12.4Experimental Techniques and Chemical Analysis
Separation and purification — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 11 parts Lesson 4 of 5 1 quiz
Lesson summary
12.4 Separation and purification — Notes — IGCSE Chemistry notes: Assess the purity of a substance and choose and describe methods of separating mixtures.TermMeaningExampleSolventThe liquid in which a solute dissolvesWater in sea waterSoluteThe substance that dissolves in a liqui…
- Saturated solution
- Crystallisation
- Simple distillation
- Fractional distillation
- Pure substance
- Mixture: melts and boils over a range of temperatures because its components change state at different temperatures
Section 1 of 11Terms used for solutions
| Term | Meaning | Example |
|---|---|---|
| Solvent | The liquid in which a solute dissolves | Water in sea water |
| Solute | The substance that dissolves in a liquid to form a solution | Salt in sea water |
| Solution | Mixture formed when a solute dissolves in a solvent | Sea water |
| Saturated solution | Maximum concentration of solute dissolved at a given temperature | Dead Sea water |
| Soluble / insoluble | Will / will not dissolve | Salt / sand in water |
| Filtrate | Liquid or solution that has passed through a filter | Coffee in a cup |
| Residue | Substance remaining after evaporation, distillation, filtration or a similar process | Coffee grounds in filter paper |
Section 2 of 11Assessing purity
1
Pure substance: melts and boils at specific, sharp temperatures (water: melts at 0 °C, boils at 100 °C).
2
Mixture: melts and boils over a range of temperatures because its components change state at different temperatures.
3
Melting and boiling data show whether a sample is pure or a mixture.
4
Identifying a substance: measure its melting and boiling points and compare them with literature values or data tables.
- Reference boiling points are quoted at a given pressure, so compare under the same conditions.
- Boiling points are commonly found by distillation and melting points with a melting point apparatus.
5
Effect of impurities: the melting point appears lower and the boiling point appears higher than the true values.
6
The closer the measured value is to the true value, the purer the sample.
Section 3 of 11Measuring melting point
Measuring melting point
Method
- Place a small sample in the melting point apparatus.
- Heat it slowly so the thermometer and sample stay at nearly the same temperature.
- Record the temperature at which melting starts and finishes.
- Compare the range with the data-table value.
Melting point is routinely used to assess the purity of drugs.
Section 4 of 11Filtration
1
All separation techniques rely on a difference in a physical property such as solubility or boiling point.
2
Filtration separates an undissolved solid from a liquid or solution, such as sand from sand and water.
3
Large solid particles cannot pass through the filter paper and stay behind as the residue,
- and the liquid or solution that passes through is the filtrate.
4
Centrifugation can also separate an undissolved solid from a liquid.
5
Separating two solids: dissolve the soluble one in a suitable solvent, filter off the insoluble one, then crystallise the filtrate.
6
Water dissolves salt but not sand.
Section 5 of 11Crystallisation
Crystallisation
Method
- Heat the solution to evaporate some of the solvent, removing no more than about half, until it is saturated.
- Test for saturation by dipping a clean, dry, cold glass rod in, as crystals form on the rod as it cools.
- Leave the saturated solution to cool slowly so crystals grow as solubility falls.
- Collect the crystals by filtration.
- Wash the crystals with a little cold distilled water to remove impurities.
- Dry them between sheets of filter paper or in a drying oven.
Use: for solids more soluble in hot solvent than in cold, such as copper(II) sulfate from its solution.
Section 6 of 11Simple distillation
Simple distillation
Use: separates a solvent from a solution, such as pure water from salt water, or a pure liquid from a mixture of liquids.
Method
- Heat the solution, so the pure solvent evaporates and the vapour rises into the condenser.
- The vapour cools and condenses to pure liquid, which is collected in a beaker.
- The solid solute is left behind in the flask.
Simple distillation is less effective than fractional distillation when boiling points are close.
Section 7 of 11Fractional distillation
1
Use: separates two or more miscible liquids, such as ethanol and water, or the fractions of crude oil.
2
Principle: the liquid with the lowest boiling point evaporates first, is condensed and collected.
3
Ethanol boils at 78 °C and water at 100 °C, so stop heating when the temperature rises towards 100 °C.
4
Safety: an electric heater is safer than a flame with flammable liquids.
5
Crude oil is separated industrially, and it is not done in school because some components are toxic, but a synthetic crude oil can be used.
6
A gas mixture can be cooled until it liquefies and then separated by fractional distillation.
7
Diffusion can be used when boiling points are very close or distillation is impractical.
Section 8 of 11Choosing a separation method
| Mixture | Method |
|---|---|
| Insoluble solid and liquid | Filtration |
| Soluble solid from solution | Crystallisation or evaporation |
| Solvent from a solution | Simple distillation |
| Miscible liquids with different boiling points | Fractional distillation |
| Coloured soluble substances | Paper chromatography |
| Soluble and insoluble solids | Dissolve, filter, then crystallise the filtrate |
Section 9 of 11More from the specification
1Locating agents
- Chemical substances sprayed onto a chromatogram to produce visible coloured spots from originally colourless components like amino acids.
2Distance moved (Rf value)
- The ratio of the distance travelled by a solute spot relative to the distance travelled by the solvent front.
3Solubility separation (Calcium vs Magnesium)
- Insoluble calcium sulfate is separated from soluble magnesium chloride by dissolving in water and filtering, then evaporating the filtrate.
4Crystallisation heating
- A solution is gently heated until saturated crystals begin to produce along a glass rod, then left to cool.
5Chloride
- Salt of hydrochloric acid.
6Locating agent
- Chemical that makes colourless spots visible.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Filtrate | the liquid or solution that passes through a filter. |
| Residue | the solid left on the filter paper. |
| Saturated solution | a solution holding the maximum solute at a given temperature. |
| Crystallisation | forming crystals by cooling a saturated solution. |
| Simple distillation | separating a solvent from a solution by evaporating and condensing it. |
| Fractional distillation | separating miscible liquids by their different boiling points. |
| Pure substance | a substance with sharp, specific melting and boiling points. |
Section 11 of 11Exam technique
1
Saying the salt evaporatesthe water evaporates and the salt is left behind.
2
Mixing up filtrate and residuethe residue is the solid left on the paper.
3
Heating the crystallisation solution to dryness instead of removing no more than half.
4
Using simple distillation to separate miscible liquids with close boiling points.
5
Saying a pure substance melts over a range or that impurities raise the melting point: it has a sharp melting point and impurities lower it.
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