Separation and Purification Techniques

Mixtures contain two or more substances jumbled together. To separate them, choose a method based on their properties, such as solubility and boiling point. A pure substance has fixed melting and boiling points.

Choosing a method

• Solubility: does it dissolve in a solvent (like water)?
• State/size: is it a solid mixed with a liquid?
• Boiling point: how easily does it evaporate?
• Colour: can spots be separated by chromatography?

Core techniques

Using a suitable solvent

Dissolve only the substance you want. Example: sugar + sand. Sugar dissolves in water, sand does not. Stir, then filter to remove sand. Finally, crystallise sugar from the solution.

Filtration

Separates an insoluble solid from a liquid using filter paper. The solid left is the residue. The liquid that passes through is the filtrate. Example: sand from water.

Crystallisation

Obtains pure crystals of a dissolved solid. Heat gently to evaporate some solvent until the solution is concentrated, then cool to form crystals. Filter and dry between filter papers. Do not evaporate to complete dryness if the salt is hydrated.

Simple distillation

Separates a solvent from a solution, or liquids with very different boiling points. The liquid boils, vapour passes into a condenser, and cools to a liquid (distillate). Example: water from salty water.

Fractional distillation

Separates miscible liquids with close boiling points using a fractionating column. Lower-boiling liquid distils first. Example: ethanol (≈78°C) from water (100°C).

Paper chromatography

Separates soluble coloured substances. Draw a pencil baseline, place small spots, dip the paper so the solvent is below the spots, let the solvent rise. Different substances travel different distances.

Use the Rf value to compare substances:

[ R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}} ]

Assessing purity

• A pure substance melts and boils at a single, sharp temperature.
• Impurities lower the melting point, raise the boiling point, and cause a wider temperature range.

Quick guide: which method?

• Insoluble solid + liquid → Filtration
• Soluble solid + liquid → Crystallisation (to get solid) or Simple distillation (to get solvent)
• Two miscible liquids → Simple or Fractional distillation (use fractional if boiling points are close)
• Coloured dissolved substances → Paper chromatography
• Mixture where only one part dissolves → Suitable solvent + Filtration + Crystallisation