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1.4 Fractional Distillation and Paper Chromatography

GCSE Chemistry · AQA · Chapter 1: Atomic Structure

A free GCSE Chemistry lesson for AQA (8462) from OBA School: separating liquids with similar boiling points, and separating dissolved substances with paper chromatography. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Explain when fractional distillation is used and how the fractionating column works
  • Describe how to carry out paper chromatography
  • Explain how chromatography separates substances using a mobile phase and a stationary phase
  • Interpret a chromatogram and calculate Rf values

Key words

  • Fractional distillation: Separating a mixture of liquids that have different (but often close) boiling points.
  • Miscible: Liquids that mix completely and do not form layers, e.g. ethanol and water. (Oil and water are immiscible.)
  • Fraction: A part of the mixture collected over a small range of boiling points.
  • Chromatography: A method that separates dissolved substances as a solvent moves through paper.
  • Stationary phase: The part that does not move: the chromatography paper.
  • Mobile phase: The solvent that moves through the paper, carrying the substances.
  • Solvent front: The furthest point the solvent reaches up the paper.
  • Rf value: Distance moved by a substance ÷ distance moved by the solvent.

Fractional distillation

Simple distillation works when the boiling points are very different. When two liquids have boiling points that are close together, such as ethanol (78 °C) and water (100 °C), both evaporate together. To separate them we use fractional distillation, which adds a fractionating column packed with glass beads.

Fractional distillation is used for miscible liquids – liquids that dissolve in each other completely, so you cannot see two layers. Liquids start to evaporate well below their boiling points, so in simple distillation the vapour would contain both liquids.

  • The column is hottest at the bottom and coolest at the top.
  • Vapours rise and condense on the beads, then evaporate again many times. Each time, the vapour gets richer in the liquid with the lower boiling point.
  • The liquid with the lowest boiling point reaches the top first and passes into the condenser. The thermometer stays at its boiling point.
  • When the temperature rises to the next boiling point, a new beaker is used to collect the next fraction.

Exam tip: You can check that a fraction is fairly pure by measuring its boiling point: a pure liquid boils at one fixed temperature. If a fraction boils over a range, it still contains some of the other liquid.

Key fact: Fractional distillation is used to separate crude oil into fractions, to separate oxygen and nitrogen from liquid air, and to make spirits such as whisky.

Carrying out paper chromatography

Paper chromatography separates substances that are dissolved in a solvent, such as the dyes in an ink or a food colouring.

1. Draw a start line in pencil about 2 cm from the bottom of the paper. (Pencil does not dissolve; ink would run.) 2. Put small, concentrated spots of each sample on the line. 3. Stand the paper in a beaker of solvent. The solvent level must be below the start line, or the spots would dissolve into the solvent in the beaker. 4. Put a lid on to stop the solvent evaporating. Wait for the solvent to soak up the paper. 5. Take the paper out before the solvent reaches the top. Mark the solvent front in pencil and let the paper dry.

How chromatography separates substances

Every chromatography method has a stationary phase (the paper) and a mobile phase (the solvent). Each substance is shared between the two phases. A substance that is more soluble in the solvent spends more time in the mobile phase and moves further up the paper. A substance that is more strongly attracted to the paper moves a shorter distance.

  • A pure substance gives one spot, whatever solvent is used.
  • A mixture may give two or more spots.
  • A substance that does not dissolve in the solvent stays on the start line.

Interpreting chromatograms and Rf values

The Rf value of a substance compares how far it moved with how far the solvent moved. Both distances are measured from the start line:

Rf = distance moved by substance ÷ distance moved by solvent

X has two spots, so it is a mixture. Its spots match the heights of C and B, so X contains dyes B and C, but not A.

Worked example: calculating an Rf value

On the chromatogram above, the solvent front is 5.0 cm above the start line. The spot from dye B is 3.5 cm above the start line.
Rf = 3.5 ÷ 5.0 = 0.70

Answer: 0.70 (no units)

Exam tip: An Rf value is always between 0 and 1. If you get a value bigger than 1, you have divided the wrong way round. Different compounds have different Rf values in the same solvent, so Rf values help to identify substances.

Common mistakes

  • Drawing the start line in pen. The ink would dissolve and run up the paper with the samples.
  • Putting the start line below the solvent level. The samples dissolve into the solvent in the beaker.
  • Measuring distances from the bottom of the paper. Always measure from the start line.
  • Saying a substance moves further because it is “heavier” or “lighter”. It depends on solubility in the solvent and attraction to the paper.
  • Using simple distillation for liquids with close boiling points. You need a fractionating column.

Revision questions

Try these, then check your answers below each one.

Question 1. Why is fractional distillation used, rather than simple distillation, to separate ethanol from water? (Choose from: Their boiling points are close together, Ethanol is a solid, Both liquids are insoluble in each other, Water does not boil.)

Answer: Their boiling points are close together

Working: With close boiling points, both liquids evaporate together, so a fractionating column is needed to separate them.

Question 2. On a chromatogram, a spot has moved 3.0 cm from the start line and the solvent front has moved 7.5 cm. Calculate the Rf value.

Answer: 0.4

Working: 3.0 ÷ 7.5 = 0.40.

Question 3. True or false? A pure substance gives only one spot on a chromatogram.

Answer: True

Working: True. A pure substance contains one compound, so it gives one spot in any solvent.

Related lessons

Continue in the full course

This lesson is section 1.4 (AGCCH0104) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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