Homework activity: Chromatography crime solving

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Chromatography is a separation technique used by forensic scientists to separate mixtures and identify substances found at a crime scene. In this simple investigation students can use paper chromatography to identify the pen used to write a note.

Each student will need:

  • Pieces of kitchen paper (white with no pattern is best)
  • At least 3 different black felt-tip pens
  • Pencils
  • Sticky tape and scissors
  • Glasses

Student instructions

Give a friend or a member of your family the black pens and a sheet of kitchen paper. Tell them that you will leave the room and then they should choose a pen and write a short message on the paper. You will use the power of science to tell them which pen they used!

  1. Collect the note and the pens.
  2. Cut the note to form a long strip which has a small amount of ink at one end (see diagram).
  3. Cut out more strips from another sheet of kitchen paper – one for each of the pens.
  4. Onto one end of each strip use a pen to draw a small dot about 1 mm across and 2 cm from one end of the strip. Repeat for all the pens so you have several strips of paper.
  5. Use sticky tape to tape the end of each strip to a pencil and place it in a glass of water (see diagram). Make sure the water does not cover the ink dot.
  6. Repeat this for all the strips (including the one from the note) and leave them for a few minutes. The water will travel up the paper, carrying the ink with it.

Black ink is a mixture of different coloured pigments. The pigments differ in their solubility. The more soluble inks travel further up the paper. This results in them separating out.

Which pen was used to write the note? How do you know?

Students need to compare the pattern of the strip taken from the note with that obtained from each of the pens. The pen that matches the note is the one that was used.

Find out more

Your students can find out how a similar technique using DNA can help forensic scientists solve crimes by reading Smart Scientist: Professor Sir Alec Jeffreys and DNA fingerprinting

Students can test their understanding of pure substances and separating techniques by completing Structured questions: Forensic science.