Before the lesson, write the note using one of the black pens. Example of what it can read: "What you seek is buried under the old cedar at the back of the house". Something more applicable to the classroom or school building would be better.
In the lesson, read the note to the students, and show them the black pens that might have written the note, and that they will use chromatography to figure out which one. Demonstrate the steps below, before allowing them to proceed.
Prepare chromatography strips for the black pens:
Cut out filter paper, using the template to make the correct size. Draw a pencil line across the paper, where indicated on the template (see 1st photo).
With each black pen make a line across the strip of the filter paper, over the pencil line.
Prepare chromatography strip for the ink on the note:
Cut off a small piece of the note with a word on it, lay it word-side down on a filter paper strip, at the same level as the line on the template, and use a paperclip to attach them together. (2nd photo.)
Run the chromatograms:
Clip the top of the filter paper strips with the small binder clips (for all the black candidate pens, and for the note). Then thread the coffee stir stick through the binder clip arms and lay the sticks across the top of the tub. The filter paper should dip into the water, but the black lines and the piece of note are above the water line. Allow the chromatograms of the three candidate pens to run (3rd photo). Wait three our four minutes until the colours have separated up the filter paper to within 1cm from the binder clip.
Although the chromatogram with the note is not as distinct as the chromatograms from the black lines, the colours should still be distinguishable, so that the pen that wrote the note can be identified (see 4th photo comparing the chromatogram from a line and from a note).
Many black pens have a different chromatography pattern (see 5th image). Note that marker brands change their ink composition now and again (e.g. crayola now has more ink colours in some of their black markers).
How does chromatography work?
The coloured dye molecules in the ink of the pen are attracted to both the water that it is in, but also the surface of the filter paper. Each different colour is attracted to the water or the filter paper to different extents. As the water moves up, the dye molecules that are most attracted to the water will move along fast with it. If the dye molecules are mostly attracted to the paper, they will get stuck to the paper and not move along with the water at all. Most colours are attracted to both the water and the paper, so will travel with the water for a while, then stick to the paper for a while. Depending on the relative attraction of a dye to the water and the paper, a colour will travel at its own rate. The differing rates of travel separate out the colours.