Chromatography

A reliable Paper 2 mark — if your method is precise, your R_f formula isn’t inverted, and you can interpret a chromatogram cleanly.

SPEC 4.8.1.3 • PAPER 2 • RP6

Why This Lesson Matters

Chromatography appears in almost every Paper 2 sitting, usually as a 6–10 mark question cluster. The pattern is consistent:

Recurring asks

1. Plan or fix the method (often 6 marks).

2. Calculate an R_f — sometimes backwards (given R_f, find a distance).

3. Explain why two substances travel different distances.

4. State whether a sample is pure or impure from the chromatogram.

Where students lose marks

Inverting the R_f formula. Saying ‘solubility’ instead of attraction. Saying ‘less pure’ instead of impure. Drawing the start line in pen. Solvent above the start line.

The Two Phases

Spec definition

Chromatography separates mixtures by distributing substances between a stationary phase and a mobile phase.

Stationary phase

Does not move. In paper chromatography, this is the paper.

Mobile phase

Moves through the stationary phase, carrying dissolved substances along. In paper chromatography, this is the solvent (e.g. water or ethanol).

The mechanism in one line

Each substance has a different balance of attractions to the two phases. That difference is what produces separation.

RP6 Apparatus

The diagram below shows the setup (sample on start line; solvent level below it) plus the positions you measure after the experiment has run (solvent front at top; substance spot partway up). Each detail is mark-bearing.

RP6 Method — Each Step Has a Reason

1
Draw a start line in pencil ~1 cm from the bottom of the paper. Pen ink would dissolve in the solvent and contaminate the chromatogram.
2
Place a small concentrated spot of the sample on the start line.
3
Pour solvent into the beaker so the solvent level is BELOW the start line. If the solvent is at or above the line, the spot dissolves directly into the solvent pool and there is no separation.
4
Cover the beaker with a lid. Stops solvent evaporating and keeps the beaker saturated with solvent vapour.
5
Wait for the solvent to rise up the paper by capillary action, carrying dissolved components with it.
6
Before the solvent reaches the top, remove the paper and mark the solvent front in pencil immediately. Solvent evaporates quickly — mark before you lose it.
7
Measure both distances from the start line: distance moved by the substance (centre of spot) and distance moved by the solvent.

Examiner Alert: Setup Mistakes

Examiner Alert P2 Jun23 Q06.1

Question showed a setup with a faulty diagram. Two mistakes had to be identified for 2 marks. The mark scheme accepted:

1. ‘the start line is drawn in ink’ — should be pencil.

2. ‘the start line is below the water level’ — should be above.

Examiner Alert P2 Nov21 Q03.1 (RP6 plan, 6 marks)

Examiner verdict: The main barriers to a Level 3 mark were claiming the solvent should start at or above the start line, or omitting how distances should be measured. Almost two-fifths reached Level 3.

Drill

If a question gives you a diagram and asks you to find mistakes, the two highest-yield checks are pencil vs ink and solvent level vs start line.

How Separation Happens

Two substances travelling up the same paper in the same solvent end up at different heights. Why?

The attraction balance

Each substance has a particular attraction to the paper (stationary phase) and a particular attraction to the solvent (mobile phase).

Weaker attraction to paper

Substance is carried further up the paper. Travels far → high R_f.

This is the spec / mark scheme phrasing.

Stronger attraction to paper

Substance stays closer to the start line. Travels little → low R_f.

Equivalent: weaker attraction to the solvent.

Examiner Alert P2 Jun24 Q04.1

Question explicitly said ‘refer to forces of attraction between the dyes and the chromatography paper’. Around 30% scored both marks. Most who lost marks talked about solubility or attraction to the solvent — ignoring the instruction to discuss attraction to the paper.

Lesson

Read the prompt. If it says attraction to paper, write about attraction to paper — not solubility, not attraction to solvent.

Reading Purity from a Chromatogram

The rule

A pure compound produces a single spot in all solvents. A mixture separates into multiple spots (one for each component).

