Separation and purification Revision Notes

A suitable solvent is chosen based on the solubility of the substances involved. When separating a mixture, one component should dissolve in the solvent while the other does not. This difference allows separation by dissolving the soluble component and leaving the insoluble behind.

For example, to separate a mixture of salt and sand, water is used as the solvent because salt dissolves in water but sand does not.

The choice of solvent affects the efficiency of separation and purity of the product. The solvent should not react chemically with the substances and should be easy to remove after separation, often by evaporation or crystallisation.

• Solvent must dissolve one component but not the other(s).
• Should be chemically inert with respect to the substances.
• Should be easy to remove after separation (e.g., by evaporation).

Filtration separates an insoluble solid from a liquid or gas by passing the mixture through a filter medium that allows only the fluid to pass.

In the laboratory, filter paper folded into a cone is placed in a funnel. The mixture is poured through the filter paper. The solid particles are trapped on the paper as the residue, while the liquid passes through as the filtrate.

Filtration is commonly used to separate sand from water or to remove precipitates from reaction mixtures.

• Apparatus: funnel, filter paper, beaker or flask to collect filtrate.
• Procedure: fold filter paper, place in funnel, pour mixture slowly.
• Observation: solid remains on filter paper; clear liquid collected below.
• Residue = solid left on filter paper; filtrate = liquid collected.

Crystallisation purifies a solid dissolved in a solvent by exploiting differences in solubility at different temperatures.

The solution is heated to dissolve the solid completely, then allowed to cool slowly. As the solution cools, the solubility decreases and pure crystals form, leaving impurities in the solution.

The crystals are collected by filtration and dried. This method is used to obtain pure samples of soluble salts from their solutions.

• Heat solution to dissolve solute completely.
• Allow solution to cool slowly to form crystals.
• Filter to separate crystals from remaining solution (mother liquor).
• Dry crystals to remove surface solvent.
• Impurities remain dissolved in the mother liquor.

Simple distillation separates a liquid from a solution based on differences in boiling points.

The mixture is heated until the component with the lower boiling point vaporises. The vapour passes through a condenser where it cools and condenses back to liquid, collected in a separate container.

This method is suitable when the boiling points of the components differ significantly (usually more than 25°C). For example, separating pure water from salt solution.

• Apparatus: distillation flask, heat source, condenser, receiver.
• Heat mixture gently to boil the more volatile liquid.
• Vapour condenses in the condenser and is collected as distillate.
• Residue remains in the distillation flask.
• Suitable for liquids with large boiling point differences.

Fractional distillation separates mixtures of liquids with closer boiling points (less than 25°C difference).

It uses a fractionating column between the distillation flask and condenser. The column provides a large surface area for repeated vaporisation-condensation cycles, improving separation.

The vapour rises through the column, cooling and condensing repeatedly. The component with the lower boiling point reaches the top first and passes into the condenser to be collected.

• Apparatus: distillation flask, fractionating column, condenser, receiver.
• Fractionating column allows multiple vaporisation-condensation cycles.
• Separates liquids with similar boiling points (e.g., ethanol and water).
• Collect fractions at different temperatures corresponding to boiling points.

Filtration is used to separate an insoluble solid from a liquid. It relies on the difference in particle size: solid particles are trapped by filter paper, while the liquid passes through.

Typical apparatus includes filter paper folded into a cone, placed in a funnel, with a beaker below to collect the filtrate.

The mixture is poured onto the filter paper. The solid residue remains on the paper, and the clear liquid (filtrate) is collected below.

• Used when solid does not dissolve in the liquid.
• Common in separating sand from water.
• Filter paper pore size must be appropriate to trap solid particles.
• Example: Separating sand from salt solution before evaporating the solution to obtain salt crystals.

Crystallisation separates a soluble solid from a solution by exploiting differences in solubility at different temperatures.

The solution is gently heated to evaporate some solvent until it becomes saturated, then cooled to allow pure crystals to form.

Crystals are collected by filtration, washed with cold solvent to remove impurities, and dried.

• Used to purify solids dissolved in liquids.
• Requires careful control of temperature to avoid impurities.
• Example: Purifying impure copper sulfate by crystallisation from aqueous solution.
• Avoid overheating to dryness to prevent decomposition of crystals.

Evaporation involves heating a solution to remove the solvent as vapour, leaving the dissolved solid behind.

It is suitable when the solid does not decompose on heating and when the solvent has a relatively low boiling point.

