Paper Chromatography in IGCSE Chemistry — How to Read Spots and Calculate Rf

A black pen leaves a single dot on paper. Run that ink through paper chromatography, and the dot might separate into yellow, red and blue spots. Those three spots give you a clue about what was in the ink.

In Cambridge IGCSE Chemistry (0620), Core students learn to separate soluble coloured substances and interpret the resulting pattern. Extended students also learn to calculate (R_f) values and use locating agents to reveal colourless substances. Cambridge IGCSE Chemistry syllabus, Section 12.3

How does paper chromatography separate ink?

A solvent travels up the paper and carries the dissolved dyes with it. Different dyes interact differently with the solvent and the paper, so they can travel different distances. The pattern left on the paper is called a chromatogram.

Two details matter when setting up the experiment.

• Draw the starting line in pencil. Pencil graphite stays in place under typical classroom conditions. A pen line could dissolve and add extra spots.

• Keep the starting line above the solvent. The bottom of the paper touches the solvent, while the ink spots sit above it. If the spots are submerged, the samples can wash into the solvent before they separate.

When you take the paper out, mark the solvent front immediately. This is the furthest point the solvent reached, and it can become hard to see as the paper dries.

Worked example — what’s in the black ink?

A student runs an unknown black ink alongside three known dyes on the same sheet of paper. All the samples start on the same pencil line. The solvent front reaches 8.0 cm above that line.

Which known dyes match spots in the unknown ink?

Dyes A and B match the yellow and red spots in the unknown ink. Each pair has the same colour and travelled the same distance under the same conditions.

Dye C is blue, but it travelled 5.0 cm. The blue spot from the unknown ink travelled 6.0 cm. The colour is the same, but the spots are at different heights, so dye C doesn’t match.

What is the Rf value of the red spot?

Measure both distances from the pencil starting line. Measure the dye distance to the centre of the spot.

Rf = distance travelled by the substance ÷ distance travelled by the solvent

For the red spot, Rf = 4.0 cm ÷ 8.0 cm = 0.50. The centimetres cancel, so (R_f) has no unit.

Try it yourself

On a different chromatogram, the solvent front travels 10.0 cm and a purple spot travels 6.5 cm from the starting line.

1. What is the purple spot’s (R_f) value?

2. A second purple spot on the same chromatogram travels 5.0 cm. Does it match the first?

Answers — 1. 0.65, from 6.5 ÷ 10.0. 2. No. The second spot has an (R_f) of 0.50, so the two purple spots travelled different distances under the same conditions.

Frequently asked questions

Does one spot mean the sample is pure?

One spot is consistent with a pure substance in a simple exam chromatogram. In a real experiment, two substances can travel together and appear as one spot, so a single spot alone cannot prove purity.

How do you see colourless substances on a chromatogram?

A suitable locating agent makes the separated spots visible. Once you can see them, you can compare their positions or measure how far they travelled.

Can I compare Rf values from two different solvents?

Compare (R_f) values measured under the same conditions. Changing the solvent or paper can change how far a dye travels. Running the unknown and known samples side by side is a straightforward way to make a fair comparison.

For revision planning across subjects, read TheTuitionE’s IGCSE exam preparation guide for UAE parents.

If your child can use the formula but gets stuck deciding which distances to measure, bring a recent chromatogram question to a free TheTuitionE demo class.