Ch 3 · Chromatography · IGCSE Chemistry 0620
Topic 3 of 5 in Experimental Techniques and Chemical Analysis — IGCSE Chemistry 0620 — 38 lessons.
Introduction
Paper chromatography separates the substances in a mixture by how far they are carried up a piece of paper by a solvent. It is quick, needs only a drop of sample, and shows at a glance whether a substance is pure and what a mixture contains. You must be able to describe the method, read a chromatogram, and (Extended) calculate Rf values and deal with substances you cannot see.
Key ideas — The method
The substances separate because each is carried a different distance: a substance that is more soluble in the solvent and less strongly attracted to the paper travels further. The finished paper is a chromatogram.
🧑🏫 Teacher's explanation
Imagine a crowd of people walking through a shopping mall. Some people are very fast and move straight through (highly soluble, low affinity for the floor), while others stop frequently to look at shop windows (less soluble, high affinity for the paper). Because they move at different speeds, they eventually spread out across the mall. Chromatography does exactly this with molecules.
📖 Definitions
🔍 Worked example — Easy
A student performs chromatography on a blue dye. The solvent travels 5.0 cm from the baseline, and the blue dye travels 4.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Standard
A chromatogram shows a solvent front at 8.0 cm and a red dye spot at 2.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Exam-style
An unknown substance forms a spot at 4.5 cm from the baseline. The solvent front is at 9.0 cm. A known standard of the same substance has an R_f of 0.50. Determine if the unknown is the same substance and explain why.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Interpreting chromatograms
🧑🏫 Teacher's explanation
Imagine you are at a race where everyone starts at the same line, but some runners are much faster than others. As the race progresses, the fast runners pull ahead, while the slow runners stay near the start. Chromatography is essentially a race for molecules. The paper is the track, and the solvent is the wind pushing the runners forward. Because different molecules have different 'affinities' (preferences) for the paper versus the solvent, they move at different speeds.
📖 Definitions
🔍 Worked example — Easy
A student runs a chromatogram. The solvent front moves 8.0 cm from the baseline. A blue dye spot moves 6.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Standard
An unknown ink produces two spots. Spot A has an R_f of 0.45. Spot B has an R_f of 0.80. A known reference ink produces a single spot with an R_f of 0.45. Identify the composition of the unknown ink.
🔍 Worked example — Exam-style
A student uses a solvent that does not move the pigment from the baseline. The distance from the baseline to the solvent front is 10.0 cm. Calculate the R_f value of the pigment.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Calculate the R_f value for the yellow spot. [2] (b) Explain why the food coloring is considered a mixture. [1] [3 marks]
Key ideas — Rf values and colourless substances
🧑🏫 Teacher's explanation
Imagine a crowd of people walking through a shopping mall. Some people are very fast and walk straight to the exit, while others stop frequently to look at shop windows. If you know how far the 'exit line' moved and how far a specific person moved, you can calculate a ratio that describes their 'speed' or affinity for the shops. In chromatography, the 'shops' are the solvent, and the 'people' are the chemical substances.
📖 Definitions
🔍 Worked example — Easy
A blue dye travels 4.0 cm from the baseline. The solvent front travels 10.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Standard
A student performs chromatography on a mixture. A spot of unknown substance X travels 3.2 cm from the baseline. The solvent front is at 8.0 cm from the baseline. Calculate the R_f value.
🔍 Worked example — Exam-style
In an experiment to identify an amino acid, the solvent front moves 12.0 cm from the baseline. An unknown spot moves 9.0 cm from the baseline. A known standard of the amino acid has an R_f of 0.75. Is the unknown the amino acid?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. A student runs a chromatogram of an unknown food colouring X and three permitted dyes A, B and C. The solvent front is 8.0 cm above the baseline. X gives two spots, at 2.4 cm and 6.0 cm; A, B and C give single spots at 2.4 cm, 4.4 cm and 6.0 cm. (a) State what the chromatogram shows about the purity of X, and which dyes it contains. [2] (b) (Supplement) Calculate the R_f value of dye B. [2] (c) Explain why the baseline is drawn in pencil and why it must be above the solvent level at the start. [2]
🔍 Worked example — Easy
A student runs a chromatogram of an unknown food colouring X and three permitted dyes A, B and C. The solvent front is 8.0 cm above the baseline. X gives two spots, at 2.4 cm and 6.0 cm; A, B and C give single spots at 2.4 cm, 4.4 cm and 6.0 cm. (a) State what the chromatogram shows about the purity of X, and which dyes it contains.
🔍 Worked example — Standard
(Supplement) Calculate the R_f value of dye B, given it traveled 4.4 cm and the solvent front traveled 8.0 cm.
🔍 Worked example — Exam-style
A substance travels 3.0 cm from the baseline. The solvent front travels 12.0 cm. Calculate the R_f value.
⚠️ Common errors
Common mistakes
Practice
Answers Draw a pencil baseline on chromatography paper and put spots of the black ink and each known ink on it [B1]; stand the paper in a suitable solvent with the level below the baseline and cover [B1]; let the solvent rise, then mark the solvent front and dry the paper [B1]; compare the heights of the spots from the black ink with those of the known inks; matching heights show the same dye [B1]. Y is pure (a single substance). [B1] To stop the solvent evaporating, which would change the distance it travels or stop it rising evenly. [B1] R_f = 3.6/9.0 [M1] = 0.40 [A1]. Run the chromatogram as usual; the amino acids are colourless so no spots are visible [B1]; spray the dried paper with a locating agent, which reacts with the amino acids to give coloured spots [B1]; measure the spots and calculate R_f values, then compare them with known values for the same solvent [B1].