Separation and Purification

1. Separation and purification using different techniques

(a) Separation using a suitable solvent

• Principle: Different substances have different solubilities in a given solvent.
• Purpose: To separate a soluble substance from an insoluble one, or to selectively dissolve one component from a mixture.
• Steps:
  1. Add the chosen solvent to the mixture.
  2. Stir/shake to dissolve the soluble substance.
  3. Use filtration to remove the insoluble component.
  4. Evaporate the solvent from the filtrate to obtain the pure substance.
• Example:
  • Mixture of salt and sand – add water → salt dissolves, sand remains insoluble → filter → evaporate water to get pure salt.
• Key points:
  • Choice of solvent is important – it should dissolve the target substance but not others.
  • Can be used in extraction processes in chemistry and industry.

(b) Filtration

• Principle: Separates an insoluble solid from a liquid or solution.
• Apparatus: Filter funnel, filter paper, beaker/conical flask.
• Steps:
  1. Place folded filter paper in a funnel.
  2. Pour the mixture into the filter.
  3. Liquid passes through the paper (filtrate).
  4. Solid remains on the paper (residue).
• Applications:
  • Removing sand from water.
  • Removing precipitate from a reaction mixture.
• Limitations:
  • Cannot separate dissolved substances from a liquid – requires evaporation, distillation, or crystallisation.

(c) Crystallisation

• Principle: Obtains pure solid from its solution by forming crystals as the solvent evaporates.
• Steps:
  1. Prepare a solution of the substance in a suitable solvent.
  2. Heat gently to evaporate part of the solvent until the solution is concentrated.
  3. Test by dipping a clean glass rod – crystals form on cooling.
  4. Leave the hot saturated solution to cool slowly → crystals form.
  5. Filter crystals from the solution.
  6. Dry crystals between filter papers or in a warm oven.
• Applications:
  • Purifying copper(II) sulfate crystals from an impure solution.
  • Producing pure sugar crystals.
• Key points:
  • Cooling slowly produces larger, purer crystals.
  • Rapid cooling produces small crystals and may trap impurities.

(d) Simple distillation

• Principle: Separates a solvent from a solution (or a liquid from solids dissolved in it).
• Apparatus: Distillation flask, heat source, thermometer, condenser, receiving flask.
• Steps:
  1. Solution is heated in a distillation flask.
  2. Solvent evaporates at its boiling point.
  3. Vapour passes into the condenser, where it is cooled and collected as a liquid (distillate).
  4. Non-volatile solutes remain in the flask.
• Applications:
  • Purifying water from seawater.
  • Separating ethanol from water if boiling points differ significantly.
• Limitations:
  • Not efficient for separating mixtures of liquids with close boiling points – requires fractional distillation.

(e) Fractional distillation

• Principle: Separates a mixture of two or more miscible liquids with different boiling points.
• Extra equipment: Fractionating column between distillation flask and condenser.
• Steps:
  1. Heat the mixture slowly.
  2. Vapour rises up the fractionating column.
  3. Liquids with lower boiling points condense higher up and pass into the condenser first.
  4. Liquids are collected separately as the temperature rises.
• Applications:
  • Separation of ethanol and water in industrial processes.
  • Fractional distillation of crude oil into petrol, kerosene, diesel, etc.
• Key points:
  • Fractionating column improves separation by repeated condensation and evaporation cycles.
  • Thermometer used to monitor the boiling point and identify fractions.

2. Choosing suitable separation and purification techniques

The correct method depends on:

• State of the substances (solid, liquid, dissolved).
• Solubility in different solvents.
• Boiling points.
• Purity requirements.

Examples:

• Salt + sand: Dissolve in water → filter → evaporate water (solvent extraction + filtration + evaporation).
• Sugar solution: Heat to concentrate → crystallisation.
• Ethanol + water: Fractional distillation.
• Oil + water: Use separating funnel (immiscible liquids).

3. Testing purity of substances

Melting point

• Pure substances have a sharp melting point (melts at a single temperature).
• Impure substances melt over a range of temperatures and at lower temperatures than pure samples.
• Example: Pure ice melts at 0°C; salt-contaminated ice melts at lower temperatures over a range.

Boiling point

• Pure substances have a fixed boiling point.
• Impurities change the boiling point (raise for liquids, lower for solids dissolved in liquid).
• Example: Pure water boils at 100°C; salt water boils above 100°C.

Chromatography (secondary check)

• Pure substances give a single spot on a chromatogram.
• Impurities give multiple spots.

Why purity is important:

• In food, medicines, and chemicals, impurities can be harmful or reduce effectiveness.
• In industry, purity ensures consistent product quality and performance.