Separation Techniques

Filtration separates an INSOLUBLE solid from a liquid. The mixture is poured through filter paper — the solid is caught on the paper (the residue); the liquid passes through (the filtrate).

Use when:

• Sand in water → filter; sand stays on paper.
• Tea leaves in tea → strainer (a coarse filter).
• Particles in muddy river water → makes water look clearer.

Evaporation (crystallisation) recovers a SOLUBLE solid from solution. Heat the solution gently in an evaporating basin until most of the water boils off. The dissolved solid is left behind as crystals.

Use when:

• Salt from salt water (a coastal salt-evaporation pond).
• Sugar from a sugar solution.
• Pure copper sulfate crystals from a CuSO₄ solution.

For best crystals: evaporate slowly. Fast evaporation gives small powdery crystals; slow gives larger, more regular ones.

Simple distillation separates a liquid from a dissolved solid. The mixture is heated; the liquid boils, the vapour rises, passes through a Liebig condenser (cooled by surrounding water), and condenses back into a clean liquid in the receiver.

Use when:

• Pure water from salt water (desalination).
• Distilled water for batteries.
• The component you want is the LIQUID.

Fractional distillation separates TWO OR MORE liquids that have different boiling points. It uses a tall fractionating column packed with glass beads. As vapour rises, lower-BP liquid passes through and reaches the condenser first; higher-BP liquid condenses back down.

Use when:

• Separating ethanol from water (ethanol BP 78 °C, water 100 °C).
• Separating crude oil into fractions (petrol, kerosene, diesel, etc.) at an oil refinery.
• Producing pure liquid oxygen and nitrogen from liquid air.

Use this decision tree:

1. One solid + one liquid, solid INSOLUBLE in liquid → filtration.
2. Solid dissolved in liquid, want solid → evaporation.
3. Solid dissolved in liquid, want liquid → simple distillation.
4. Two liquids, different BPs → fractional distillation.
5. Magnetic and non-magnetic solids → magnetism.
6. Heavy and light particles in liquid (settling) → decanting / centrifuging.
7. Coloured substances on paper → chromatography (covered in the next subtopic).

Examples in industry:

• Sewage treatment uses filtration + sedimentation + biological breakdown.
• Petroleum refineries use fractional distillation as the very first major step.
• Salt production uses evaporation (sun-driven in salt pans, or boiling).
• Sugar refining uses crystallisation.
• Air separation (for liquid O₂, liquid N₂) uses fractional distillation of liquefied air.

MYP criterion B/C lab work involves choosing the right method for a given mixture, drawing apparatus correctly, and processing the products.