Launch practical
About 50 minutes
- Set up the bench with the 11 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 12 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
How to read it. The start line, close up.
What it shows
Rf = distance moved by substance ÷ distance moved by solvent
The solvent (the mobile phase) soaks up the paper (the stationary phase) and carries each dye with it. A dye that is more soluble in the solvent and less attracted to the paper spends more time moving, so it travels further. Each dye in a mixture moves its own fraction of the solvent’s distance, its Rf value, so the mixture separates.
The graph to expect. A dye moves a fixed fraction of the distance the solvent moves, so its distance is proportional to the solvent’s: the fraction is its Rf.
Recall: learn it. Measure both from the pencil start line. Rf has no unit and is always less than 1.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Chromatography paper, cut to fit the beaker | The stationary phase the dyes move along. | — |
| Pencil | Draws the start line and the solvent front: graphite does not dissolve. | — |
| 30 cm ruler | Draws the start line and measures each distance. | 1 mm |
| Glass capillary tubes | Put small, concentrated spots on the start line. | — |
| Food colourings A, B, C, D and the unknown U | The samples to separate. | — |
| 250 cm³ beaker | Holds the solvent and the hanging paper. | — |
| Glass rod and tape | The paper hangs from it across the top of the beaker. | — |
| Solvent: water (or ethanol and water) | The mobile phase that carries the dyes up the paper. | — |
| Watch glass | A lid, so the solvent does not evaporate from the paper. | — |
| White tile | A clean, flat surface to spot and measure the paper on. | — |
| Safety goggles | Protect your eyes from splashes and glass. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Boards set this practical slightly differently. Your course setting picks one; you can change it before you start.
Why: The method is the same; the samples, the readings and the exam questions follow your board.
Check the paper hangs clear of the beaker
Hold the paper against the outside of the beaker. It must be narrow enough to hang straight down without touching the glass on either side.
Why: Where the paper touches the glass, the solvent creeps up the wall faster and the front bends.
Put on safety gogglesSafety
Put on goggles and keep them on until everything is cleared away.
Why: Food colourings, inks and solvent can splash into your eyes, and capillary tubes can break.
Draw the start line
With a ruler, draw a horizontal line 2 cm from the bottom of the paper. Mark five dots on it, at least 1 cm from each edge, and label them A, B, C, D and U underneath.
Do: In pencil.
Why: Every distance is measured from this line, so it must stay where it is and not add colours of its own.
Spot the colourings
Dip a clean capillary tube in each colouring and touch it on its dot. Use a new tube for each colour.
Do: Small spots: touch, let it dry, touch again (no more than 5 mm).
Why: The smaller the starting spot, the smaller each spot stays as it moves, so close colours can separate.
Choose the solvent
AQA’s method uses water. An ethanol and water mixture is the alternative AQA suggests for comparing solvents.
Do: Water.
Why: How far each dye moves depends on how soluble it is in the solvent and how strongly it is attracted to the paper, so Rf values depend on the solvent.
Pour the solvent into the beaker
Pour solvent into the 250 cm³ beaker.
Do: No more than 1 cm deep: below the start line.
Why: The bottom of the paper must dip into the solvent, but the start line must stay above it.
Get a lid ready
Have a watch glass ready to cover the beaker once the paper is in.
Why: A lid keeps the air in the beaker full of solvent vapour, so the solvent does not evaporate from the paper as it rises.
Take the readings
One chromatogram: the distance moved by the solvent, then the distance moved by each spot (A–D and each spot in U), all from the start line, in mm.
Repeats: AQA’s method makes one chromatogram, with every colouring on the same paper. You cannot repeat a reading on the same paper (it is dry and the spots have stopped), but you can run a second chromatogram, or compare your Rf values with the class’s, to check they are reproducible.
Make safe and clear upSafety
Pour the solvent away: water down the sink, ethanol into the residue bottle. Put used and broken capillary tubes in the broken-glass bin. Let the chromatogram dry completely, then keep it flat for your results. Wipe the bench, wash your hands, then take your goggles off.
While you take readings
Choose how long to leave it
Leave the beaker where it will not be knocked and do not move it while the solvent rises.
Do: Until the solvent is about 1 cm from the top.
Why: The further the solvent travels, the further apart the spots end up, and a 1 mm reading is a smaller fraction of each distance.
Mark the solvent front straight away
As soon as you lift the paper out, draw a pencil line along the wet edge before it dries.
