Section 1 of 5Choosing a Method
The best separation method depends on whether the wanted part is a solid or a liquid, and on its solubility and boiling point.
Filtration
- Separates an insoluble solid from a liquid: the solid stays on the filter paper as the residue and the liquid passes through as the filtrate.
Crystallisation
- Separates a dissolved solid from its solution by evaporating some solvent, cooling the saturated solution and filtering off the crystals.
Simple distillation
- Separates a solvent from a dissolved solid,
- or a liquid from a mixture of liquids with very different boiling points, by evaporating and condensing.
Fractional distillation
- Separates two or more miscible liquids with different boiling points by collecting each vapour at its own boiling temperature.
Section 2 of 5Steps of Crystallisation
Crystallisation gives pure crystals of a solid that is much more soluble in hot solvent than in cold.
1
Heat the solution to evaporate some of the solvent, which leaves a saturated solution behind.
2
Test whether the solution is saturated by dipping a clean, dry, cold glass rod into it and seeing whether crystals form on the rod.
3
Allow the saturated solution to cool slowly, so that crystals grow as the solubility falls.
4
Filter off the crystals, rinse them with a small amount of cold distilled water to remove impurities, and then leave them to dry.
Section 3 of 5Distillation Apparatus
Distillation Apparatus
Simple set-up
- The solution is heated in a flask, the vapour rises into a condenser, and the cooled liquid is collected in a beaker.
Water flow
- Cold water enters at the bottom of the condenser near the beaker and leaves at the top,
- so the condenser stays cold where the vapour condenses.
Fractionating column
- A column between the flask and condenser lets the vapour of the higher-boiling liquid condense and fall back, improving separation.
Ethanol and water
- Ethanol boils at about 78 °C and water at 100 °C, so heating to 78 °C collects the ethanol first and separates it from the water.
Section 4 of 5Worked Example
Question
- A mixture contains a soluble salt dissolved in water.
- Describe how to obtain dry crystals of the salt.
- Heat the solution to evaporate some water and form a saturated solution.
- Test for saturation by dipping a cold glass rod into the solution.
- Allow the solution to cool slowly so that crystals form, then filter off the crystals.
- Rinse the crystals with a little cold distilled water and leave them to dry.
Answer Use crystallisation: evaporate to saturation, cool slowly, filter, wash with cold distilled water and dry.
Section 5 of 5Exam Checklist & Tips
1.2.3 – Key Points for the Exam
- Filtrate The liquid that passes through the filter paper.
- Residue The solid left behind on the filter paper.
- Crystallisation Separating a dissolved solid by making a saturated solution and cooling it so that crystals form.
- Condenser Apparatus that cools vapour so that it turns back into a liquid.
- Miscible Describes liquids that mix together completely to form one layer.
- Fractional distillation Separation of miscible liquids by collecting each vapour at its own boiling temperature.
Common Mistakes
- Mixing up filtrate and residue;
- the residue is the solid left on the paper, and the filtrate is the liquid that passes through.
- Connecting the water to the top of the condenser, which stops the condenser cooling the vapour efficiently.
- Evaporating all of the solvent when collecting crystals;
- only partial evaporation is needed to form a saturated solution.
- Using simple distillation for two liquids with close boiling points, which needs fractional distillation to separate them properly.