12.2.2Experimental Techniques and Chemical Analysis
Separation and Purification Techniques
15 min read 8 parts Lesson 6 of 9 1 quiz
Section 1 of 8Separation and Purification Techniques
1
A pure substance is one substance only; it has a sharp (fixed) melting point and boiling point. Impurities lower the melting point and make it melt over a range; they raise the boiling point.
2Filtration
Filtration the mixture is poured through filter paper in a funnel; insoluble solid stays as the residue and the liquid or solution passes through as the filtrate.
3
Evaporation to dryness heats a solution until all solvent is gone; crystallisation stops heating at the point of saturation (test: crystals form on a glass rod dipped and cooled),
- then cools the solution so crystals grow,
- which are filtered, washed with a small amount of cold distilled water and dried between filter papers or in a warm oven.
4
Simple distillation separates a solvent from a dissolved solid: the solvent boils, the vapour is cooled in a condenser, and pure liquid (distillate) is collected; the solute remains in the flask.
5
Fractional distillation separates miscible liquids with different boiling points: vapour rises up a fractionating column (packed with glass beads) and repeatedly condenses and re-evaporates; the most volatile (lowest b.p.) component reaches the top first.
- The temperature on the thermometer stays constant while one liquid distils and rises when the next begins.
6Uses of fractional distillation
Uses of fractional distillation separating crude oil into fractions, and separating ethanol from water after fermentation.
7Paper chromatography
Paper chromatography the solvent moves up the paper by capillary action carrying the substances; each separates according to its solubility in the solvent and attraction to the paper. The baseline is drawn in pencil and kept above the solvent level.
8Interpreting a chromatogram
Interpreting a chromatogram a pure substance gives one spot; a mixture gives several. Matching spots (same Rf, same solvent) with a known reference identifies components. A spot left on the baseline is insoluble in that solvent.
9
Colourless substances (e.g. amino acids, sugars) are made visible with a locating agent (sprayed then dried/heated) or by viewing under UV light.
10
Mixtures of two solids can be separated using a difference in solubility: add water, stir (warm) to dissolve the soluble one, filter off the insoluble one, then crystallise the soluble one from the filtrate.
11
Purity is essential in foodstuffs and drugs (medicines) because impurities could be toxic or harmful; purity is checked by melting/boiling point and chromatography.
12Choosing a suitable solvent
Choosing a suitable solvent the substance must dissolve in the solvent (for chromatography) or be separated because of different solubility; organic solvents such as ethanol are used when substances are insoluble in water.
13Core
A mixture of gases (e.g. air) is separated by first cooling it until it liquefies, then fractionally distilling the liquid; each gas boils off at its own boiling point (nitrogen −196 °C before oxygen −183 °C).
14Core
When fractionally distilling ethanol and water, heat with an electric heater or water bath rather than a naked flame because ethanol is flammable. Collect the distillate while the thermometer stays near 78 °C and stop once the temperature starts climbing towards 100 °C.
Section 2 of 8Process & sequence
- Draw a pencil baseline about 1–2 cm above the bottom of the chromatography paper (pencil does not dissolve in the solvent).
- Place small concentrated spots of the unknown mixture and known reference substances on the baseline, spaced apart.
- Stand the paper in a beaker with solvent below the baseline (so spots do not dissolve into the solvent pool);
- cover with a lid to reduce evaporation.
- Allow the solvent to rise until it is near the top;
- remove the paper and immediately mark the solvent front in pencil.
- Dry the paper; spray with a locating agent if the substances are colourless.
- Measure the distance from the baseline to the centre of each spot and to the solvent front;
- calculate Rf values.
- Compare spots/Rf values with the references: same Rf in the same solvent means likely the same substance;
- one spot suggests a pure substance.
Simple distillation of sea waterwrite these steps in this order; each step is a marking point.
- Place sea water in a round-bottomed flask with anti-bumping granules.
- Fit a thermometer with its bulb level with the side arm leading to a Liebig condenser.
- Run cold water into the condenser at the bottom and out at the top.
- Heat the flask; water boils at 100 °C and the steam enters the condenser.
- Steam condenses to pure water (distillate), collected in a beaker; salt remains in the flask.
Section 3 of 8Compare
| Mixture | Method | Physical property used |
|---|---|---|
| Insoluble solid + liquid (sand + water) | Filtration | Particle size / solubility |
| Soluble solid from its solution (salt from water) — want the solid | Evaporation or crystallisation | Solvent has lower boiling point |
| Solvent from a solution (pure water from sea water) | Simple distillation | Big difference in boiling points |
| Miscible liquids (ethanol + water; crude oil) | Fractional distillation | Different boiling points |
| Coloured / soluble substances (inks, dyes, amino acids) | Paper chromatography | Different solubility in solvent / attraction to paper |
| Two solids, one soluble (salt + sand) | Dissolve, filter, then evaporate/crystallise | Different solubility |
Crystallisation is preferred to evaporating to dryness when the salt decomposes on strong heating or contains water of crystallisation.
| Test | Pure substance | Impure substance |
|---|---|---|
| Melting point | Sharp, at the exact data-book value | Lower, melts over a range |
| Boiling point | Sharp, fixed value | Higher, boils over a range |
| Chromatogram | One spot only | Two or more spots |
Section 4 of 8Worked examples
0.65
On a chromatogram the solvent front moved 8.0 cm from the baseline. Spot A moved 5.2 cm. Calculate the Rf value of A.
