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

Solution: Solids exhibit rigidity due to the presence of very 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 rather than moving freely. This immobility of particles is the fundamental reason for the rigid nature of solids.

Solution: The definite volume of solids is a consequence of the strong intermolecular forces of attraction that bind the constituent particles together. These forces ensure that the particles are held in fixed positions and maintain a specific, closely packed arrangement. Because the particles cannot move past each other or change their relative distances significantly, the overall volume occupied by the solid remains constant and well-defined.

Solution: Solids can be classified as either amorphous or crystalline based on the arrangement of their constituent particles. Crystalline solids have a regular, long-range, ordered arrangement, while amorphous solids have a disordered, short-range arrangement.

Amorphous solids:

• Polyurethane
• Teflon
• Cellophane
• Polyvinyl chloride
• Fibre glass

Crystalline solids:

• Naphthalene
• Benzoic acid
• Potassium nitrate
• Copper

Solution: Glass is considered a supercooled liquid because, like liquids, it possesses the ability to flow, albeit at an extremely slow rate over long periods. This slow flow is not easily observable in everyday life but can lead to structural changes over time. For instance, old glass windows and doors are often found to be slightly thicker at the bottom than at the top, indicating that the glass has flowed downwards under gravity, a characteristic behavior of liquids.

Solution: A solid that exhibits the same value of physical properties, such as refractive index, along all directions is known as an isotropic solid. This property of isotropy is characteristic of amorphous solids, which have a random, disordered arrangement of their constituent particles. In contrast, crystalline solids are typically anisotropic, meaning their physical properties vary with direction due to their ordered atomic structure.

Regarding cleavage property, amorphous solids do not exhibit a well-defined cleavage property. When an amorphous solid is cut with a sharp-edged tool, it tends to break into pieces with irregular, uneven surfaces, rather than splitting along specific planes.

Solution: Solids can be classified into different categories based on the type of intermolecular forces or bonding that holds their constituent particles together. Here is the classification of the given substances:

• Potassium sulphate (K₂SO₄): This is an ionic compound formed by electrostatic attraction between K⁺ and SO₄²⁻ ions. Therefore, it is an Ionic solid.
• Tin (Sn): Tin is a metal where atoms are held together by metallic bonds, characterized by a sea of delocalized electrons. Thus, it is a Metallic solid.
• Benzene (C₆H₆): Benzene molecules are non-polar due to symmetry. The intermolecular forces are weak London dispersion forces. Hence, it is a Molecular (non-polar) solid.
• Urea (CO(NH₂)₂): Urea molecules are polar due to the presence of electronegative atoms and an asymmetrical structure. They are held together by dipole-dipole interactions. Thus, it is a Polar molecular solid.
• Ammonia (NH₃): Ammonia molecules are polar and can also form hydrogen bonds with each other. Therefore, it is classified as a Polar molecular solid (specifically, capable of hydrogen bonding).
• Water (H₂O): Water molecules are polar and strongly interact through hydrogen bonds. Solid water (ice) is a prime example of a Hydrogen bonded molecular solid.
• Zinc sulphide (ZnS): Zinc sulphide is formed by the electrostatic attraction between Zn²⁺ and S²⁻ ions. It is an Ionic solid.
• Graphite: Graphite has a layered structure where carbon atoms within each layer are strongly bonded by covalent bonds. The layers are held together by weaker van der Waals forces. It is considered a Covalent or network solid.
• Rubidium (Rb): Rubidium is an alkali metal. Its atoms are held together by metallic bonds. Thus, it is a Metallic solid.
• Argon (Ar): Argon is a noble gas. In its solid state, the atoms are held together by very weak London dispersion forces. Hence, it is a Non-polar molecular solid.
• Silicon carbide (SiC): Silicon carbide has a crystal structure similar to diamond, with strong covalent bonds between silicon and carbon atoms throughout the crystal lattice. It is a Covalent or network solid.