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CGB IGCSE Chemistry
Separation Techniques
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Separation methods for mixtures
 
Sedimentation and Decantation - Solid particles dispersed in a liquid are allowed to settle.  The liquid is decanted (carefully poured off).  Equipment : depends on the size of the sample - test tube or beaker.
 
Filtration - Solid particles dispersed in a liquid can be separated by pouring liquid through a filter.  The solid particles will remain on the filter as a residue, the liquid (the filtrate) will pass through the filter to be collected.  If you are quantitatively filtering it is important to wash the residue with water.  Equipment : filter, filter paper, beaker or conical flask.
 
Separating Funnel: A separating funnel is used to separate immiscible (don't mix) liquids.  The less dense liquid floats above the more dense liquid.  The more dense liquid can be removed by a tap at the bottom of the separating funnel. Equipment : separating funnel, beaker.
 
Evaporation : Solutions (solvent and solute) can be separated by evaporation.  The solution is poured into the evaporating basin and the solvent is allowed to evaporate leaving the solute.  Evaporation can be speeded up by heating the solution, care must be taken not to heat the solution to dryness as this may result in solute "popping" out of the evaporating basin.  Equipment : evaporating basin, tripod, bunsen burner.
 
Distillation : Miscible liquids( liquids that do mix) and solvents from solutions can be separated by distillation.  Solutions are placed in distillation flask.  The distillation flask is heated, liquids are separted by boiling points.  Liquids with lower boiling points are distilled first.  The liquid with the lower boiling point forms a gas first.  The gas flows into a condenser.  Cold water flows into the condenser, lowering the temperature, the gas condenses into a liquid and flows into a beaker. Equipment : distillation flask, condenser, beaker.
 
Fractional distillation :  Liquids that have boiling points very close together can be separated by fractional distillation.  In fractional distillation a fractionating column is used.  The column is filled with glass beads.  The glass beads act as condensers.  The liquid with the higher boiling point will condense and drip back into the distillation flask when they hit the glass beads.  Equipment : distillation flask, fractionating column, condenser, beaker.
 
Chromatography : When more than one solid is dissolved in solution they can be separated by chromatography.  The simplest form of chromatography is paper chromatography.  A piece of filter paper is marked with a pencil line about 1 inch from the bottom of the paper.  Concentrated solutions are then dotted onto the paper and dried.  The bottom of the paper is placed into the solvent (it is important that the dots of solution do not touch the solvent).  The solvent travels up the paper and carries the dissolved solid with it.  The distance that the solid travels up the paper depends on how soluble it is in the solvent.  The more soluble the solid the further up the paper it will travel.  In this way the solids dissolved in the solution are separated.  The distance moved by a particular spot is measured and related to the solvent front.  The ratio of these distances is called the Rf.  The Rf can be determined by dividing the distance moved by the substance by the distance moved by the solvent front. Equipment : filter paper, pencil, solvent, ruler
Locating agents can be used to visualize spots which are colourless.
 
 

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