Specification Point 1.10:
Describe these experimental techniques for the separation of mixtures:
The simple distillation of a mixture of salt and water
Use: To separate a liquid and soluble solid from a solution (e.g. water from a solution of salt water)
- Solution is heated and water vapours will rise and evaporate
- Water vapours will pass through the condenser, where it cools and condenses, turning into a liquid that will be collected in a beaker
- After all the water is evaporated from the solution, the solute will be left behind
The fractional distillation of a mixture of ethanol and water
Use: To separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two)
- Solution is heated at temperature of substance with the lowest boiling point
- This substance will rise and evaporate, and vapours will pass through a condenser, where it cools and condenses, turning into a liquid that will be collected in a beaker
- All of this substance is evaporated and collected, leaving behind a mixture or a substance
- For water and ethanol: Ethanol has a boiling point of 78°C and water of 100°C. The mixture is heated until it reaches 78°C, the ethanol boils and distils out of the mixture and condenses into the beaker. When temperature starts to increase to 100°C heating should be stopped. Water and ethanol are separated.
The filtration of a mixture of sand and water
Use: 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)
- 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
The process of crystallisation
Use: 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)
- Solution is heated, allowing the solvent to evaporate to leave a saturated solution. Test the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form in the glass rod.
- Saturated solution is allowed to cool and solids will come out of the solution, as the solubility increases, and crystals will grow
- Crystals are collected by filtering the solution. Then the crystals are washed with cold, distilled water to remove impurity. Dry the crystals.
The paper chromatography of ink and plant dye
Use: To separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink)
- Pencil line is drawn on chromatography paper and spots of ink / dye is placed on it.
- Paper is lowered into a bucket of solvent, allowing the solvent to travel up the paper, taking some of the coloured substances with it.
- Different substances will have different solubilities so will travel at different rates, causing the substances to spread apart. Those with higher solubility will spread more than the others.
- This will show the different components of the ink / dye.
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