2.1.5 Separation Techniques

Filtration

• 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)
• Centrifugation can also be used for this mixture
• A piece of filter paper is placed in a filter funnel above a beaker
• A mixture of insoluble solid and liquid is poured into the filter funnel
• The filter paper will only allow small liquid particles to pass through as filtrate
• Solid particles are too large to pass through the filter paper so will stay behind as a residue

Crystallisation

• 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, allowing the solvent to evaporate, leaving a saturated solution behind
• Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution
  • If the solution is saturated, crystals will form on the glass rod
• The saturated solution is allowed to cool slowly
• Crystals begin to grow as solids will 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

Simple 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 pure water evaporates producing a vapour which rises through the neck of the round bottomed flask
• 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 water is evaporated from the solution, only the solid solute will be left behind

Fractional Distillation

• This is used to separate two or more liquids that are miscible with one another (e.g., ethanol and water from a mixture of the two)
• The solution is heated to the temperature of the substance with the lowest boiling point
• This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker
• All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture
• For water and ethanol
  • Ethanol has a boiling point of 78 ºC and water of 100 ºC
  • The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker

• When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated

• You might be given an example of a mixture and asked to work out which separation techniques would be required 

Answer:

• • Filtration to separate sand (undissolved solid) and water
  • Crystallisation to separate the salt (dissolved solid) and water