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

Solution: The existence of a substance in the solid state is favoured by conditions that allow the constituent particles to be held together by strong intermolecular forces, minimizing their motion.

At low temperatures, the thermal energy of the particles is reduced. This decrease in kinetic energy allows the strong cohesive forces between the particles to dominate over their tendency to move randomly. Consequently, the particles become fixed in position, leading to the formation of a solid structure.

Conversely, high temperatures and high thermal energy increase particle motion, favouring the liquid or gaseous states. Weak cohesive forces would also not be sufficient to hold particles together in a fixed structure.

Answer: (b) Low temperature

Solution: Crystalline solids are characterized by a highly ordered, three-dimensional arrangement of their constituent particles (atoms, ions, or molecules) that repeats periodically over long distances. This ordered structure leads to several distinct properties:

• They possess a definite and characteristic heat of fusion because the same amount of energy is required to overcome the same type and strength of intermolecular forces in breaking down the crystal lattice.
• They exhibit anisotropic nature. This means that their physical properties, such as electrical conductivity, refractive index, and mechanical strength, vary when measured along different directions within the crystal. This is a direct consequence of the ordered arrangement of particles.
• They are considered true solids due to their rigid structure and definite shape.

The statement that crystalline solids are isotropic in nature is incorrect. Isotropy, where properties are the same in all directions, is a characteristic of amorphous solids. Answer: (b) Isotropic nature

Solution: Amorphous solids are characterized by a lack of long-range order in their particle arrangement. Their constituent particles are arranged randomly, exhibiting only short-range order. Among the given options:

• Graphite (C), Chrome alum, and Silicon carbide (SiC) are examples of crystalline solids, possessing a regular, repeating three-dimensional structure.
• Quartz glass (SiO₂) is an amorphous solid. While quartz (the mineral) is crystalline, the glass form lacks the long-range order typical of crystals.

The random arrangement in amorphous solids means they do not have a sharp melting point and are isotropic. Answer: (b) Quartz glass (SiO₂)

Solution: Antiferromagnetic substances are characterized by the presence of magnetic domains where the magnetic moments of adjacent atoms or ions are aligned in opposite directions. These opposing alignments effectively cancel each other out, resulting in a substance that exhibits very weak or no net magnetism at room temperature.

The schematic arrangement for antiferromagnetic substances shows an equal number of magnetic moments pointing up and down, arranged in a regular pattern such that neighbouring moments oppose each other. This results in a net magnetic moment close to zero.

For example, a pattern like ↑↓↑↓ or ↑↓↑↓

↓↑↓↑ would represent this opposing alignment.

Answer: (d) (This refers to the option depicting opposing magnetic moments cancelling each other out.)

Solution: Quartz glass is an amorphous solid. Amorphous solids lack a long-range, ordered arrangement of their constituent particles. Instead, they possess only a short-range order, meaning the arrangement is ordered over very small distances but becomes random over larger distances.

A key characteristic of amorphous solids is their isotropic nature. This means that their physical properties, such as refractive index, electrical resistance, and thermal conductivity, are the same regardless of the direction in which they are measured. Therefore, the refractive index of quartz glass is the same in all directions.

In contrast, crystalline solids are anisotropic, and their refractive index would differ depending on the direction of measurement.

Answer: (a) Same in all directions

Solution: Let's analyze each statement regarding amorphous solids:

• (a) On heating, they may become crystalline at certain temperature: This is true. When heated to a sufficiently high temperature (often called the 'transition temperature' or 'softening point'), amorphous solids can rearrange their particles into a more stable, ordered crystalline structure.
• (b) They may become crystalline on keeping for a long time: This is also true. Over extended periods, especially at slightly elevated temperatures, amorphous solids can undergo slow structural changes leading to crystallization.
• (c) Amorphous solids can be moulded by heating: This is true. Because they lack a sharp melting point and gradually soften upon heating, amorphous solids can be easily shaped and moulded into various forms, which is why materials like glass are widely used.
• (d) They are anisotropic in nature: This statement is not true. Amorphous solids are characterized by a random arrangement of their constituent particles, which results in isotropic behaviour. Their physical properties are the same in all directions. Anisotropy is a characteristic feature of crystalline solids.

Answer: (d) They are anisotropic in nature

Solution: Crystalline solids are defined by the highly ordered, repeating, three-dimensional arrangement of their constituent particles (atoms, ions, or molecules). This ordered structure extends throughout the entire crystal, exhibiting long-range order.

Because of this regular arrangement over a long distance, the forces of attraction between the particles are uniform throughout the crystal. Consequently, a specific, fixed amount of thermal energy is required to overcome these forces and break down the crystal lattice, leading to a sharp melting point. All particles experience similar bonding environments and melt at the same temperature.

In contrast, amorphous solids have only short-range order, leading to variations in bonding and a gradual softening over a range of temperatures, rather than a sharp melting point.

Answer: (b) a regular arrangement of constituent particles observed over a long distance in the crystal lattice