Mixtures and Separation Techniques
~8 min read
- Mixtures: Two or more substances physically combined. No fixed ratio. Components retain identity.
- Types: Homogeneous (solution — air, sugar in water). Heterogeneous (mixture with visible distinct phases — sand+water, oil+water).
- Separation: Filtration, evaporation, distillation, fractional distillation, sublimation, chromatography, centrifugation, magnetic separation, separating funnel.
Nature presents materials as mixtures. CDS/OTA tests how we separate them — filtration, distillation, chromatography and more — and the distinction between mixtures and pure compounds.
Mixtures, Compounds and Elements
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Composition | One type of atom | Fixed ratio by mass | Variable ratio |
| Made by | — | Chemical reaction | Physical mixing |
| Separation | — | Chemical methods only | Physical methods |
| Examples | Fe, Au, O₂ | H₂O, NaCl, CO₂ | Air, sea water, soil, brass |
Separation Techniques
| Technique | Used for | Example |
|---|---|---|
| Filtration | Insoluble solid from liquid | Tea leaves from tea; sand from water |
| Evaporation | Soluble solid from liquid (where solid is the product) | Salt from brine |
| Distillation | Liquid from solid; or two miscible liquids with very different BPs | Pure water from sea water |
| Fractional distillation | Mixture of miscible liquids with close BPs | Crude oil refining; liquefied air → N₂, O₂ |
| Sublimation | Solid that sublimes mixed with non-subliming solid | Camphor + salt; iodine + sand |
| Chromatography | Components of a solution that differ in adsorption | Ink colours, plant pigments |
| Centrifugation | Suspended particles by spinning | Cream from milk; blood plasma |
| Magnetic separation | Magnetic + non-magnetic mix | Iron filings from sulphur |
| Separating funnel | Immiscible liquids | Oil + water; petrol + water |
| Winnowing | Lighter chaff from heavier grain | Husk from wheat |
| Crystallisation | Pure solid from impure solid | Pure copper sulphate |
Solutions
- Solution = homogeneous mixture of solute (dissolved) in solvent (dissolves).
- Saturated solution: cannot dissolve more solute at given temperature.
- Concentration = mass or volume of solute per mass or volume of solution.
- Tyndall effect: scattering of light by colloidal particles makes the path of a beam visible (e.g., through fog or smoke). Distinguishes colloid from true solution.
- Colloid: particles 1-1000 nm. Examples — milk (fat in water), fog, smoke, butter, gel.
CDS/OTA PYQ Examples
Q: Separation of salt from sea water is by:
(a) Filtration (b) Evaporation (c) Sublimation (d) Distillation
Answer: (b) Evaporation.
Q: Crude oil is separated into petrol, diesel and kerosene by:
(a) Filtration (b) Distillation (c) Fractional distillation (d) Sublimation
Answer: (c) Fractional distillation.
Q: To separate iron filings from sulphur powder we use:
(a) Magnetic separation (b) Filtration (c) Distillation (d) Sublimation
Answer: (a) Magnetic separation.
Q: Tyndall effect is shown by:
(a) Pure water (b) Salt solution (c) Colloidal solution (d) Sugar solution
Answer: (c) Colloidal solution — scatters light.
Drill Mixtures and Separation Techniques for CDS/OTA
CDS/OTA-pattern items on Mixtures and Separation Techniques with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
How does fractional distillation differ from simple distillation?
Fractional distillation uses a vertical column packed with surfaces that allow multiple condensation-vaporisation cycles. This separates liquids whose boiling points are close (e.g. petrol 35-200 °C, diesel 200-350 °C from crude oil).
Why do we use chromatography to test for adulteration?
Components of an ink, dye or food colour separate at different rates on paper, revealing how many pigments are present and which ones.
Is air a compound or a mixture?
Mixture — composition of N₂ (~78%), O₂ (~21%), Ar (0.9%), CO₂ (~0.04%), and traces. Each retains its own properties; ratio varies slightly with altitude and pollution.