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Notes for the Instructor

Today students will be introduced to the concept of class evidence. Class evidence is anything in an investigation that points to a group of people rather than a specific individual. While class evidence cannot pinpoint a perpetrator, it can be used to narrow down a field of suspects. Currently your students have a field of four suspects who could have possibly broken the cookie jar.

The class evidence that you will look at in this activity involves the pen used to write the crime scene note. Remember that many people could have owned (and used) a pen like the one that wrote the note, so the results today will not point a finger at one person. However, you can see if any of the suspects falls into the group of people that have access to a particular type of pen.

Although it might appear that black ink is black ink, this is not the case. Each pen manufacturer uses its own unique mix of pigments. A process called chromatography, can help us better see the differences between pens. Chromatography is the process of using a liquid or gas to separate a mixture into its components. Today you will use isopropyl alcohol (rubbing alcohol) to separate the different color inks within the black ink. The different colors will start all together in a dot of black ink at the end of a piece of chromatography paper just as they come out of the pen.

Because the ink is able to dissolve into, or get “picked up” by, the alcohol solution, the two will travel together across the paper. Different colored inks will travel different distances and at different speeds depending on the size of the molecules of each color and how attracted the molecules are to the paper. Each color of ink will stop at its own spot during the trip from the bottom of the paper to the top. The pattern, or bands, of pigments left behind on the paper is called a chromatogram.

Black pens made with the same mixture of colored inks will have similar chromatograms. When comparing chromatograms in this activity, students should look for the same colors appearing in the same order. However the distance up the paper that each color travels may vary from one chromatogram to the other.

In forensics, chromatography of ink can help investigators in a variety of ways. It can show if more than one pen was used on a document, revealing a forgery or changing of the original document. Linking a document to the pen that wrote it can also determine when something was written. For example, an alleged antique document from the 1900s could not have been written by a pen that was first made in the 2000s. In the case of the cookie jar mystery, we can compare the pen used to write the note with pens available to each of the suspects.

The activities in this lesson address Next Generation Science Standards practices of Planning and Carrying Out Investigations and Analyzing and Interpreting Data. In addition, they address Common Core Learning Standards. See the appendix on page 105 for more details.