Why: The wet edge fades within a minute as the solvent evaporates; the solvent distance is measured to this line.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Capillary tubes break easily | Cuts to the fingers from sharp glass | Handle them gently, do not use chipped ones and put broken glass in the broken-glass bin. |
| Ethanol (if used as the solvent) is highly flammable | It could catch fire and cause burns | No flames in the room; use a small volume and keep the lid on the beaker. |
| Food colourings and solvent splashed in the eyes | Sore, irritated eyes | Wear safety goggles throughout. |
| Spilt solvent on the floor | Slips and falls | Wipe up spills straight away. |
Variables
- Independent: you change
- The colouring (A, B, C, D or U)
- Dependent: you measure
- Distance moved by each spot in mm
- Control: you keep the same
- The paper and the solvent. All five samples on the same sheet, in the same solvent.
- Depth of solvent and height of the start line. Solvent no more than 1 cm deep; start line 2 cm up.
- Size of the spots. Small spots, each no more than 5 mm across.
- Temperature. Everything at room temperature.
Results from a specimen run
| Looking at | Observation |
|---|---|
| As the solvent rose | U splits into separate coloured spots |
| The start line | A sharp grey line, still in place |
| Lane U | 3 spots |
| Lanes A to D | One spot each |
| Spot | Distance moved by solvent in mm | Distance moved by spot in mm | Rf value |
|---|---|---|---|
| A | 56 | 11 | 0.20 |
| B | 56 | 49 | 0.88 |
| C | 56 | 18 | 0.32 |
| D | 56 | 31 | 0.55 |
| U, top | 56 | 24 | 0.43 |
| U, middle | 56 | 17 | 0.30 |
| U, bottom | 56 | 11 | 0.20 |
Processing the data
- Rf values
- A 0.20 · B 0.88 · C 0.32 · D 0.55 · U1 0.43 · U2 0.30 · U3 0.20
- What is in U?
- A, C and one other colouring
- Pure or a mixture?
- A, B, C and D: each gives a single spot
| Quantity | Specimen run | Accepted |
|---|---|---|
| Rf of A (dark blue) in water | 0.20 | 0.19 |
| Rf of B (blue) in water | 0.88 | 0.88 |
| Rf of C (crimson) in water | 0.32 | 0.32 |
| Rf of D (red) in water | 0.55 | 0.56 |
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: It will be fine
The paper touches the glass on one side. The solvent front bends up on that side, so spots near it (lane U) move further than they should and their Rf values come out too high (a systematic error).
The fix: Check the paper hangs clear of the beaker. Where the paper touches the glass, the solvent creeps up the wall faster and the front bends.
Method: Skip them
No effect on your data, but a splash or a glass chip could reach your eyes: flagged in the safety review.
The fix: Put on safety goggles. Food colourings, inks and solvent can splash into your eyes, and capillary tubes can break.
Method: In felt-tip pen (it is easier to see)
The ink dissolves in the solvent and runs up the paper as a violet smear across every lane, so you see extra coloured patches that are not in your samples.
The fix: Draw the start line. Every distance is measured from this line, so it must stay where it is and not add colours of its own.
What happens as the solvent rises: Recorded “U splits into separate coloured spots”
The fix: Look again at lane U: the colours have not pulled apart into separate spots.
What happens as the solvent rises: Recorded “U moves up but stays as one spot”
The fix: Look again at lane U: it has pulled apart into more than one colour.
Exam answer: Leaving out: Ink would dissolve (in the solvent) and run up the paper / mix with the spots
The fix: Say what ink does in the solvent: it dissolves and runs up the paper.
Exam answer: Leaving out: Pencil (graphite) is insoluble / does not dissolve (so the line does not move)
The fix: Then say what pencil does: graphite is insoluble, so the line stays where it is.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Pencil start line · Knowledge[2 marks]
Explain why the start line on chromatography paper is drawn in pencil and not in ink.
Worked answer and how the marks are given
A full-mark answer
Ink would dissolve in the solvent and run up the paper, giving extra colours. Pencil (graphite) is insoluble, so the line does not move.
Mark scheme
- Ink would dissolve (in the solvent) and run up the paper / mix with the spots
- Pencil (graphite) is insoluble / does not dissolve (so the line does not move)
Where marks are usually lost
- Missing: Ink would dissolve (in the solvent) and run up the paper / mix with the spots. Say what ink does in the solvent: it dissolves and runs up the paper.
- Missing: Pencil (graphite) is insoluble / does not dissolve (so the line does not move). Then say what pencil does: graphite is insoluble, so the line stays where it is.
