Learning Objectives
- Distinguish between pure substances and mixtures
- Classify pure substances into elements and compounds
- Differentiate between homogeneous and heterogeneous mixtures
- Learn various methods of separation of mixtures
- Understand the difference between physical and chemical changes
Key Concepts
Pure Substances and Mixtures
Pure substance: Has a fixed composition and definite properties. Examples: gold, water (H₂O), sodium chloride (NaCl).
Mixture: Contains two or more pure substances mixed in any proportion. Properties vary with composition. Examples: air, soil, sea water.
Types of Mixtures
Homogeneous mixture (Solution): Uniform composition throughout. Particles are not visible. Examples: salt in water, air, alloys.
Heterogeneous mixture: Non-uniform composition. Different components are visible. Examples: oil and water, sand and iron filings.
Solutions
A solution has two components: solute (substance being dissolved — present in smaller amount) and solvent (substance doing the dissolving — present in larger amount).
Properties of a solution: Homogeneous, particles smaller than 1 nm (not visible), stable, does not scatter light, cannot be separated by filtration.
Concentration: Mass by mass % = (Mass of solute / Mass of solution) × 100
Suspensions and Colloids
Suspension: Heterogeneous mixture with large particles (> 100 nm) that settle down on standing. Can be separated by filtration. Scatters light. Example: muddy water.
Colloid: Appears homogeneous but is heterogeneous. Particle size between 1 nm and 100 nm. Does not settle, cannot be separated by filtration, but scatters light (Tyndall effect). Examples: milk, fog, smoke, blood.
Separation Methods
- Evaporation: Separates a volatile solvent from a non-volatile solute (salt from sea water).
- Centrifugation: Separates suspended particles from liquid using a centrifuge (cream from milk).
- Separating funnel: Separates immiscible liquids of different densities (oil and water).
- Sublimation: Separates a sublimable substance from a non-sublimable one (camphor from salt).
- Chromatography: Separates components based on differential rates of movement through a medium (dyes in ink).
- Distillation: Separates miscible liquids with different boiling points.
- Fractional distillation: Separates miscible liquids with close boiling points (petroleum).
- Crystallisation: Obtaining pure solid from a solution (pure copper sulphate).
Elements and Compounds
Element: A substance that cannot be broken down into simpler substances by chemical means. Examples: iron, oxygen, carbon.
Compound: A substance formed by chemical combination of two or more elements in a fixed ratio. Properties differ from constituent elements. Example: water (H₂O) is formed from hydrogen and oxygen.
Summary
Matter can be classified as pure substances (elements and compounds) or mixtures (homogeneous and heterogeneous). Solutions, colloids, and suspensions differ in particle size and properties. Various separation techniques are used based on the physical and chemical properties of the components.
Important Terms
- Solution: A homogeneous mixture of two or more substances
- Colloid: A mixture with particle size between 1-100 nm showing Tyndall effect
- Tyndall Effect: Scattering of light by colloidal particles
- Chromatography: Technique to separate components based on differential adsorption
- Distillation: Separation based on difference in boiling points
- Crystallisation: Obtaining pure crystals of a solid from its solution
Quick Revision
- Pure substances: Elements (cannot be broken down) and Compounds (fixed ratio)
- Solution particles < 1 nm; Colloid 1-100 nm; Suspension > 100 nm
- Tyndall effect shown by colloids, not by true solutions
- Distillation separates liquids with different boiling points
- Chromatography separates components based on different solubilities