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Study Hack Lab

RP 6Chemistry

Chromatography

Write-up template

Name
Date
Course
GCSE Chemistry
Board and specification
AQA GCSE Chemistry (8462), 4.8.1.3AQA GCSE Combined Science: Trilogy (8464), 5.8.1.3Apparatus and techniques: AT 1, AT 4
Class and teacher

Aim

Separate four food colourings (A–D) and an unknown mixture (U) by paper chromatography, calculate Rf values and find out which colourings are in U.

Contents

  1. Plan and risk assessment
  2. Apparatus
  3. Method
  4. Results
  5. Graph
  6. Analysis
  7. Conclusion and evaluation
  8. Make safe and clear up

A blank write-up for use in a real lesson. The headings follow the AQA required practical; the readings are yours to take. Layout v1, practical v2. Original material; not affiliated with AQA, which has not reviewed it.

1 Plan and risk assessment

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.

Rf = distance moved by substance ÷ distance moved by solvent

Variables

Independent (you change)
Dependent (you measure)
Control (you keep the same)

Prediction

U is a mixture of colourings. What do you expect its spot to do as the solvent rises?

Dye X is very soluble in the solvent and only weakly attracted to the paper. Compared with other dyes, X will…

How many spots will a pure substance give?

Risk assessment

HazardHarm it could causeControl

2 Apparatus

  • 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 (resolution 1 mm) Draws the start line and measures each distance.
  • 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.

3 Method

Write numbered steps someone else could follow. Say what you measure, with which instrument, and how you keep it a fair test.

4 Results

The table is drawn before you measure. Record each reading to the resolution of the instrument, then work out the mean of your repeats, leaving out any anomaly.

Observations

Looking atObservation
As the solvent rose
The start line
Lane U
Lanes A to D

Distances on the chromatogram · Every distance from the pencil start line: to the solvent front, and to the centre of each spot.

SpotDistance moved by solvent in mmDistance moved by spot in mmRf value
A
B
C
D
U, top
U, middle
U, bottom

Anomalies

Ring any anomalous result. Say what probably went wrong and whether you repeated it.

5 Graph

This practical has no graph: summarise your results in a sentence.

6 Analysis

  1. Rf values

    Answer

  2. What is in U?

    Answer

7 Conclusion and evaluation

Conclusion

Conclusion: what do your results show? Use your own numbers.

Sources of error

For each, say whether it is systematic or random, and how it affected your results.

Finding the centre of each spot (spots spread out as they move)

Solvent front marked after it had dried

Start line drawn in ink

Solvent above the start line

Paper touching the beaker wall

Ruler resolution (1 mm) on short distances

Solvent evaporating from the paper (no lid)

Improvement

How could you improve the method? Say why it would help.

8 Make safe and clear up

  • 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