Specification Point 2.7:
Explain the experimental techniques for separation of mixtures by:
a) simple distillation
b) fractional distillation
e) paper chromatography
- 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 pure liquid H2O that is collected in a beaker.
- After all the water is evaporated from the solution, only the solid solute will be left behind.
Diagram showing the distillation of a mixture of salt and water
- 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 of a mixture of ethanol and water
- Fractional distillation is the technique used to separate the components of crude oil.
- Crude oil is a mixture of hydrocarbon molecules of different chain sizes.
- The mixture is evaporated by heating and its vapours condense at different heights corresponding to different temperatures in the fractionating column.
- Each fraction contains hydrocarbon molecules with a similar number of carbon atoms and a similar range of boiling points, making it easier to produce more useful chemicals and products.
Fractional distillation of crude oil occurs in a fractionating column which has a range of temperatures with the hottest at the bottom
- 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.
- Mixture of insoluble solid and liquid is poured into the filter funnel.
- Filter paper will only allow small liquid particles to pass through as the filtrate.
- Solid particles are too large to pass through the filter paper so will stay behind as a residue.
Filtration of a mixture of sand and water
- 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).
- The solution is heated, allowing the solvent to evaporate to leave 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 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 cold, distilled water to remove impurities and allowed to dry.
Diagram showing the process of crystallisation
- 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).
- It is also used to identify unknown substances.
- 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.
- 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.
The colours in ink or plant dye can be separated using paper chromatography
Choosing a Technique
Specification Point 2.8:
Describe an appropriate experimental technique to separate a mixture, knowing the properties of the components of the mixture.
Methods of purification
- 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 b.p., between the substances being separated.
Mixtures of solids
- Differences in density, magnetic properties, sublimation and solubility can be used.
- 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.
Mixtures of liquids
- Immiscible liquids can be separated using a separating funnel or by decanting (pouring carefully).
- Examples include when an organic product is formed in aqueous conditions.
Separating funnel being used to separate kerosene and water
Edexcel GCSE Chemistry Notes
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