Calculate Rf values and decide what a chromatogram can tell you about a mixture.
Read the chemistry notes and worked example
Think through the method
Place the sample spots on a pencil baseline above the starting solvent level. The solvent carries dissolved substances up the paper. Mark the solvent front before it disappears.
Calculate Rf using distances from the same baseline: distance to the centre of the spot divided by distance to the solvent front. An Rf value is tied to the solvent and conditions.
Matching one Rf value supports a possible identity under matching conditions; it does not prove that two substances are identical. Edexcel also requires a separate ink-distillation part, included when that profile is selected.
Worked example
A spot travels 3.2 cm while the solvent front travels 8.0 cm from the baseline. Rf = 3.2 ÷ 8.0 = 0.40.
A mixture can produce more than one visible spot. Colourless components may need an appropriate locating method.
Exam-board practical requirements
AQA 8.2.6 / RP6
Specification printed pages 106.
separate coloured substances by paper chromatography
calculate Rf
Edexcel 2.11
Specification printed pages 59.
paper chromatography of ink dyes
simple distillation to separate water from ink
Bunsen burner and safe liquid handling
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Explain how a chromatogram supports an identification while leaving uncertainty, and state the additional part required by Edexcel’s ink practical.
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Check your explanation for
Measure both distances from the same baseline.
Keep solvent and conditions comparable when using reference Rf values.
Avoid claiming that one Rf match proves identity.
Include simple distillation of water from ink for Edexcel.
Example answer
Rf compares the spot-centre distance with the solvent-front distance from the same pencil baseline. Matching a reference under the same solvent and conditions is consistent with that dye, but another substance may have the same Rf. Edexcel’s ink investigation also needs simple distillation to collect water separately from non-volatile dye; chromatography alone does not cover that part.
An answer to improve
The spots have the same colour, so they are definitely the same pure substance.
Improve your answer
Add the ratio definition and a qualified conclusion. Keep the board-specific second part explicit.
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