Separation and purification techniques for mixtures

Atomic structure and the periodic table • Atomic models and isotopes

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Why might fractional distillation be chosen over simple distillation?

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Fractional distillation is chosen when boiling point differences are small and higher separation efficiency is required.

Key concepts

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Filtration

Filtration separates an insoluble solid from a liquid by passing the mixture through a porous medium. The solid particles become trapped on the filter paper while the liquid passes through, producing a filtrate. Particle size and filter pore size limit effectiveness; very fine suspensions require centrifugation or fine filters.

Evaporation and Crystallisation

Evaporation removes a volatile solvent by heating so that the solute remains as a solid. Heating increases vapour pressure of the solvent; therefore the solvent leaves and the solute concentrates. Crystallisation relies on cooling or controlled evaporation to reduce solubility; therefore crystals form as the solution becomes supersaturated. Thermal decomposition of solutes and solubility curves limit applicability.

Simple Distillation

Simple distillation separates a liquid from a non-volatile solute or from another liquid with a large boiling point difference. Heating causes the more volatile component to vaporise first; the vapour condenses and collects as distillate. A large boiling point gap increases purity; small differences reduce effectiveness and require fractional distillation.

Fractional Distillation

Fractional distillation separates two or more miscible liquids with closer boiling points using a fractionating column. Repeated vaporisation and condensation along the column cause enrichment of the lower boiling component in the vapour phase; therefore components separate more completely. Column efficiency, boiling point differences and azeotrope formation constrain separation.

Paper and Thin-Layer Chromatography

Chromatography separates dissolved substances by differential affinity between a mobile phase and a stationary phase. Components that bind more strongly to the stationary phase move more slowly; therefore spots separate along the paper or plate. Solvent choice, polarity differences and adsorption strength determine resolution; closely similar substances may require different solvent systems or columns.

Separating Funnel and Decanting

A separating funnel separates immiscible liquids by density difference. The denser layer settles to the bottom and drains away when the tap opens; therefore the two liquids separate. Decanting pours off a liquid from a settled solid or from less dense liquid; therefore it provides a simple separation when layers are clearly defined. Emulsions and small density differences limit effectiveness.

Sieving and Mechanical Separation

Sieving separates solid particles by size through a mesh or sieve. Larger particles remain above the mesh while smaller particles pass through; therefore particle size distribution determines separation quality. Mechanical techniques suit coarse solid mixtures and fail for very fine particles.

Centrifugation

Centrifugation applies rapid rotation to produce a strong outward force that accelerates sedimentation of dense particles. Denser components move outward and form a pellet; therefore separation occurs much faster than by gravity alone. Balance, rotor speed and thermal sensitivity of samples constrain use.

Magnetic Separation

Magnetic separation removes magnetic materials from a mixture using a magnet. Magnetic particles experience an attractive force and separate from non-magnetic material; therefore iron or steel contaminants separate quickly from sand. Non-magnetic particles require different methods.

Choosing a Suitable Technique

Selection depends on measurable properties. Insoluble solids and liquids indicate filtration followed by evaporation or crystallisation. Liquids with large boiling point differences indicate simple distillation; smaller differences indicate fractional distillation. Mixtures of dissolved organic compounds with polarity differences indicate chromatography. Heat-sensitive substances require low-temperature or non-thermal methods. Required purity, sample size and available equipment also determine choice.

Key notes

Important points to keep in mind

Select method based on particle size, solubility, boiling point and polarity.

Use filtration for insoluble solids; follow with evaporation or crystallisation for dissolved solutes.

Choose simple distillation for large boiling point differences and fractional distillation for small differences.

Avoid heating methods for heat-sensitive substances; consider chromatography or low-temperature techniques.

Chromatography resolves similar compounds by differing affinities to stationary and mobile phases.

Magnetic separation works only for magnetic materials; use other methods for non-magnetic components.

Centrifugation speeds up sedimentation but requires balanced rotors and suitable equipment.

Azeotropes prevent complete separation by simple distillation; alternative methods or additives may be necessary.

Controlled cooling in crystallisation yields purer crystals than rapid cooling.

Consider required purity, sample size, cost and equipment availability when selecting a technique.