Chromatography
Chromatography separates a mixture of soluble substances and can identify them. The chemistry is simple; the marks are in the practical details and the Rf calculation.
1. How it works
Substances separate because they have DIFFERENT SOLUBILITIES in the solvent and are attracted to the paper by different amounts.
The MORE SOLUBLE a substance is (and the less it is attracted to the paper), the FURTHER it travels.
The two phases:
- Stationary phase — the paper
- Mobile phase — the solvent that moves up it
2. The method
- Draw a BASELINE in PENCIL, about 1 cm from the bottom
- Place a small, concentrated spot of the mixture on the baseline
- Stand the paper in a container with the solvent level BELOW the baseline
- Cover the container (to stop the solvent evaporating)
- Allow the solvent to rise near the top
- Remove the paper, immediately mark the SOLVENT FRONT in pencil, and allow to dry
The two practical details that carry marks:
1. The baseline MUST be drawn in PENCIL. Ink would dissolve in the solvent and travel up the paper, contaminating the result. 2. The solvent must start BELOW the baseline. If the solvent covers the spots, they dissolve into the solvent reservoir instead of travelling up the paper.
Mark the solvent front before the paper dries — once the solvent evaporates you cannot see where it reached, and the Rf becomes impossible.
For a better result: use a small, concentrated spot (a large one gives smeared, overlapping results), and let the solvent rise as far as practical.
3. Interpreting a chromatogram
ONE spot = a PURE substance. MORE THAN ONE spot = a MIXTURE.
Identifying substances: run known reference substances alongside the unknown on the same paper. If a spot in the mixture travels the same distance as a reference, it is likely to be that substance.
Comparison must be on the SAME chromatogram, because Rf values depend on the solvent and conditions used.
Counting components: the number of spots gives the minimum number of substances present — two substances with identical Rf would overlap.
4. Rf values
Rf = distance travelled by the SPOT ÷ distance travelled by the SOLVENT
Both distances are measured from the BASELINE, and to the CENTRE of the spot.
Worked example: a spot travels 3.6 cm; the solvent front is 8.0 cm from the baseline.
Rf = 3.6 ÷ 8.0 = 0.45
Rf is always between 0 and 1 and has NO UNITS — it is a ratio of two lengths.
If your Rf is greater than 1, you have divided the wrong way round — the spot can never travel further than the solvent that carries it.
Measure to the CENTRE of the spot, not its edge — and always from the pencil baseline, not the bottom of the paper.
Rf values are characteristic of a substance in a given solvent, so they can be compared with published values to identify it.
5. Colourless substances
If the substances are colourless, spray with a LOCATING AGENT (or view under UV light) to make the spots visible.
Example: ninhydrin reveals amino acids as coloured spots.
6. Uses
- Separating and identifying dyes and inks
- Analysing food colourings
- Testing for drugs and pollutants
- Separating amino acids from proteins
7. Mistakes that cost marks
Drawing the baseline in ink.
Starting with the solvent above the baseline.
Not marking the solvent front before it dries.
Measuring from the bottom of the paper rather than the baseline.
Measuring to the edge of the spot rather than its centre.
Dividing the wrong way round, giving Rf > 1.
Giving Rf units.
Comparing Rf values from different chromatograms.
Saying several spots means a pure substance.
Frequently asked questions
What does chromatography separate? Mixtures of soluble substances.
Why do substances separate? They have different solubilities in the solvent and different attractions to the paper.
Which substance travels furthest? The most soluble one.
Why is the baseline drawn in pencil? Ink would dissolve and travel up the paper.
Why must the solvent be below the baseline? Otherwise the spots dissolve into the solvent instead of moving up.
How do I know a substance is pure? It produces one spot.
What is the Rf value? Distance moved by the spot ÷ distance moved by the solvent.
What range can Rf take? Between 0 and 1, with no units.
Where do I measure from and to? From the baseline, to the centre of the spot.
How do I see colourless spots? Use a locating agent or UV light.
Quick revision checklist
- I can explain why substances separate
- I know the most soluble travels furthest
- I draw the baseline in pencil
- I keep the solvent below the baseline
- I mark the solvent front before drying
- I know one spot means pure
- I compare with references on the same paper
- I can calculate Rf correctly
- I measure from the baseline to the centre of the spot
- I know Rf is between 0 and 1 with no units
- I know about locating agents
These notes cover chromatography in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for your own exam series.
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