2.1.5Elements, compounds and mixtures
Separation Techniques — OCR Gateway GCSE Combined Science A J250 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 6 of 18
Part 1 of 5Choosing a method
| Mixture to separate | Technique | Property used |
|---|---|---|
| Insoluble solid and liquid (sand and water) | Filtration | Particle size: solid is too big to pass the paper |
| Dissolved solid from solution (copper sulfate crystals) | Crystallisation | Solubility falls as the solution cools |
| Solvent from a solution (pure water from salt water) | Simple distillation | Different boiling points of solvent and solute |
| Miscible liquids (ethanol and water) | Fractional distillation | Different boiling points of the liquids |
| Dyes in an ink | Paper chromatography | Different solubilities in the solvent |
Part 2 of 5Filtration and crystallisation
Filtration
- Filtration Pour the mixture through filter paper in a funnel.
- The liquid that passes through is the filtrate;
- the solid left behind is the residue.
- The liquid that passes through is the filtrate;
- Centrifuging can also separate a fine solid from a liquid.
Crystallisation
- Crystallisation Heat the solution to evaporate some solvent until it is saturated.
- Test by dipping in a cold glass rod: crystals form on it if the solution is ready.
- Leave the solution to cool slowly so that large crystals form, filter them off, wash with a little cold distilled water, then dry.
Part 3 of 5Simple distillation
1
Heat the solution in a flask. The solvent has the lower boiling point, so it evaporates first.
2
The vapour travels into a condenser where it is cooled by water running through the outer jacket.
3
Cooling water enters at the bottom and leaves at the top so the jacket stays full.
4
The vapour condenses and the pure liquid drips into a collecting beaker.
5
The dissolved solid stays behind in the flask.
Part 4 of 5Fractional distillation
1Heat the mixture
- Heat the mixture Heat gently using a fractionating column fitted above the flask.
2Lowest boiling point first
- Lowest boiling point first Ethanol boils at 78 °C and water at 100 °C, so ethanol vapour reaches the top of the column first.
3Condense and collect
- Condense and collect The vapour is cooled in the condenser and collected as a liquid.
4Change collector
- Change collector When the thermometer rises towards 100 °C, switch containers: the remaining liquid is mostly water.
Part 5 of 5Paper chromatography
Chromatography
Set up
- Set up Draw a pencil start line (pencil does not dissolve), place a spot of sample on it and stand the paper in solvent with the start line above the liquid.
Separation
- Separation The solvent rises by capillary action and carries dissolved substances with it at different rates.
More soluble travels further
- More soluble travels further Substances more soluble in the solvent move higher up the paper.
Reading the result
- Reading the result One spot suggests a pure substance;
- several spots mean a mixture.
- The finished paper is called a chromatogram.
Key terms
| Term | Meaning |
|---|---|
| Filtrate | The liquid that passes through filter paper. |
| Residue | The solid left on the filter paper. |
| Saturated solution | A solution that cannot dissolve any more solute at that temperature. |
| Miscible | Able to mix completely to form a single liquid. |
| Chromatogram | The paper showing the separated spots after chromatography. |
Avoid these mistakes
1
Using filtration to separate a dissolved solid; dissolved particles pass through the filter paper.
2
Evaporating a solution to dryness during crystallisation, which gives a powder instead of crystals.
3
Drawing the start line in pen, whose ink would run up the paper with the solvent.
4
Putting the start line below the solvent level so the sample washes into the beaker.
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