1-chromatography

• what is the definition for chromatography?

  separation of mixtures of soluble substances by running a solvent through the mixture on paper, causing the substances to move at different rates over the paper
• what are the steps

  • water and ethanol solution is heated
  • as the paper is lowered into the solvent some of the dye spreads up the paper
  • the paper has absorbed the solvent , and the dye has spread further up the paper
• what does separation by chromatography produce?

  a chromatogram
• what can a paper chromatogram be used for?

  distinguish between impure and pure substances

  • pure produces one spot
  • impure produces 2 or more spots
• how are two substances likely to be the same?

  if they have the same number of spots and match in colour

  they have the same Rf value
• how is Rf value calculated?

  distance travelled by substance/ distance travelled by solvent
• how are Rf values given?

  rounded as decimals and always between 0 and 1
• what does a Rf value of 0 mean?

  no separation- substance in insoluble so the spot remains at the starting line
• what does a Rf value of 1 mean?

  substance has moved as far as the solvent , very rare
• why is it important to draw the lines and write labels on the chromatography paper in pencil and not in ink?

  pencil marks are non-soluble, while ink can dissolve and interfere with the separation of compounds.
• why should the spots of ink be above the level of the solvent in the beaker?

  otherwise the solvent would simply wash it off into the solution
• what is meant by the term ‘solvent front’?


  the solvent front is the furthest point reached by the solvent in a chromatography experiment.
• What would happen if you used permanent ink instead of water soluble ink? How could you overcome this problem?

  there would be a poor separation and you should test the ink to confirm its solubility in the chosen solvent before you carry out the experiment
• which is the mobile phase?

  the solvent used in chromatography.
• which is the stationary phase?

  the liquid or solid phase