Chromatography and separation techniques: Rf values explained
Choose between filtration, crystallisation and distillation, and calculate Rf values in chromatography, with worked examples.
KS3GCSEMiddle SchoolHigh SchoolCurriculum fit: GCSE Chemistry and Combined Science (separation techniques). US: middle and high school physical science (properties of matter).
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What is it?
A mixture contains two or more substances that are not chemically joined, so they can be separated by physical methods. The best method depends on the properties of the substances, such as solubility and boiling point.
Chromatography separates substances, such as the dyes in ink, by how far they travel up a paper with a solvent.
Key formulas and ideas
- Filtration separates an insoluble solid from a liquid.
- Crystallisation obtains a soluble solid from a solution.
- Simple distillation separates a solvent from a solution. Fractional distillation separates liquids with different boiling points.
- Rf = distance moved by the substance ÷ distance moved by the solvent front.
Worked examples
Example 1: Choosing a method
Question: Which method separates sand from water?
- Sand is an insoluble solid.
- Filtration holds back the sand and lets the water through.
Answer: Filtration
Example 2: Rf value
Question: A spot travels 3.0 cm and the solvent front travels 6.0 cm. Find the Rf value.
- Rf = 3.0 ÷ 6.0.
- = 0.5.
Answer: Rf = 0.5
Example 3: Fractional distillation
Question: How can ethanol be separated from a mixture with water?
- Ethanol boils at about 78 °C and water at 100 °C.
- Heat the mixture and collect the ethanol first, as it evaporates at the lower temperature.
Answer: Fractional distillation
Common mistakes
- Drawing the pencil baseline in ink or in a place where it dips into the solvent.
- Measuring to the wrong point of the spot or solvent front.
- Using filtration for a dissolved solid.
- Giving an Rf value greater than 1.
Practice questions
Frequently asked questions
Why use pencil for the baseline?
Pencil does not dissolve in the solvent, so it does not run up the paper.
What does the number of spots tell you?
It shows the minimum number of different substances in the mixture.
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Reviewed by StudyMorrow editorial review on 5 Nov 2026.