‘In all solvents’ matters

The spec says a pure compound gives a single spot in all solvents. One spot in one solvent is suggestive but not proof — the components of a mixture might overlap by chance in that solvent. To confirm purity, repeat in a second solvent.

Examiner Alert P2 Jun24 Q04.3

Two-mark question comparing the purity of two dyes. Mark scheme requires the absolute terms: ‘A is impure’ and ‘B is pure’. Around a quarter scored both marks. The most common loss was writing ‘A is less pure’ or ‘B is purer’ — comparative language is not enough.

The R_f Formula

R_f = distance moved by substancedistance moved by solvent

Three rules

1. Both distances measured from the start line (not from the bottom of the paper).

2. Substance distance is to the centre of the spot.

3. R_f is between 0 and 1 — a substance can never travel further than the solvent that carries it.

Sanity check

If your R_f is greater than 1, you have inverted the formula. Flip it and try again.

Worked Example: R_f Forwards

In a chromatography experiment, the solvent front travels 8.0 cm from the start line. A blue spot is found 6.4 cm from the start line. Calculate the R_f value of the blue substance. 2 marks

Step 1

Write the formula: R_f = distance moved by substance ÷ distance moved by solvent.

Step 2

Substitute: R_f = 6.4 ÷ 8.0.

Step 3

R_f = 0.80. Sanity check: 0 < 0.80 < 1 ✓

No units

R_f is a ratio of two distances in the same unit, so the units cancel. Never write ‘cm’ after an R_f value.

Worked Example: R_f Backwards

Examiners often give you the R_f and one distance, then ask you to rearrange.

A purple pigment has an R_f value of 0.30. In an experiment, the solvent front travels 9.0 cm from the start line. Calculate the distance moved by the purple pigment. 3 marks

Step 1

Start from R_f = substance ÷ solvent → 0.30 = substance ÷ 9.0.

Step 2

Rearrange: substance = 0.30 × 9.0.

Step 3

Distance moved by purple pigment = 2.7 cm. Sanity check: substance < solvent ✓

Examiner Alert P2 Jun24 Q04.4

Three-quarters of candidates scored full marks on a question of exactly this type. Of those who lost all three marks, the dominant cause was incorrect substitution — putting numbers in the wrong slots of the formula.

Why R_f Doesn’t Depend on How Far the Solvent Travels

If you let the experiment run longer (or use a longer paper), the solvent front travels further and so does the substance. The two distances grow together — their ratio stays the same.

In one sentence (mark scheme phrasing)

The ratio of distance moved by the spot to distance moved by the solvent is fixed/constant; the spot distance is directly proportional to the solvent distance.

Visual intuition

Imagine doubling the time. The solvent goes from 8 cm to 16 cm. The substance, carried by the solvent in the same way, goes from (say) 6 cm to 12 cm. R_f = 6/8 = 12/16 = 0.75. Same ratio.

Examiner Alert P2 Jun24 Q04.5

Only around 10% of students answered this correctly. Most just stated that ‘both travel further’, which doesn’t engage with the maths. The mark scheme rewards anything along the lines of: ratio is constant / spot distance is proportional to solvent distance.

What Actually Changes R_f?

If R_f is independent of solvent travel distance, what makes it change?

ChangeDoes it change R_f?Why

Different solventYesDifferent attraction between substance and mobile phase. (Spec: ‘different R_f values in different solvents’.)

Different type of chromatography paperYesDifferent attraction between substance and stationary phase. (Tested in P2 Jun23 Q06.)

Longer paper / more timeNoBoth distances scale together; ratio fixed.

Bigger beakerNoGeometry of the container is irrelevant to attractions.

Different length of paperNoSame as longer paper / more time.

Quick MCQ pattern

If a question lists four possible reasons two students got different R_f values, pick the one that changes a phase — usually different solvent.

Practice 1: R_f Calculation

A student spots a yellow food dye on chromatography paper. After the experiment, the solvent has travelled 10.0 cm from the start line and the centre of the yellow spot is 7.5 cm from the start line.

Calculate the R_f value of the yellow dye. 2 marks

1R_f = distance moved by substance ÷ distance moved by solvent.

2R_f = 7.5 ÷ 10.0.

