Separating Mixtures (SL IB Chemistry)

• The choice of the method of separation depends on the nature of the substances being separated
• All methods rely on there being a difference of some sort, usually in a physical property such as boiling point between the substances being separated

Separating a mixture of solids

• Differences in solubility can be used to separate solids
• For a difference in solubility, a suitable solvent must be chosen to ensure the desired substance only dissolves in it and not other substances or impurities, e.g. to separate a mixture of sand and salt, water is a suitable solvent to dissolve the salt, but not the sand

Solvation & 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
• Filter paper is placed in a filter funnel above another beaker
• The mixture of insoluble solid and liquid is poured into the filter funnel
• Filter paper will only allow small liquid particles to pass through in the filtrate
• Solid particles are too large to pass through the filter paper so will stay behind as a residue

Filtration Diagram

Filtration enables the separation of insoluble solids from mixtures. The quality and speed of the filtration depends on the choice of filter paper and the size of the suspended solid particles. Vacuum filtration can be used for very finely suspended solids, which can clog up the pores in the filter paper using gravity filtration alone

Crystallisation

• Used to separate a dissolved solid from a solution, when the solid is 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 and leaving a saturated solution behind
• You can 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 when it is removed and allowed to cool
• The saturated solution is allowed to cool slowly and solids will come out of the solution as the solubility decreases, and crystals will grow
• Crystals are collected by filtering the solution
• They are then washed with distilled water to remove any impurities and then allowed to dry

Crystallisation Technique Diagram

Crystallisation is carried out slowly using gentle heating or just leaving a saturated solution to evaporate so that larger crystals are formed which are easier to separate

Recrystallisation

• Recrystallisation is used to purify impure solids
• The principle is that a hot solvent is used to dissolve both the organic solid and the impurities and then as the solution cools the solid crystallises out and leaves behind the impurities in the solution
• The key is using the minimum amount of solvent to dissolve the solid and avoid loss of the product
• If any solid impurities remain in the solution, a hot filtration can be carried out
• Once the solution has cooled down to room temperature and crystallised then the product crystals can be recovered by filtration
• This is faster using Buchner apparatus in which filtration occurs under reduced pressure

Recrystallisation Equipment Diagram

 

The steps involved in recrystallisation of an impure solid. After filtration the product is washed with fresh cold solvent and then allowed to dry on filter paper

Simple Distillation

• Used to separate a liquid and soluble solid from a solution (e.g. water from a solution of saltwater) 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 pure water which is collected in a beaker
• After all the water is evaporated from the solution, only the solid solute will be left behind

Simple Distillation Diagram

Simple distillation can be used to separate the products of fermentation, such as alcohol and water. However, more effective separation is to use fractional distillation where the liquids are closer to boiling point and a higher degree of purity is required

Fractional distillation

• 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

Fractional Distillation Diagram

 

Separation of a mixture of ethanol and water is best achieved by fractional distillation when the two components are close together in boiling point or there are multiple components

• An electric heater is safer to use when there are flammable liquids present
• The separation of the components in petroleum is achieved by fractional distillation on an industrial scale
• Fractional distillation of crude oil is not carried out in school laboratories due to the toxic nature of some of the components of the crude oil, but it can sometimes be simulated using a synthetic crude oil made specially for the demonstration

Paper Chromatography

• This technique is used to separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink)
• A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples
• The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent so the samples don´t wash into the solvent container
• The solvent travels up the paper by capillary action, taking some of the coloured substances with it
• This will show the different components of the ink / dye

Paper Chromatography Equipment Diagram

Analysis of the composition of ink can be carried out using paper chromatography. Different substances have different solubilities so will travel at different rates, causing the substances to spread apart. Those substances with higher solubility will travel further than the others