A famous painting has been stolen from the Fitzwilliam Museum in Cambridge. The thief left a note at the scene mocking the police and saying they plan to strike again

Use chromatography to discover which black pen was used in the crime, and find the culprit.

Skill development

Learners will develop their working scientifically skills by:

  • Using appropriate scientific language and ideas to explain, evaluate and communicate their methods and findings.
  • Drawing conclusions and raising further questions that could be investigated, based on their data and observations.
  • Asking their own questions about scientific phenomena.

Learning objectives

  • Use knowledge of solids, liquids and gases to decide how mixtures might be separated.
  • Compare and group together everyday materials on the basis of their properties, including their solubility.

Concepts supported

Children will learn:

  • That certain colours and dyes are made up from a mixture of colours, and these can be separated by using chromatography.

Equipment

  • Filter paper (e.g. a coffee filter)
  • Plastic or paper bowl
  • Black pen x 3
  • Water
  • Stapler

The best pens for this activity are (1) a black felt tip pen (2) a black permanent pen (3) a non-permanent black pen. These look the same on paper, but have different chromatography results.

Procedure

  1. Draw a spot along the line of the filter paper with each of the suspect’s pens.
  2. Staple the ends of the filter paper together without allowing them to touch.
  3. Place the paper into the bowl so that the marker spots are above the water.
  4. Leave the paper in the water until the patterns become clear.
  5. Compare the ink pattern from the pen found at the crime scene with the three suspect’s pens.
  6. Tell us who committed the crime! 

Suggested activity use

This activity could be used with a whole class. Learners could work in small groups investigating how to separate the colours in the black pens provided. The activity provides a useful exercise for children applying their previous knowledge of chromatography to help solve the ‘crime scene’ problem you have set.

Practical considerations

You may need to experiment with a range of black pens beforehand, as some pens work better than others. You will also need to prepare the letter, using a pen that matches one of the suspect’s pens.

Filter paper will be required as certain papers are not absorbent enough to separate the colours out effectively.

Previous work on chromatography may be beneficial.

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