- Question 2Describing the method · Knowledge[6 marks]
A student wants to find the Rf value of a food colouring using paper chromatography.
Describe a method the student could use to find the Rf value of the food colouring.
Worked answer and how the marks are given
A full-mark answer
1. Draw a start line in pencil about 2 cm from the bottom of the chromatography paper.
2. Use a capillary tube to put a small spot of the food colouring on the start line.
3. Pour a suitable solvent such as water into a beaker, about 1 cm deep.
4. Hang the paper in the beaker so the bottom dips in the solvent but the start line is above the solvent level.
5. Put a lid on the beaker.
6. Wait for the solvent to travel up the paper until it is near the top.
7. Take the paper out and mark the solvent front in pencil, then dry the paper.
8. Measure the distance from the start line to the centre of the spot.
9. Measure the distance from the start line to the solvent front.
10. Rf = distance moved by the spot ÷ distance moved by the solvent.Mark scheme
Indicative content:
- Draw a start line in pencil on the chromatography paper
- Put a (small) spot of the food colouring on the start line
- Put a suitable solvent (water) in a beaker and stand the paper in it
- The solvent level is below the start line
- Use a lid
- Wait for the solvent to travel up the paper (until it is near the top)
- Mark the solvent front
- Dry the paper
- Measure the distance between the start line and the centre of the spot
- Measure the distance between the start line and the solvent front
- Use the measurements to work out Rf = distance moved by the spot ÷ distance moved by the solvent
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
- Question 3Calculating an Rf value · Applying knowledge[3 marks]
On your chromatogram, the solvent moved 56 mm and the red spot in U moved 24 mm from the start line.
Calculate the Rf value of the red spot in U.
Give your answer to 2 significant figures.
Rf =
Worked answer and how the marks are given
A full-mark answer
Rf = 24 ÷ 56
Rf = 0.43Mark scheme
- Rf = 24 ÷ 56 (method)
- Rf = 0.429
- Answer given to 2 significant figures: 0.43 (precision)
- Allow answers from 0.426 to 0.431 (the marker's tolerance; correct rounding of the student's own working is also accepted).
- One mark for giving the answer to 2 significant figures.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 2.3 (no marks). Upside down: Rf = spot distance ÷ solvent distance = 24 ÷ 56 = 0.43. An Rf value lies between 0 and 1, so a value above 1 shows the division is the wrong way up.
- Answer 0.58 (no marks). Both distances are measured from the start line, not the bottom of the paper: Rf = 24 ÷ 56 = 0.43.
- Answer 43 (gains 1 mark). Rf is a ratio, not a percentage: 24 ÷ 56 = 0.43, with no unit.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Finding the centre of each spot (spots spread out as they move) | Random | Each spot distance can be a millimetre or two out either way, more for big smeared spots, so Rf values scatter. | Use small, concentrated spots (touch, dry, touch again) so they stay compact. |
| Solvent front marked after it had dried | Systematic | Once dry, the front cannot be seen, so the line is a guess. Every Rf on that paper is out in the same direction: too big if the line is below the real front (solvent distance too small), too small if it is above. | Draw a pencil line along the wet edge as soon as the paper comes out. |
| Start line drawn in ink | Systematic | The ink dissolves and runs up every lane, adding a coloured patch that is not in any sample. | Draw the start line in pencil: graphite does not dissolve. |
| Solvent above the start line | Systematic | The colourings dissolve into the solvent in the beaker, so every spot is faint and a faint dye may not show. | Keep the solvent no more than 1 cm deep, below the start line. |
| Paper touching the beaker wall | Systematic | The solvent front bends up on that side, so spots there move further and their Rf values are too high. | Use paper narrow enough to hang clear of the walls. |
| Ruler resolution (1 mm) on short distances | Random | Each distance is only to the nearest millimetre, so each Rf can be a few hundredths out, more when the solvent has not moved far. | Let the solvent travel further (to about 1 cm from the top), so each distance is longer. |
| Solvent evaporating from the paper (no lid) | Random | The front rises unevenly, so Rf values vary from lane to lane. | Put a lid on the beaker so the air inside stays full of solvent vapour. |
Repeatable. If you run a second chromatogram of the same colourings with the same method and equipment, your Rf values agree closely.
Reproducible. Another group, with different paper from the same box, or a different person measuring, gets similar Rf values for the same colourings in the same solvent.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 6 | This page | — |
| Edexcel | 2.11 | Similar, with differences | Chromatogram of an ink, then simple distillation of the ink to collect water. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.