3R_f = 0.75 (no units; 0 < 0.75 < 1 ✓).

Practice 2: R_f Backwards

A red dye has an R_f value of 0.32 in a particular solvent. In a fresh experiment using the same solvent, the solvent travels 9.5 cm from the start line.

Calculate the distance moved by the red dye. Give your answer to 2 significant figures. 3 marks

1R_f = substance ÷ solvent → 0.32 = substance ÷ 9.5.

2Rearrange: substance = 0.32 × 9.5.

3Distance moved by red dye = 3.04 cm = 3.0 cm (to 2 s.f.).

Practice 3: Identify a Substance

A student tests an unknown food dye U against a reference dye X. The solvent travels 12.0 cm. The dye spots travel:

U: 4.8 cm X: 4.8 cm

Are U and X the same substance? Justify with a calculation. 3 marks

PYes — U and X could be the same substance.

ER_f(U) = 4.8 ÷ 12.0 = 0.40. R_f(X) = 4.8 ÷ 12.0 = 0.40.

EThe two R_f values are identical, run on the same paper in the same solvent. To confirm they are the same substance, the spec says they must give the same R_f in all solvents — so a second test in a different solvent would be needed for proof.

Common Errors

WrongCorrectWhy it loses marks

R_f = solvent ÷ substanceR_f = substance ÷ solventInverted; gives R_f > 1

Pen for the start linePencil for the start linePen ink dissolves and contaminates

Solvent at or above start lineSolvent below the start lineSpot dissolves directly into solvent pool

“Less pure” / “purer”“Pure (one spot)” / “impure (two or more spots)”Comparative language is not enough

“More soluble in the solvent” (when prompt says attraction to paper )“Weaker attraction to the paper”Doesn’t answer the question asked

R_f = 0.80 cmR_f = 0.80 (no units)R_f is a ratio — units cancel

“Both travel further” (asked why R_f is independent of travel)“The ratio of distances is constant”Doesn’t engage with the mathematical relationship

Phrases the Mark Scheme Rewards

Stationary phase

“The paper” (MS Nov21 P2 Q03.3 — MCQ)

Why R_f differs

“Different solvent used” (MS Nov21 P2 Q03.2)

Travel further

“Weaker attraction to the (chromatography) paper” (MS Jun24 P2 Q04.1)

Pure vs impure

“A is impure / a mixture”, “B is pure” / “A produces two spots, B produces one spot” (MS Jun24 P2 Q04.3)

R_f independence

“The ratio of spot distance to solvent distance is constant” / “directly proportional” (MS Jun24 P2 Q04.5)

Setup mistakes

“Start line drawn in ink” / “start line below water level” (MS Jun23 P2 Q06.1)

The 6-Mark RP6 Plan: Checklist

If asked to plan an investigation to determine an R_f value, hit every item on this list (mark scheme indicative content):

M
Method: draw pencil start line; spot of sample on the line; suitable solvent in beaker; paper in beaker so paper is in solvent but solvent is below the start line; lid on; wait for solvent to travel up; mark solvent front; dry the paper.
D
Distances: measure distance from start line to centre of spot; measure distance from start line to solvent front.
C
Calculation: use the two distances to determine the R_f value (R_f = substance ÷ solvent).

Level 3 trigger

To reach Level 3 (5–6 marks), the plan must be logically sequenced and would actually produce a valid R_f. Missing the ‘solvent below start line’ or how distances are measured tends to cap students at Level 2.

Lesson Recap

The two phases

Stationary = paper. Mobile = solvent. Different attractions to each phase produce separation.

Travel rule

Stronger attraction to solvent → travels far → high R_f. Stronger attraction to paper → stays low → low R_f.

R_f formula

R_f = substance ÷ solvent. Always between 0 and 1. No units. If >1, you’ve inverted.

Purity

One spot → pure. Multiple spots → impure / mixture. Use absolute language (“pure”/“impure”), not comparative (“purer”).

Up next

Lesson 8.03: Identification of common gases — the ‘splint, lime water, damp litmus’ tests, where examiners punish missing single words like ‘damp’.