Methods of Purification
- Mixtures can contain elements and / or compounds.
- Each constituent of the mixture retains its chemical properties.
- The parts of a mixture are not chemically bonded together and so they can be separated by physical means.
- 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 melting point, 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 that only the desired substance dissolves in the solvent 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 oil and water or when an organic product is formed in aqueous conditions.
- Miscible liquids need to be separated by fractional distillation.
Separating funnel being used to separate kerosene and water
- 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 a 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 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 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.
Diagram showing the process of crystallisation
- 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.
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 distils 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
- 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.
- 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.
- This will show the different components of the ink / dye.
- If two or more substances are the same, they will produce identical chromatograms.
- If the substance is a mixture, it will separate on the paper to show all the different components as separate spots.
- An impure substance will show up with more than one spot, a pure substance should only show up with one spot.
Analysis of the composition of ink using paper chromatography
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.