Revision notes for AQA GCSE Chemistry Chromatography. Open the guide for explanations and worked examples. Written against the AQA GCSE Chemistry (8462) specification, so the content matches what's examinable rather than general Chemistry background.
Chromatography
What you'll learn
- How chromatography separates mixtures using a stationary phase and a mobile phase.
- How to carry out the GCSE required practical for paper chromatography.
- How to read a chromatogram and calculate an RfR_fRf value.
- How chromatography can help decide whether a substance is pure or impure.
The big idea: separating a mixture
A mixture contains two or more substances that are not chemically bonded together. Because the substances are not bonded, they can often be separated by physical methods.
A pure substance is a single element or compound, not mixed with anything else. In chromatography, purity is judged by how many spots appear on the chromatogram.
Definition
Chromatography
Chromatography is a separation technique used to separate substances in a mixture and give information to help identify them.
Paper chromatography is especially useful for separating coloured substances, such as inks or food colourings. A mixture that looks like one colour may actually contain several different dyes.
Stationary phase and mobile phase
Chromatography always involves two phases. A phase here means a part of the system that a substance can spend time in.
The stationary phase is the phase that stays still. In GCSE paper chromatography, this is the chromatography paper.
The mobile phase is the phase that moves. This is the solvent, meaning the liquid that dissolves and carries the substances up the paper.
Key Idea
Why substances separate
Substances separate because they spend different amounts of time in the stationary phase and the mobile phase. This is called their distribution between the phases.
If a dye dissolves well in the solvent and is not strongly attracted to the paper, it spends more time in the mobile phase and moves further. If a dye is more strongly attracted to the paper, it is held back and moves a shorter distance.
The diagram shows the basic paper chromatography setup and how distances are measured after the solvent has moved up the paper.
Example
Predicting which dye travels further
A dye mixture contains dye A and dye B. Dye A dissolves very well in the solvent. Dye B is strongly attracted to the paper.
- Dye A spends more time in the mobile phase, so the moving solvent carries it further up the paper.
- Dye B spends more time in the stationary phase, so it is held back by the paper.
- Dye A will make a spot higher up the chromatogram than dye B.
Required practical 6: paper chromatography
You need to be able to describe how paper chromatography can be used to separate and tell the difference between coloured substances.
Key Idea
Required practical 6: method
- Draw a pencil line near the bottom of a strip of chromatography paper. This is the origin line, where the samples start.
- Place small spots of the coloured substances on the origin line. Let the spots dry, then add another small spot if needed to make them more concentrated.
- Put a shallow layer of solvent in a beaker. The solvent level must be below the origin line.
- Suspend the paper in the beaker so the bottom touches the solvent, but the spots do not go under the solvent.
- Allow the solvent to rise up the paper. Remove the paper before the solvent reaches the top.
- Immediately mark the solvent front, which is the furthest point reached by the solvent.
- Measure the distances from the origin line to the centre of each spot, and from the origin line to the solvent front.
Use a pencil for the origin line because pencil graphite does not dissolve in the solvent. If you used ink, the line might separate into dyes and interfere with the results.
Common Mistake
Putting the spots under the solvent
If the coloured spots start below the solvent level, they may dissolve straight into the solvent in the beaker instead of travelling up the paper. Keep the origin line above the solvent level.
A chromatogram is the finished paper or record showing the separated spots. Each spot represents one substance, or sometimes more than one substance if two substances happen to travel the same distance.
Calculating RfR_fRf values
The retention factor, written as RfR_fRf, compares how far a substance moved with how far the solvent moved.
Rf=distance moved by substancedistance moved by solventR_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}Rf=distance moved by solventdistance moved by substanceThe distance moved by the substance is measured from the origin line to the centre of the spot. The distance moved by the solvent is measured from the origin line to the solvent front.
Because RfR_fRf is a ratio of two distances, it has no units. In normal paper chromatography, the spot should not move further than the solvent front, so RfR_fRf values are usually between 0 and 1.
