Chemistry Class 12: Guess Questions & Detailed Answers

Chapter 1: The Solid State

1. What is the solid state? Explain the characteristics of solids.

The solid state refers to one of the fundamental states of matter where particles (atoms, molecules, or ions) are tightly packed. In solids, the particles have fixed positions and can only vibrate. This leads to definite shape and volume.
Characteristics of solids: Solids have strong intermolecular forces, making them incompressible and rigid. They also have high density compared to liquids and gases, and their thermal expansion is usually very low. Solids resist shear forces and have structural integrity.

2. On what basis are solids classified? Explain.

Solids are classified into crystalline and amorphous solids based on the arrangement of their particles. Crystalline solids have a well-ordered, repeating arrangement of particles, leading to sharp melting points. Examples include salt and diamond. Amorphous solids lack long-range order and melt over a range of temperatures. Examples include glass and plastic. Crystalline solids are further categorized into molecular, ionic, metallic, and covalent network solids based on bonding.

3. What is the difference between metallic and non-metallic solids?

Metallic solids consist of metal atoms that are held together by a 'sea' of delocalized electrons, which allows them to conduct electricity and be malleable. Examples include copper and iron. Non-metallic solids are typically covalently bonded and do not conduct electricity, except in special cases like graphite. Non-metallic solids tend to be brittle and include substances like sulfur and diamond.

4. Differentiate between crystalline and amorphous solids.

Crystalline solids: These have a long-range order of atoms or molecules in a repeating pattern, sharp melting points, and distinct cleavage planes. Examples include quartz and salt.
Amorphous solids: These lack long-range order, do not have sharp melting points, and can flow, though very slowly (considered pseudo-solids). Examples are glass and rubber.

5. Explain the significance of unit cells in crystals.

A unit cell is the smallest structural unit of a crystal lattice that repeats in three-dimensional space to form the entire crystal. The arrangement of atoms in a unit cell determines the physical properties of the crystal, such as density, symmetry, and melting point. The study of unit cells helps in understanding crystallography and predicting the behavior of materials.

6. What are semiconductors? Discuss their uses.

Semiconductors are materials that have electrical conductivity between conductors (metals) and insulators (non-metals). Their conductivity can be controlled by doping with impurities.
Semiconductors play a crucial role in modern electronics. They are used in the manufacture of transistors, diodes, solar cells, and integrated circuits, which are essential components of computers, smartphones, and other electronic devices.