Chromatography with ink pens (black) and ink on a note

Students use chromatography to separate the colours in black ink.
The unique colours in different black pens can be used to identify which pen was used to write a note.
Science content
Chemistry: Physical Changes, Solutions, Mixtures and Separating (2, 4, 5, 6)
  • Pot e.g. large yogurt pot
  • Water to fill pot to a depth of 1cm
  • White coffee filter, cut into strips (add size) (use a template for students to make them the correct length for the pot)
  • Bamboo skewer or other thin stick
  • Small binder clip
  • Water soluble black pens. For the Honey Mystery three pens with different chromatography patterns are Pentel, Crayola Washable and Smile Maker

Do chromatography with 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 image).
With each black pen make a line across the strip of the filter paper, over the pencil line.
Clip the other end of the filter paper into the small binder clip, then thread it through the skewer. Balance the skewer on top of the pot so that the filter paper dips into the water (see 2nd image)
Wait three our four minutes until the colours have separated up the filter paper to within 1cm from the binder clip (see 3rd image).
Many black pens have a different chromatography pattern (see 4th 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).

Do chromatography with the ink on the note:
Cut a small piece of the note and use a paper clip to secure it, ink side down, to a new strip of filter paper.
Separate the colours as before.
Compare the colours to those with the three pens. Although the pattern will be less intense, the colours in it should identify it as one of the pens. (See 5th image for the chromatography patterns of two pens: the two right-hand chromatography patterns in each case were from a written note).
(If this does not work too well, the note can be written directly on filter paper with enough room to cut it into strips that can hang in the pot).

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.

Grades taught
Gr K
Gr 1
Gr 4
Gr 5
Gr 6
Teaching site
Brock Elementary
Gordon Elementary
ingridscience afterschool