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Paper chromatography separates substances by their attraction to the paper and their solubilities in the solvent. Free notes and 12 questions answered.
Paper Chromatography - Principles and Applications is part of Purity, formulations and chromatography in GCSE AQA Higher Chemistry.
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Paper chromatography separates substances by their attraction to the paper and their solubilities in the solvent. Free notes and 12 questions answered.
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Use this lesson to stay inside Purity, formulations and chromatography while connecting the detail back to the wider Topic 8: Chemical analysis topic.
Read the core explanations from Paper Chromatography - Principles and Applications before testing yourself from memory.
Picture a drop of black ink on wet paper - you'll notice it spreads out into different colours. That's chromatography in action, and it reveals something crucial: what looks like a single substance is often a mixture.
Paper chromatography works because different substances have different attractions to paper and different solubilities in the solvent. Think of it like a race where each runner has different abilities - some are good at climbing (attracted to paper), others are good at swimming (soluble in solvent).
The separation happens because of the balance between two competing forces. Substances strongly attracted to the paper move slowly. Substances very soluble in the solvent move quickly with the solvent front.
Every substance has a unique 'fingerprint' in chromatography called the Rf value. This tells you exactly how far a substance travels compared to the solvent front.
Rf values are like chemical fingerprints - each substance has a characteristic value in a particular solvent. A substance with Rf = 0.8 travels 80% as far as the solvent front. One with Rf = 0.2 barely moves from the baseline.
Worked example: Calculating Rf Values
A chromatogram shows the solvent front travelled 10.0 cm. Substance X travelled 6.0 cm and substance Y travelled 3.5 cm. Calculate the Rf values.
Answer: Rf(X) = 0.60, Rf(Y) = 0.35
Here's the key insight: the same substance will always have the same Rf value in the same solvent under the same conditions. This is how we identify unknown substances - we compare their Rf values to known standards.
This is where chromatography becomes a detective tool. A pure substance produces one spot in any solvent. A mixture produces multiple spots because different components separate out.
Think of it like this: if you have pure blue paint, it stays blue no matter what you mix it with. But if your 'blue' paint is actually a mixture of blue and yellow dyes, the right solvent will separate them and reveal the truth.
The pattern is clear: **One spot = pure substance. Multiple spots = mixture.** But there's a catch - you need to try different solvents to be certain, because some mixtures might not separate in every solvent.
**Trap**: Many students think a single spot always means pure. Not true! If the components of a mixture have very similar Rf values in your chosen solvent, they might not separate. Always test with different solvents to be sure.
Worked example: Required Practical: Paper Chromatography Setup
Separate the dyes in black ink using paper chromatography
Answer: Chromatogram showing separated dye components with calculated Rf values
The key variables in this practical: **Independent variable** = type of solvent used. **Dependent variable** = Rf values of separated components. **Control variables** = temperature, paper type, sample size, solvent depth.
Chromatography questions test your understanding of the principles, not just your ability to calculate. Examiners love to ask about the 'why' behind the technique.
**Common exam mistakes**: Writing Rf values greater than 1 (impossible - substances can't travel further than the solvent). Forgetting to mention both phases when explaining separation. Measuring from the wrong baseline or to the wrong part of the spot.
The power of chromatography lies in its simplicity and reliability. Whether you're testing food dyes, identifying drugs, or checking the purity of pharmaceuticals, the same principles apply. Different substances travel at different rates, and those rates are their unique fingerprints.
When quoting an Rf value, you must state the solvent because the same substance can have a different Rf in a different solvent. One spot suggests a pure substance under those conditions, while multiple spots suggest a mixture.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Paper chromatography separates mixtures because different substances have different attractions to the paper (the stationary phase) and different solubilities in the solvent (the mobile phase). Substances strongly attracted to the paper move slowly, while very soluble substances move quickly with the solvent.
In paper chromatography, the stationary phase is the paper, which stays fixed, and the mobile phase is the solvent, which moves up the paper carrying dissolved substances with it. Separation depends on the balance between attraction to the paper and how soluble each substance is in the solvent.
In the chromatography required practical, a pencil baseline is drawn 2 cm from the bottom of the paper, the ink is spotted on it, and the paper is stood in solvent about 1 cm deep. The beaker is covered, the paper taken out before the solvent reaches the top, the solvent front marked in pencil, and distances measured when dry.
A pure substance produces one spot on a chromatogram, while a mixture produces multiple spots. A single spot is not always proof of purity, because components with very similar Rf values may not separate, so different solvents should be tried.
The baseline is drawn in pencil because pencil will not dissolve in the solvent. The solvent level must also be under the baseline, or the sample would dissolve away into the solvent.
An Rf value shows how far a substance travels compared with the solvent front in chromatography: Rf = distance moved by substance ÷ distance moved by solvent. Each substance has a characteristic Rf value in a particular solvent, like a chemical fingerprint.
The solvent level must be lower than the baseline, or the sample will dissolve away into the solvent instead of moving up the paper. The solvent is poured to a depth of about 1 cm, with the pencil baseline 2 cm from the bottom.
The beaker is covered with a watch glass during chromatography to prevent the solvent evaporating. The paper is then left until the solvent nearly reaches the top, but not until the solvent reaches the top edge.
A mixture can give only one spot if its components have very similar Rf values in the chosen solvent, so they don't separate. A single spot does not always mean a pure substance, which is why chromatography should be tried with different solvents.
In the chromatography practical, the independent variable is the type of solvent used and the dependent variable is the Rf values of the separated components. Control variables include temperature, paper type, sample size and solvent depth.
The solvent must be stated with an Rf value because the same substance can have a different Rf in a different solvent. An Rf value is only characteristic of a substance in a particular solvent under the same conditions.
The solvent front is marked with a pencil immediately because it disappears when the paper dries. Distances are then measured from the baseline to the centres of the spots once the paper is dry.