CBSE Class 12 Chemistry Chapter 1: The Solid State - NCERT Solutions

Solution: Solids are characterized by strong intermolecular forces of attraction between their constituent particles. These strong forces hold the particles in fixed positions, allowing them only to vibrate about their mean positions. This restricted movement is the reason why solids exhibit rigidity.

Solution: The constituent particles in solids are held together by very strong intermolecular forces. These forces ensure that the particles are closely packed and occupy fixed positions. Consequently, solids maintain a constant volume regardless of the container they are placed in.

Solution:

The classification of the given substances into amorphous and crystalline solids is as follows:

Amorphous solids: These solids lack a long-range, ordered arrangement of their constituent particles. Examples from the list include:

Polyurethane, Teflon, Cellophane, Polyvinyl chloride, Fibre glass.

Crystalline solids: These solids possess a regular, repeating three-dimensional arrangement of their constituent particles. Examples from the list include:

Naphthalene, Benzoic acid, Potassium nitrate, Copper.

Solution: Glass is considered a supercooled liquid because, like liquids, it possesses a very slight tendency to flow over extended periods, although this flow is extremely slow. This characteristic is sometimes inferred from the observation that old glass panes are often found to be slightly thicker at the bottom than at the top, suggesting a slow downward movement of the glass material over many years.

Solution:

Nature of the solid: When a physical property, such as refractive index, has the same value when measured along all directions, the solid is said to be isotropic. Isotropy is a characteristic feature of amorphous solids, which have a disordered arrangement of their constituent particles. Therefore, the solid described is an amorphous solid.

Cleavage property: Crystalline solids typically exhibit cleavage property, meaning they can be split into two pieces with smooth, flat surfaces when cut with a sharp-edged tool, along specific planes. Amorphous solids, however, do not have such ordered planes. When an amorphous solid is cut with a sharp-edged tool, it breaks into pieces with irregular, uneven surfaces. Thus, this solid would not show a distinct cleavage property.

Solution:

The classification of the given solids based on the nature of intermolecular forces is as follows:

• Ionic solids: These solids are formed by ions held together by electrostatic forces. Examples: Potassium sulphate, Zinc sulphide.
• Metallic solids: These solids consist of metal cations arranged in a lattice and held together by a 'sea' of mobile electrons. Examples: Tin, Rubidium.
• Molecular solids: These solids are formed by discrete molecules held together by intermolecular forces. They can be further classified:
• Non-polar molecular solids: Held together by weak London dispersion forces. Examples: Benzene, Argon.
• Polar molecular solids: Held together by dipole-dipole interactions. Examples: Urea, Ammonia.
• Hydrogen bonded molecular solids: Held together by hydrogen bonds. Example: Water.
• Covalent or network solids: These solids have atoms joined together by a network of covalent bonds. Example: Graphite, Silicon carbide.