Unlike crystallisation, evaporation continues until all solvent is gone, so crystals do not form but a solid residue remains.

• Used to obtain solids from solutions where crystallisation is not required.
• Example: Obtaining solid salt from saltwater by evaporating water.
• Care must be taken to avoid overheating and decomposition of the solid.

Distillation separates liquids with different boiling points by heating the mixture to vaporise the more volatile component, then condensing the vapour back to liquid.

Simple distillation is used when boiling points differ significantly (usually >25°C).

Fractional distillation is used when boiling points are closer, using a fractionating column to improve separation.

• Simple distillation apparatus includes a distillation flask, condenser, and receiver.
• Fractional distillation uses a fractionating column packed with glass beads to provide surface area for repeated condensation and vaporisation cycles.
• Example: Separating ethanol (bp 78°C) from water (bp 100°C) by fractional distillation.
• Observation: Distillate collected changes composition as temperature rises during fractional distillation.

Chromatography separates substances based on their different affinities for a stationary phase and a mobile phase.

Common types include paper chromatography and thin-layer chromatography (TLC).

Substances move at different rates, producing distinct spots or bands that can be identified by their retention factor (Rf).

• Used to separate mixtures of coloured or colourless substances.
• Rf value = distance moved by substance ÷ distance moved by solvent front.
• Example: Separating plant pigments or inks.
• Requires careful spotting and solvent choice for effective separation.

The melting point of a pure solid is the fixed temperature at which it changes state from solid to liquid under atmospheric pressure. Similarly, the boiling point of a pure liquid is the fixed temperature at which it changes state from liquid to gas at atmospheric pressure.

Pure substances have characteristic melting and boiling points that are sharp and reproducible. For example, pure water melts at 0°C and boils at 100°C at 1 atm pressure.

These fixed points are due to the uniform structure and consistent intermolecular forces in pure substances, requiring a specific amount of energy to change state. The melting and boiling points are physical constants used to identify substances.

• Melting point: temperature solid → liquid at 1 atm
• Boiling point: temperature liquid → gas at 1 atm
• Pure substances have sharp, fixed melting and boiling points
• These points depend on intermolecular forces and molecular structure

Impurities disrupt the regular lattice of a solid and the intermolecular interactions in liquids, altering melting and boiling points.

Impurities lower and broaden the melting point range because they weaken the solid structure, causing melting to start at a lower temperature and occur over a wider range.

Impurities raise the boiling point because they cause boiling point elevation by reducing the vapour pressure of the liquid, requiring a higher temperature to boil (a colligative property).

• Impurities lower melting point and broaden melting range
• Impurities raise boiling point above that of pure substance
• Melting point depression and boiling point elevation indicate impurity presence

Melting point is measured using a melting point apparatus or a Thiele tube. A small sample of solid is packed into a capillary tube and heated slowly. The temperature at which the solid starts to melt and when it completely melts is recorded to determine the melting range.

Boiling point is measured by heating a liquid in a distillation apparatus with a thermometer placed just above the liquid surface. The temperature stabilises at the boiling point when the liquid boils steadily.

Key variables include heating rate (slow heating improves accuracy), atmospheric pressure (should be standard or noted), and sample purity. Repeated measurements improve reliability.

• Melting point apparatus or capillary tube and Thiele tube used
• Boiling point measured by distillation with thermometer
• Slow heating rate for accurate melting point
• Record start and end of melting to find melting range
• Note atmospheric pressure during boiling point measurement

Measured melting and boiling points can be compared with known values from data tables to identify unknown substances.

A close match in melting and boiling points indicates the substance is likely pure and correctly identified.

If the melting point is lower and broader than expected, or the boiling point is higher, the sample is impure or a mixture, affecting identification accuracy.

• Compare measured points with reference data
• Close match indicates pure substance and correct identity
• Deviations suggest impurities or incorrect identification

A solid sample melts between 128°C and 132°C. The known melting point of pure benzoic acid is 122°C.

The sample’s melting point is higher and the range is narrow, suggesting it is not pure benzoic acid but possibly a different compound or a mixture.

If the sample melted between 118°C and 122°C, the lower and broader range would indicate impurities in benzoic acid.

• Sample melting point range: 128–132°C
• Pure benzoic acid melting point: 122°C
• Higher melting point and narrow range suggests different substance
• Lower and broader melting range indicates impurities