All Resources

Chromatography Caper

In this activity, students will explore chromatography, and solve a mystery.

Inks, paints, and dyes consist of particles of colour that are dissolved or suspended in a liquid base. When you write, the liquid part dries and leaves just the colour behind.

When the water creeps up filter paper on which a note has been written in felt marker, it contacts the dried colour. The colour molecules in the ink are dissolved and wicked up the strip with the water by capillary action. The more soluble a molecule is in water, the farther it will be carried by the water, higher up the paper.

Different colours are made of different sized molecules, that disolve differntly into water, some get carried further and faster than others. This process is called chromatography: the separation of a mixture by passing it in solution or suspension through a medium in which the components move at different rates.

In this activity: A mystery culprit has left behind a note and the students are asked to determine which of the 4 suspects wrote the note. Each suspect owns a different marker. Each brand of marker will have a different combination of pigments; by separating out these pigments and comparing them we can find out which pen was used to write the note.

Teacher Tip:Permanent marker and dry-erase board inks are not water soluble, so the water will not be able to dissolve its pigments to carry them up the strip. Be sure to use Washable felt pens with water soluble ink.

Objectives

  • Examine the separation of coloured chemicals via chromatography.

Materials

  • Per Group of 3-4 students:
    4 coffee filters cut into strips (4 per group
    4 types/brands of black markers, labelled 1–4 (e.g. a couple of water-based markers from different brands, a permanent marker, a white board marker); each group has the same marker variety
    water
    tape
    plastic or paper cups

Key Questions

  • What colours did the black ink separate into?
  • Which colour traveled up the filter paper furthest? Why do some inks not travel up the filter paper?
  • Which number marker matches the marker used for the original ransom note?

What To Do

Preparation:

  1. Use one of the 4 black pens to write a fake ransom note on a piece of coffee filter e.g. “I have your cookies, if you want them back bring a bag of candy to the playground.”
    Be sure and let the ink dry equally on all notes. Preparation the day before is ideal.

Activity:

  1. Divide students into groups of 3 or 4.
  2. Tell the students that there are 4 suspects who could have written the note, and that you have a marker from each of them. In order to find out who took the cookies, we have to find out whose marker matches the ink in the note.
  3. Demonstrate how to set up a chromatography test using a strip cut from the ransom note. Stick this strip up on the board for the students to look at. The students will compare their results to this strip.
  4. Number your filter strips 1–4.
  5. With pen #1, draw a horizontal line across filter #1, three finger widths from the bottom.
  6. Pour water into a paper cup to a depth of 0.5 cm. Dip the end of filter #1 into the water and tape it to the side of the cup so that it remains upright.
  7. Wait 2–3 minutes for the ink to stop moving and remove the filter to let it dry.
  8. Repeat the steps 3–5 using pen numbers 2, 3, and 4. Each student in the group gets a turn.
  9. Compare the filters to the sample taken from the ransom note to determine whose pen wrote the note.

Extensions

  • Try doing a chromatography experiment using different coloured markers. What colours does a brown, orange, purple, or green marker separate into? Are all browns the same?
  • Compare markers that come from different brands. Do the same colours travel equal distances? Why or why not?