- Rf = distance moved by substance ÷ distance moved by solvent = 5.2 ÷ 8.0 = 0.65 (no units).What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
5.0 cm
Substance X has Rf = 0.40. The solvent front travelled 12.5 cm. How far from the baseline is the spot of X?
- distance of X = Rf × solvent distance = 0.40 × 12.5 = 5.0 cm.
Below
A sample of aspirin melts between 128 °C and 133 °C. The data-book value is 136 °C. Is it pure? Explain.
- Not pure
- an impure substance melts below the pure value and over a range of temperatures (128–133 °C);
- pure aspirin would melt sharply at 136 °C.
- an impure substance melts below the pure value and over a range of temperatures (128–133 °C);
Section 5 of 8Keep going
Separation & Purification IGCSE Chemistry 0620: what to do next
These Separation & Purification IGCSE Chemistry 0620 notes covered the key ideas, worked examples and exam technique for this topic. Check your Separation & Purification IGCSE Chemistry 0620 understanding with the practice questions below, then reread any Separation & Purification IGCSE Chemistry 0620 point you were unsure about before moving on.
Separation & Purification IGCSE Chemistry 0620: revision checklist
- Revise the main Separation & Purification IGCSE Chemistry 0620 definitions until you can write them from memory.
- Practise Separation & Purification IGCSE Chemistry 0620 exam questions under timed conditions.
- Link each Separation & Purification IGCSE Chemistry 0620 idea to a diagram or a worked example.
- List the common Separation & Purification IGCSE Chemistry 0620 mistakes and how to avoid them.
- Explain one Separation & Purification IGCSE Chemistry 0620 idea out loud as if teaching a friend.
- Summarise Separation & Purification IGCSE Chemistry 0620 in five key points on a revision card.
- Check which command words appear most often in Separation & Purification IGCSE Chemistry 0620 questions.
- Practice: Separation & Purification IGCSE Chemistry 0620 practice questions
- Previous lesson: 11.3 Experimental Techniques — 0620 Notes
- Next lesson: 11.5 Identification of Ions & Gases — 0620 Notes
- Course: Cambridge IGCSE Chemistry 0620 — all units
- Official syllabus: Cambridge IGCSE Chemistry 0620 syllabus
Section 6 of 8Key terms
Section 7 of 8Command terms
Command terms
State / Name
Give a short answer only; no explanation is needed (1 mark = 1 fact).
Define
Give the precise meaning. Learn the syllabus definitions word-for-word: every keyword in the definition is a marking point.
Describe
Say what happens / what the data show, with key features and figures. Do not give reasons.
Explain
Give reasonswhat happens AND why/how, linked with “because”, “so”, “therefore”.
Suggest
Apply knowledge to an unfamiliar situation; any sensible, scientifically-based answer can score.
Compare
Give similarities AND differences, each as a direct comparison (“X is … whereas Y is …”).
Outline
Set out the main points briefly.
Calculate
Work out from given facts; show working, give the unit and the requested significant figures.
Identify
Name/select from given information.
Predict
Give an expected result without a full justification.
Evaluate
Judge the importance or quality of the evidence/method, with reasons.
Section 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Pure substance · Mixture · Solvent · Solute · Solution · Saturated solution · Residue · Filtrate · Crystallisation · Distillation · Fractional distillation · Chromatography · Rf value · Locating agent
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Use the exact scientific term and learn syllabus definitions word-for-word.
2
Write “rate of … increases”, not “… increases”.
3
Say “concentration gradient”, never just “gradient”; name particles precisely (atoms, molecules, ions) — never “it”.
4
Give units with every numerical answer and use the significant figures asked for; show all working.
5
Compare = a similarity AND a difference; describe = what happens; explain = why it happens.
6
Repeats let you identify anomalies and calculate a mean — they do not make an experiment “valid”.
Words to avoid
1
the salt evaporatesthe water evaporates and the salt is left behind
2
the dye with the biggest spot is most solublethe substance that travels furthest (largest Rf) is most soluble in the solvent
3
draw the baseline in pen/inkdraw the baseline in pencil, because pencil (graphite) does not dissolve in the solvent
4
the solvent should cover the spotsthe solvent level must be below the baseline so the spots do not dissolve into the solvent
5
the filtrate is the solidthe residue is the solid left in the filter paper; the filtrate passes through
6
a pure substance melts at a low temperaturea pure substance has a sharp, fixed melting point