Separating Mixtures

Why this skill matters

Separation techniques form the basis of Topic 1 practical work and are tested across Papers 2-6. Questions may ask you to name the technique, describe the method, draw the apparatus, or explain why a particular technique works. Choosing the wrong technique for a given mixture is a direct mark loss.

Method and technique

Decision flowchart

1. Is the mixture a solid in a liquid?

   • Is the solid insoluble? Use filtration to separate, then dry the residue.
   • Is the solid dissolved? Use evaporation (to dryness for stable salts) or crystallisation (for hydrated salts that decompose on heating).

2. Is the mixture two liquids?

   • Are they immiscible (do not mix, form layers)? Use a separating funnel.
   • Are they miscible with very different boiling points? Use simple distillation.
   • Are they miscible with similar boiling points? Use fractional distillation.

3. Is the mixture of coloured or dissolved substances you need to identify? Use paper chromatography.

Filtration

Separates an insoluble solid from a liquid.

1. Fold filter paper into a cone and place in a filter funnel.
2. Place the funnel in a conical flask or beaker.
3. Pour the mixture into the filter paper. The liquid (filtrate) passes through; the solid (residue) stays in the paper.
4. Wash the residue with distilled water to remove impurities. Dry in an oven or warm place.

Evaporation and crystallisation

Separates a dissolved solid from a solution.

Evaporation to dryness: Heat the solution in an evaporating basin until all the water has evaporated. Suitable only for stable, non-hydrated salts (like NaCl).

Crystallisation: Heat the solution gently to evaporate some water. Test by dipping a glass rod into the solution: if crystals form on the rod when it cools, the solution is saturated. Remove from heat and allow to cool slowly. Filter the crystals, wash with cold water, and dry. Suitable for hydrated salts (like CuSO4.5H2O) that would decompose if heated to dryness.

Distillation

Separates liquids based on boiling point differences.

Simple distillation: Heat the mixture in a flask. The component with the lower boiling point boils first, its vapour travels into the condenser, cools, and condenses back to liquid, which is collected in a separate container. Monitor the thermometer to ensure you collect at the right temperature.

Fractional distillation: The same principle but with a fractionating column packed with glass beads between the flask and the condenser. The column provides repeated evaporation-condensation cycles, giving much better separation of liquids with close boiling points.

Chromatography

See the separate chromatography experiment page for full method and Rf calculation.

Separating funnel

For immiscible liquids (e.g., oil and water): pour the mixture into a separating funnel, allow the layers to settle, then open the tap to drain the denser layer (bottom) into a beaker. Close the tap and pour the upper layer from the top.

Common errors and how to avoid them

Choosing distillation when filtration is needed: If the question says “an insoluble solid in water”, filtration is the answer. Distillation separates dissolved substances or liquids.

Confusing evaporation and crystallisation: Evaporation to dryness works for NaCl but destroys CuSO4.5H2O. If the salt is hydrated, use crystallisation with slow cooling.

Forgetting the condenser in distillation: Without a condenser, the vapour escapes and you collect nothing. Always describe the condenser with cold water flowing through it.

Marking points examiners look for

• Correct technique named for the given mixture type
• Key apparatus described (filter paper, condenser, fractionating column, separating funnel)
• Method described step by step
• Explanation of why the technique works (based on physical properties)
• Residue vs filtrate correctly identified in filtration