Example
Calculating an Rf value
A spot moves 3.6 cm from the origin line. The solvent front moves 8.0 cm from the origin line.
-
Identify the two distances: the substance moved 3.6 cm and the solvent moved 8.0 cm.
-
Substitute into the ratio:
Rf=3.6 cm8.0 cmR_f = \frac{3.6\ \text{cm}}{8.0\ \text{cm}}Rf=8.0 cm3.6 cm -
Calculate the value:
Rf=0.45R_f = 0.45Rf=0.45The centimetres cancel, so the RfR_fRf value is 0.45, with no unit.
Tip
Significant figures
Give RfR_fRf values to a sensible number of significant figures. If the distances are measured to two significant figures, an RfR_fRf value to two significant figures is usually appropriate unless the question says otherwise.
Common Mistake
Measuring from the wrong place
Do not measure from the bottom of the paper. Measure from the origin line to the centre of the spot, and from the origin line to the solvent front.
Using chromatography to identify substances
Different compounds have different RfR_fRf values in different solvents. This means chromatography can help identify substances, but only if the comparison is fair.
To identify an unknown substance, run it on the same chromatogram as known substances, using the same paper and the same solvent. If a spot in the unknown sample lines up with a spot from a known substance, they may be the same substance.
The chromatogram below shows how an unknown mixture can be compared with known coloured substances.
Example
Identifying substances from a chromatogram
An unknown sample gives two spots. One spot is at the same height as known substance R. The other is at the same height as known substance B. There is no spot matching known substance Y.
- Spots at the same height have travelled the same distance in the same solvent, so they have the same RfR_fRf value.
- The unknown sample has two spots, so it contains more than one substance.
- The unknown contains substances R and B, but there is no evidence that it contains Y.
Pure and impure substances
A pure compound produces a single spot in all solvents. A mixture may separate into two or more spots, because different substances in the mixture can travel different distances.
However, one solvent may not separate every mixture clearly. Two different substances could accidentally have the same RfR_fRf value in that solvent and appear as one spot.
Common Mistake
One spot is not always final proof
A single spot in one solvent suggests a substance may be pure, but a stronger test is to repeat chromatography using different solvents. A pure compound should still produce a single spot in all solvents.
This is why the solvent matters. The same compound can have a different RfR_fRf value in a different solvent, because its distribution between the stationary and mobile phases changes.
Common Mistake
Treating Rf as fixed forever
An RfR_fRf value is only useful for comparison when the same solvent and conditions are used. Do not compare RfR_fRf values from different solvents as if they are automatically the same.
Why the solvent changes the separation
A solvent is not just “something wet”. It affects how well each substance dissolves and how strongly each substance stays on the paper.
If the solvent is suitable, the substances in the mixture separate into clear spots. If the solvent is unsuitable, the spots may not move, may all move together, or may spread out too much.
A good chromatogram has spots that are separated clearly enough to measure from the origin line to the centre of each spot.
Tip
Sanity check for chromatograms
If every spot stayed on the origin line, the solvent probably did not carry the substances well. If every spot travelled with the solvent front, the substances may have been too soluble in the solvent.
Exam technique
In the exam
- When explaining separation, use the words stationary phase, mobile phase, and distribution between the phases.
- For RfR_fRf calculations, measure from the origin line, use the centre of the spot, and remember that RfR_fRf has no unit.
- For purity questions, say that a pure compound gives a single spot in all solvents, while a mixture may give more than one spot.
Self review
Check yourself
- Why must the origin line be drawn in pencil rather than ink?
- A spot moves 2.4 cm and the solvent front moves 6.0 cm. What is the RfR_fRf value?
- Why is it important to use the same solvent when comparing an unknown sample with known substances?
Purity, formulations and chromatography
Guide 3 of 3
You've reached the end
Test yourself on this topic, or move on to the next guide.
How was this guide?