This topic accounts for approximately 5% of your exam marks.
stable
Rare
Stable5%
Definitions and separation techniques tested consistently across both papers.
Aim
Use paper chromatography to separate the components of a mixed food colouring
Identify the components by comparing Rf values and spot positions against known reference colourings
Apparatus
250 cm³ beaker (with a watch glass or piece of clingfilm as a lid)
Strip of chromatography paper sized to fit inside the beaker
Five fine capillary tubes (one fresh tube per colouring)
Four reference food colourings labelled A–D
One mixed food colouring of unknown composition labelled U
Wooden spill (or any pencil long enough to span the beaker mouth)
Paper clip
Ruler and a sharp pencil
Method
Draw a horizontal baseline in pencil about 2 cm in from one short edge of the paper
Using a fresh capillary tube for each colouring, place a small spot of each of A, B, C, D and U on the baseline, evenly spaced
Keep every spot under about 3 mm wide
Label each spot beneath in pencil
Add water to the beaker so it forms a shallow pool no deeper than 1 cm
Clip the top of the paper to the wooden spill and lay the spill across the beaker mouth so the paper hangs vertically, spotted end down
Lower the paper into the beaker so its lower edge dips into the water, with the baseline staying above the water level
Cover the beaker (watch glass or clingfilm) to keep the air inside saturated with solvent vapour
Let the solvent climb the paper undisturbed until it has risen roughly three-quarters of the paper's length, and always before the front meets the top edge
Lift the paper out, immediately mark the solvent front in pencil, and let the paper dry flat
Measure (in mm) two distances: baseline → solvent front, and baseline → each spot's centre
Calculate Rf for every spot in lanes A–D and U
Identify the unknown. A spot in U with the same Rf, and at the same height, as a reference spot is the same substance
Results — suggested table
Lane
Distance moved by spot / mm
Distance moved by solvent / mm
Rf value
A
B
C
Sources of error
Baseline drawn in pen, not pencil. Pen ink itself runs up the paper and contaminates every lane. Always use pencil for baseline and labels
Sample spots too large. Bands smear during migration and the spot centre is hard to pin down. Use a fresh capillary tube per colouring and keep each spot small
above the baseline. Samples dissolve straight into the solvent pool, so no migration occurs. Always check the solvent level before lowering the paper
Solvent reaches the top edge of the paper. Once the front has gone off the paper, you cannot tell how far it travelled, so no Rf is calculable. Lift the paper out well before the front gets there
Beaker left uncovered. Solvent evaporates from the paper, slowing the run and shifting the spot positions. Cover the beaker to keep the inside saturated with vapour
Exam tip
Describing the chromatography procedure (or explaining setup errors)
What comes up: Either "describe how to set up and carry out the experiment" (5 marks) or "explain two mistakes in the setup shown" (4 marks).
Write — procedure: (1) Draw a line in pencil near the bottom edge of the paper. (2) Place a spot of each sample on the pencil line before any contact with solvent. (3) Pour a shallow layer of solvent into the beaker. (4) Lower the paper so the bottom edge dips into the solvent with the spots above the solvent surface. (5) Leave until the solvent has nearly reached the top, then remove and immediately mark the solvent front in pencil.
Write — explaining mistakes: For each error, you need two linked points — identify the mistake, then explain the consequence. For the ink baseline: baseline drawn in ink (M1), so the ink will itself run up the paper and interfere with the results (M2). For the solvent above the baseline: solvent level is above the spots/baseline (M3), so the sample dissolves directly into the solvent pool and no migration occurs (M4).
Watch out: For the "pencil" step, the credited reason is that pencil (graphite) is insoluble in the solvent, so it will not run up the paper. Saying "pen ink would smudge" is insufficient — you need to link it to solubility.