8.1.3 Chromatography

• What is chromatography?

  Chromatography is a chemical analysis technique used to separate substances in a mixture.
• What is paper chromatography used for?

  To separate mixtures of soluble substances in liquids.
• What is the name given to the pencil line?

  Baseline.
• Why should we use pencil for the baseline rather than pen?

  Pen ink would dissolve in the solvent and move up the paper.
• Should the baseline be submerged in the solvent?

  No.
• What are some solvents commonly used in chromatography?

  Water and ethanol.
• Match the letters A to C on the diagram above with the following descriptions:

  Insoluble parts of the ink: C

  Substance with the a relatively slow rate of travel: B

  Substance with a relatively fast rate of travel: A

  
• What is the name of the resulting paper we end up with?

  A chromatogram.
• The 'mobile phase' refers to the molecules that can move. Which is the mobile phase in paper chromatography?

  The solvent.
• A substance which is more soluble in the mobile phase will spend more time in the mobile phase. This means it will move faster, and travel a further distance up the paper.
• The 'stationary phase' refers to the molecules that can't move. Which is the stationary phase in paper chromatography?

  The paper.
• A substance which is less soluble in the mobile phase will spend more time in the stationary phase, so move slower, and not travel very far up the paper.
• What is the formula to calculate the Rf value?

  Distance travelled by substance / Distance travelled by solvent
• What is the Rf value of the purple substance?

  0.8.

  
• What is the Rf value of the orange substance?

  0.5.

  
• True or false? The Rf value for a substance is specific to a particular mobile phase (solvent) and stationary phase (paper).

  If you change either the mobile or stationary phase, you'll get a different Rf value.

  True.
• How many spots will a pure substance’s chromatogram have?

  1.
• What is an Rf value?

  The ratio of the distance traveled by a compound to the distance traveled by the solvent front in chromatography.