PRELIMS

Substance and Its Nature

Chemistry deals with the composition of matter and its physical/chemical characteristics. Lavoisier (1743–1793) is regarded as the father of modern chemistry.

States of Matter

Matter is anything that occupies space, possesses mass, and can be felt by senses.

• Solid State: Definite shape and volume. Hard and rigid (e.g., metals, wood).
• Liquid State: Definite volume but no definite shape (takes shape of container). Example: water, milk.
• Gaseous State: No definite volume or shape (highly compressible). Example: air, .
• Phase Transition: Solid Liquid Gas (Vapour).

Pure Substances: Elements and Compounds

• Pure Substance: Cannot be separated by physical processes.
• Elements: Simplest pure substance.
  1. Metals: Normally hard solids (except Mercury), high MP/BP, lustrous, good conductors. Conductivity decreases as temperature increases.
  2. Non-metals: Found in all states, poor conductors (except Graphite), no lustre (except Iodine).
  3. Metalloids: Properties of both (e.g., Arsenic, Antimony, Bismuth).
• Compounds: Fixed proportion by mass of two or more elements. Properties differ from constituent elements.
  1. Organic: From living sources (Hydrocarbons/derivatives like Proteins, Fats).
  2. Inorganic: From non-living sources (Rocks/minerals like Salt, Marble).

Mixtures

Material obtained by mixing substances in indefinite proportions; component properties remain unchanged.

1. Homogeneous: Uniform composition; no visible boundaries (e.g., salt/sugar in water).
2. Heterogeneous: Non-uniform; visible boundaries (e.g., sulphur and sand).

Separation Techniques

1. Sublimation: Solid Vapour directly. Used for Naphthalene, Iodine, Ammonium Chloride.
2. Filtration: Removal of suspended solids from liquids (e.g., engine oil, tea).
3. Evaporation: Liquid Vapour at room temperature/heating. Used for salt from sea water.
4. Crystallization: Purification of solids. Solid is dissolved in solvent at boiling point, then cooled to crystallize pure solid.
5. Distillation: Vaporisation + Condensation. Separates liquids with different BPs or liquids from non-volatile solids.
6. Fractional Distillation: Similar to distillation but uses a fractionating column for volatile liquids with closer BPs (e.g., Petroleum separation, air components).
7. Chromatography: Based on different rates of absorption. Types: Paper, Column, Gas, TLC, etc.
8. Sedimentation & Decantation: Heavier insoluble solids settle (sedimentation), then clear liquid is poured off (decantation).

Colloids, Solutions, and Suspensions

• True Solution: Particle size < 1 nm; homogeneous; does not scatter light; particles not visible even under microscope (e.g., salt/sugar solution).
• Colloid (Colloidal Solution): Particle size between 1–1000 nm; appears homogeneous but is heterogeneous at microscopic level; scatters light (Tyndall Effect); particles visible under ultra-microscope; do not settle on standing (e.g., milk, blood, smoke, fog, gold sol).
  • Tyndall Effect: Scattering of light by colloidal particles, making the path of light visible (e.g., beam of light through a forest canopy, headlight beam in fog).
  • Brownian Movement: Continuous zig-zag movement of colloidal particles due to collisions with dispersion medium molecules; keeps colloid stable.
  • Types by phase: Sol (solid in liquid, e.g., paint), Gel (liquid in solid, e.g., jelly, cheese), Emulsion (liquid in liquid, e.g., milk), Aerosol (solid/liquid in gas, e.g., smoke, fog).
• Suspension: Particle size > 1000 nm; heterogeneous; particles settle on standing; visible to naked eye (e.g., mud in water, chalk in water).

Laws of Chemical Combination

• Law of Conservation of Mass (Lavoisier): Mass can neither be created nor destroyed in a chemical reaction.
• Law of Constant/Definite Proportions (Proust): A given compound always contains the same elements combined in the same fixed proportion by mass, regardless of source.
• Law of Multiple Proportions (Dalton): When two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in a simple whole-number ratio.

Confused Pairs

• Mixture vs. Compound: Mixture — variable composition, components retain properties, separable by physical means, no energy change. Compound — fixed composition, new properties, separable only by chemical means, energy change occurs on formation.
• Molecule vs. Compound: A molecule is the smallest independently-existing particle of an element or compound (e.g., is a molecule but not a compound, since it has only one element); a compound is always a molecule formed from two or more different elements (e.g., ).
• Colloid vs. Suspension vs. True Solution: Distinguished mainly by particle size and whether they scatter light (Tyndall effect) — colloids do, true solutions don't, suspensions settle out.

Concepts of State Change

• Melting Point: Equilibrium between solid and liquid forms.
• Boiling Point: Temperature where vapour pressure equals atmospheric pressure.
• Freezing Point: Temperature where liquid vapour pressure equals solid vapour pressure.
• Evaporation Factors: Nature of liquid, Temperature, Surface area. Causes cooling (high KE molecules escape).
• Vapour Pressure: Pressure of vapour in equilibrium with liquid. Increases with temperature.

UPSC Relevance

Fundamental classification of matter is a common source of conceptual questions.

• Separation Methods: Knowing which method is used for what (e.g., fractional distillation for petroleum) is essential.
• State Changes: Why evaporation causes cooling and the factors affecting it.
• Metals/Non-metals: Exceptions like Mercury (liquid metal) and Graphite (conducting non-metal) are favorite Prelims facts.
• Colloids: The Tyndall effect explains everyday visual phenomena (why the sky is blue, why headlight beams are visible in fog/dust) and is a frequently asked application-based fact.