnetic or ferrimagnetic substances?

Answer: Ferromagnetic substances, because their magnetic domains remain aligned after removing the external field.

Class 12 Chemistry – Chapter 1: The Solid State (NCERT Detailed Solutions)

Question 1.1: Define the term ‘amorphous’. Give a few examples of amorphous solids.

Answer: Amorphous solids are substances in which the constituent particles are not arranged in a regular, repeating pattern. These solids lack long-range order and are isotropic. Examples: Glass, Plastic, Rubber, Asphalt.

Question 1.2: What makes a glass different from a solid such as quartz? Under what conditions could quartz be converted into glass?

Answer: Quartz is a crystalline solid with regular arrangement, whereas glass is amorphous. Quartz can be converted into glass by melting it and cooling rapidly.

Question 1.3: Classify the following solids as: (i) Ionic (ii) Molecular (iii) Covalent (iv) Metallic — P₄, plastic, graphite, brass, Rb, LiBr, Si, SiO₂, diamond, copper.

Answer: Ionic: LiBr, Rb. Molecular: P₄, plastic. Covalent: Graphite, Si, SiO₂, diamond. Metallic: Brass, copper.

Question 1.4: What types of solids are electrical conductors, malleable and ductile?

Answer: Metallic solids, due to the presence of free electrons.

Question 1.5: Give the significance of a ‘lattice point’.

Answer: A lattice point represents a position occupied by an atom, ion, or molecule in the crystalline structure.

Question 1.6: Name the parameters that characterise a unit cell.

Answer: Edge lengths (a, b, c) and angles between edges (α, β, γ).

Question 1.7: Name the three types of cubic unit cells.

Answer: Simple Cubic (SC), Body-Centered Cubic (BCC), Face-Centered Cubic (FCC).

Question 1.8: Explain how much portion of an atom located at (i) corner and (ii) body centre of a cubic unit cell is shared by adjacent unit cells.

Answer: Corner atom: 1/8. Body-centered atom: 1.

Question 1.9: What is the two-dimensional coordination number of a molecule in square close-packed layer?

Answer: 4.

Question 1.10: A compound forms hexagonal close-packed structure. What is the total number of atoms in 1 unit cell of this structure?

Answer: In a hexagonal close-packed (hcp) structure:
12 corner atoms × 1/6 = 2 atoms
2 face-centre atoms × 1/2 = 1 atom
3 atoms completely inside = 3 atoms
Total = 2 + 1 + 3 = 6 atoms.

Question 1.11: How many lattice points are there in one unit cell of each of the following lattices? (i) Face-centred cubic (ii) Body-centred cubic (iii) Simple cubic

Answer:
(i) FCC: 8 corner atoms × 1/8 + 6 face atoms × 1/2 = 1 + 3 = 4 atoms
(ii) BCC: 8 corner atoms × 1/8 + 1 body atom = 1 + 1 = 2 atoms
(iii) SC: 8 corner atoms × 1/8 = 1 atom

Question 1.12: Explain: (i) The basis of similarities and differences between metallic and ionic crystals. (ii) Ionic solids are hard and brittle.

Answer:
(i) Similarities: Both have strong electrostatic forces of attraction and high melting points.
Differences: Metallic solids have free electrons and conduct electricity in solid state, whereas ionic solids conduct electricity only in molten or aqueous states.
(ii) Ionic solids are hard due to strong ionic bonds but brittle because slight shifting brings like charges closer, causing repulsion and fracture.

Question 1.13: Explain how vacancies are introduced in an ionic solid when a cation of higher valency is added as an impurity in it.

Answer: When a cation of higher valency replaces two cations of lower valency, one cation site becomes vacant to maintain electrical neutrality, creating a vacancy defect.

Question 1.14: Ionic solids conduct electricity in molten state but not in solid state. Explain.

Answer: In solid state, ions are immobile and fixed. In molten state, ions are free to move, allowing them to conduct electricity.

Question 1.15: What type of defect can arise when a solid is heated? Which physical property is affected by it and how?

Answer: Vacancy defect arises on heating. Atoms leave their lattice sites, reducing the density of the solid.

Question 1.16: What type of stoichiometric defect is shown by: (i) ZnS (ii) AgBr

Answer:
(i) ZnS: Frenkel defect (Zn²⁺ ions occupy interstitial positions).
(ii) AgBr: Shows both Frenkel and Schottky defects.

Question 1.17: Explain how electrical neutrality is maintained in solids with Schottky and Frenkel defects.

Answer:
In Schottky defect, equal number of cations and anions are missing.
In Frenkel defect, a cation leaves its normal site and occupies an interstitial site without affecting overall neutrality.

Question 1.18: Describe the two main types of semiconductors and contrast their conduction mechanisms.

Answer:
n-type semiconductor: Doping with Group 15 elements provides extra electrons as charge carriers.
p-type semiconductor: Doping with Group 13 elements creates holes (positive charge carriers).
In n-type, conduction is due to electrons; in p-type, it is due to holes.

Question 1.19: Non-stoichiometric cuprous oxide, Cu₂O, can be prepared in the laboratory. In this oxide, copper to oxygen ratio is slightly less than 2:1. Can you account for the fact that this substance is a p-type semiconductor?

Answer: Due to copper vacancies, some Cu⁺ ions are missing. To maintain neutrality, Cu⁺ is oxidized to Cu²⁺, creating holes which make Cu₂O a p-type semiconductor.

Question 1.20: Ferrimagnetism is observed only in solids but not in liquids and gases. Why?

Answer: Ferrimagnetism depends on a well-ordered alignment of magnetic domains, which is possible only in solids. In liquids and gases, the random motion prevents such alignment.

Question 1.21: What is meant by the term ‘coordination number’?

Answer: Coordination number is the number of nearest neighbours an atom or ion has in a crystal structure.
For example, in FCC and HCP structures, the coordination number is 12.

Question 1.22: What is the coordination number of atoms: (i) in a cubic close-packed structure? (ii) in a body-centred cubic structure?

Answer:
(i) Cubic close-packed (FCC): 12
(ii) Body-centred cubic (BCC): 8

Question 1.23: How can you determine the atomic mass of an unknown metal if you know its density and the dimensions of its unit cell? Explain.

Answer: Use the formula:
M = (d × a³ × N_A) / Z
Where: d = density, a = edge length, N_A = Avogadro number, Z = number of atoms per unit cell.

Question 1.24: A cubic solid is made of two elements, P and Q. Atoms of Q are at corners, and P at face centres. What is the formula of the compound?

Answer:
Q atoms at corners: 8 × 1/8 = 1
P atoms at faces: 6 × 1/2 = 3
Formula: P₃Q

Question 1.25: An element has a body-centred cubic structure with a cell edge of 288 pm. The density is 7.2 g/cm³. Calculate atomic mass.

Answer:
Given: a = 288 pm = 2.88 × 10⁻⁸ cm, d = 7.2 g/cm³, Z = 2.
Using the formula: M = (d × a³ × N_A) / Z
Substituting values gives atomic mass.

Question 1.26: Gold (atomic radius = 144 pm) crystallizes in a face-centred unit cell. What is the length of a side of the unit cell?

Answer:
For FCC structure: a = 2√2 × r
a = 2√2 × 144 pm = 407 pm.
Thus, the edge length is 407 pm.