Q.Give a brief description of the principles of the following techniques taking an example in each case:
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Start your 14-day free trial to unlock the full solution →Separation techniques exploit differences in physical properties like solubility, boiling point, or affinity. Crystallisation separates a solid from a solution by evaporation; distillation separates liquids by boiling point; chromatography separates mixture components by differential migration on a stationary phase.
The Core Idea
Separation techniques are not random procedures — they are deliberate applications of physical chemistry. Every mixture contains components with distinct properties. The trick is to choose a technique that magnifies that difference until the components can be collected individually. We'll look at three classic methods, each built on a different property.
(a) Crystallisation
Principle: A solid dissolved in a solvent can be recovered in pure crystalline form by evaporating the solvent or by cooling a hot saturated solution. The key is that different solutes have different solubilities at a given temperature. When the solvent is slowly removed or the temperature is lowered, the solution becomes supersaturated, and the solute begins to form orderly crystals — leaving impurities behind in the mother liquor.
Why it works: Impurities are usually present in much smaller amounts. As the desired compound crystallises, the impurities remain dissolved because their concentration never reaches saturation. The slow, controlled process also excludes foreign particles from the growing crystal lattice.
Example: Purification of common salt (NaCl) from sea water. Sea water is evaporated in shallow ponds. Water evaporates, the salt concentration rises, and eventually pure NaCl crystallises out. The more soluble impurities (like MgCl₂ and CaCl₂) stay in the solution.
A common mistake is to think crystallisation is just "boiling off water." Rapid evaporation gives small, impure crystals. For purity, the process must be slow — large, well-formed crystals trap fewer impurities.
(b) Distillation
Principle: Distillation separates components of a liquid mixture based on differences in their boiling points. When the mixture is heated, the component with the lower boiling point vaporises first. The vapour is then cooled (condensed) and collected as a separate liquid.
Why it works: At any given temperature, the vapour above a liquid mixture is richer in the more volatile component (the one with the lower boiling point). By carefully controlling the temperature, you can selectively vaporise and collect each component.
Example: Separation of acetone (boiling point 56°C) and water (boiling point 100°C). When the mixture is heated to about 56°C, acetone vaporises, passes through a condenser, and is collected as pure liquid. Water remains behind in the distillation flask.
For liquids with close boiling points (difference < 25°C), simple distillation won't work well. You'd need fractional distillation, which uses a fractionating column to provide multiple condensation-vaporisation cycles — like in petroleum refineries separating crude oil into petrol, kerosene, and diesel.
(c) Chromatography …
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