Purity, formulations and chromatography

8.1.1 Pure substances

In Chemistry, pure substance is…

  • Single element or compound
  • Not mixed with any other substance
  • Eg vaporised iodine

Pure elements and compounds

  • Melt & boil at specific temp
  • Melting & boiling point data → distinguish pure substances from mixtures

In everyday language, pure substance is…

  • Substance that has had nothing added to it
  • Unadulterated & in its natural state
  • Eg pure milk

8.1.2 Formulations


  • Mixture that has been designed as a useful product
  • Made by mixing components in carefully measured quantities → ensure product has the required properties
  • Eg fuels, cleaning agents, paints, medicines, alloys, fertilisers & foods

8.1.3 Chromatography

What is chromatography used for?

To separate mixtures & give info to help identify substance

How to set up?

  1. Draw a pencil line on the paper.
  2. Place ink on the baseline in a dot.
  3. Place the paper into the beaker which has water in it. Make sure the baseline is above water level so dye doesn’t dissolve in water.
  4. Hang paper over edge of beaker to keep it right.
  5. Put a lid on coz H2O(s) ⇌ H2O(g) & lid → closed reaction
  6. Wait for solvent to go up paper near the top.
  7. Remove paper & let it dry.
  8. Draw circles around spots.

Describe the results

  • A is made up of 1 dye
  • B is made up of 1 dye
  • C is made up of 1 dye
  • A & C matches the colouring from cake icing

Calculate the distance moved by the solvent (2)

Explain how different dyes are separated by paper chromatography. (4)

  • Solvent moves through paper
  • Different dyes have different solubility in solvent & different attractions for paper
  • So they are carried to different distances

Why do different compounds separate in a gas chromatograph column? (1)

  • Different substances travel at different speeds

Identify the errors & describe problem each error would cause (4)

  • Baseline drawn in ink – dissolves in solvent
  • Dye under solvent – dissolves in solvent
  • Water level above start line
  • Food colours would dissolve into water
  • Start line drawn in ink
  • Ink would ‘run’ on the paper