Separating Mixtures (Oxford AQA IGCSE Combined Science Double Award)

Distillation

What is a mixture? 

• A mixture consists of two or more elements or compounds not chemically joined together 

• They can therefore be separated by physical means

• The chemical nature of each element or compound within the mixture is unchanged

• The choice of the method of separation depends on the nature of the substances being separated

• All methods rely on there being a difference in a physical property such as the boiling point or solubility, between the substances being separated

Distillation

• This is used to separate a liquid and soluble solid from a solution (e.g. water from a solution of salt water) or a pure liquid from a mixture of liquids

• The solution is heated and a liquid evaporates producing a vapour which rises through the neck of the round-bottomed flask (e.g. for saltwater, this would be water boiling at 100 ^oC)

• The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker

• After all the liquid is evaporated from the solution, only the solid solute will be left behind

Distillation Set Up

Filtration

• Filtration is used to separate an undissolved solid from a mixture of the solid and a liquid / solution

  • E.g. sand from a mixture of sand and water

• A piece of filter paper is placed in a filter funnel above a beaker

• The mixture of insoluble solid and liquid is poured into the filter funnel

• The filter paper will only allow liquid particles and soluble solids to pass

  • The liquid that passes through into the beaker is called the filtrate

• Insoluble solids do not pass through the filter paper and are left behind as a residue

Filtration Set Up

Crystallisation

• Crystallisation is used to separate a dissolved solid from a solution when the solid is much more soluble in hot solvent than in cold 

  • E.g. copper sulphate from a solution of copper (II) sulphate in water

• The solution is heated which allows the solvent to evaporate and leaves a saturated solution 

• To test if the solution is saturated a clean, dry, cold glass rod is dipped into the solution

  • If the solution is saturated, crystals will form on the glass rod

• The saturated solution is then allowed to cool slowly

• Crystals begin to grow as solids come out of solution due to decreasing solubility

• The crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are then allowed to dry

Crystallisation Set Up