Mystery Science: What Chromotography Can Tell Us About the Ransom Note

A pigment is a chemical that makes something look a certain color. The brown color of a felt marker, ink pen and brown food coloring are all mixtures of different colored pigments. These pigments combined in different proportions, but all appear brown. When a piece of paper is dipped in water, the water gradually moves up the paper as it is absorbed. As the water moves through a water soluble brown stain, it dissolves the pigments in the stain. Depending on the size, shape and weight of pigment, it may be carried along by the water quite a way up the paper or may only be carried a short distance. When an ink is made of a mixture of pigments, as in the case with the ink in this mystery, each pigment will be carried up the paper in a different way, resulting in different bands of color, with each color representing a different pigment. This rainbow streak up the paper is called a chromatogram. We can recognize our mystery ink by comparing its chromatogram to that of the inks from the suspects.

We have experimented with chromatography before but this time we were going to use chromatography to identify a particular ink.

Here are some of the things we used for our chromatography experiment.

I gave my scientist-sleuths a piece cut from the ransom note (paper towel with a word written in brown), some blank paper towel strips, a small container of brown ink labeled "Ink from Mr. Green's pen" and another container (of brown food color) labeled "Ink from Prof. Plum's pen" and a brown marker that was found in the kitchen. They also had four pieces of straw and four clear cups, and a pipette.

I had them make a dot in the middle of a blank piece of paper towel with the marker. This is the marker found in the kitchen.

On another piece of paper towel, we made a dot with the ink pen from Mr. Green's pen. On a third piece of paper towel, we made a dot with the ink from Professor Plum's pen. I had them label the strips with a pencil.

They then taped each of the strips to a piece of straw-the ransom note, the ink from Prof. Plum, the ink from Mr. Green and from the marker. They then set the straws on cups of water so that the paper touched the water and started climbing up the paper.

They watched what happened when the water moved through the dots. I had them notice if the marker or one of the inks matches the mystery dot.

Who do you think wrote the ransom note? Do not be discouraged if you cannot tell. More clues will be coming up.

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GEMS: Crime Lab Chemistry

Grades 4-8

5 Sessions

Now an updated and enhanced New GEMS edition. Challenged to determine which of several black pens was used to write a ransom note, students learn and use paper chromatography, as they explore the concepts of solubility, pigments, and separation of mixtures. New sessions provide students with multiple opportunities to visualize the molecular nature of matter as they create and revise their own models and consider the limitations of models.