Separation Techniques
~8 min read
- Physical: Filtration, decantation, sedimentation, evaporation, distillation, sublimation, chromatography, centrifugation, magnetic separation.
- By property: Each technique exploits a property — particle size, density, boiling point, polarity.
- Industrial: Distillation — petroleum into fractions. Centrifugation — milk into cream and skim. Crystallisation — sugar from cane juice.
Mixtures are separated by exploiting physical differences between components. NDA tests which method for which mixture.
Common Separation Techniques
| Technique | Use | Based on |
|---|---|---|
| Filtration | Solid from liquid (e.g., sand from water) | Particle size |
| Decantation | Pouring off lighter liquid from sediment | Density |
| Sedimentation | Letting heavier solids settle | Density, time |
| Evaporation | Soluble solid from liquid (salt from sea water) | Volatility |
| Distillation | Two miscible liquids with different BP (alcohol-water) | Boiling point |
| Fractional distillation | Multiple liquids — petroleum refining | Boiling point |
| Sublimation | Sublimable substance from mixture (camphor, naphthalene from common impurities) | Sublimation |
| Crystallisation | Purification of soluble solids | Solubility vs temperature |
| Chromatography | Separation of similar substances (ink colours) | Adsorption / polarity |
| Centrifugation | Solid suspended in liquid (cream from milk; blood components) | Density at high spin |
| Magnetic separation | Iron from other materials | Magnetic property |
| Winnowing | Grain from chaff using wind | Density / drag |
Distillation in Detail
- Used to separate two miscible liquids with different boiling points.
- The mixture is heated; the more volatile liquid (lower BP) boils first, vaporises, then condenses in a separate container.
- Simple distillation: Works if BP difference > 25-30°C.
- Fractional distillation: Fractionating column with multiple condensation/vaporisation steps. Works for liquids with smaller BP gap.
- Industrial application: Petroleum refining — crude oil distilled into fractions (LPG, petrol, kerosene, diesel, lubricants, residue/bitumen).
- Alcohol distillation: Fermented liquid (ethanol + water) distilled to concentrate ethanol.
Chromatography
- Separates components of a mixture based on their differential adsorption on a stationary phase as they move with a mobile phase.
- Paper chromatography: Common demonstration. Drop ink on filter paper, let solvent rise. Different pigments travel different distances.
- Thin layer chromatography (TLC): Same idea, on silica-coated plate.
- Column chromatography: For larger scale and complex mixtures.
- Gas chromatography (GC), HPLC: Analytical lab techniques.
NDA PYQ Examples
Q: Cream is separated from milk by:
(a) Filtration (b) Evaporation (c) Centrifugation (d) Distillation
Answer: (c) Centrifugation — exploits density difference at high spin.
Q: Which technique separates a mixture of camphor and salt?
(a) Filtration (b) Sublimation (c) Evaporation (d) Distillation
Answer: (b) Sublimation — camphor sublimes; salt does not.
Q: Petroleum is refined by:
(a) Simple distillation (b) Filtration (c) Fractional distillation (d) Chromatography
Answer: (c) Fractional distillation — into petrol, kerosene, diesel, etc.
Q: Which separates pigments in ink?
(a) Filtration (b) Crystallisation (c) Chromatography (d) Sublimation
Answer: (c) Chromatography.
Drill Separation Techniques for NDA
NDA-pattern items on Separation Techniques with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why is fractional distillation used for petroleum?
Petroleum is a complex mixture of hundreds of hydrocarbons with overlapping boiling points. Fractional distillation in a tall column allows multiple vaporisation-condensation cycles, separating the mixture into useful fractions — gas, naphtha, gasoline, kerosene, diesel, lubricants, bitumen.
How does centrifugation separate milk?
Spinning at high speed creates strong centrifugal force. Denser components (skim milk) move outward; lighter components (cream/fat) stay near the centre. Industrial dairies use centrifugal separators to standardise milk fat content.
Can sea water be made drinkable?
Yes — by distillation (evaporate water, condense it back, leave salt behind) or by reverse osmosis (pressure pushes water through a semi-permeable membrane). Desalination plants in Gulf countries, Israel, Singapore use these methods at scale.
Why does paper chromatography separate ink colours?
Different pigments have different affinities for the paper (stationary phase) vs the solvent (mobile phase). Pigments more attracted to solvent travel farther; those more attracted to paper stay near origin. Result: visual separation into bands of different colours.
How is salt obtained from sea water?
Sea water is led into shallow ponds and allowed to evaporate in the sun. As water evaporates, salt crystallises out. Used since ancient times in coastal India (Marakkanam, Mumbai, Sambhar inland lake).