#7
Mixtures, chromatography and separation
Paper 1 · Higher combined
Key steps (method)
- Decide what property you’re using to separate:
- Filtration (insoluble solid from liquid)
- Evaporation/crystallisation (soluble solid from solution)
- Simple distillation (solvent from solution)
- Fractional distillation (mixture of liquids)
- Chromatography (mixture of soluble substances)
- For chromatography: measure Rf = distance moved by spot ÷ distance moved by solvent front.
Common mistakes
- Putting the baseline in ink (it must be pencil).
- Measuring Rf from the wrong points.
- Confusing evaporation with crystallisation (crystallisation is controlled to get crystals).
Exam-style questions (quick)
- Describe how to obtain a pure dry sample of salt from a solution.
- A spot moved 4.0 cm and the solvent front moved 10.0 cm. Calculate Rf.
- Explain why different substances move different distances in chromatography.