What’s in a Color
Time: 40-55 minutes
Materials (per group)
1 paper towel sheet or coffee filter
4 small cups (1 for each marker brand)
Calculators
Rulers with metric measurements
4 brands of black markers
4 toothpicks
Water
Scissors
Pencils
Paper
Background:
Chromatography is a method for analyzing mixtures by separating them into the chemicals
from which they are made. It can be used to separate mixtures like ink, blood, gasoline, and
lipstick. In ink chromatography, you are separating the colored pigments that make up the
color of the pen. Even though a pen will only write in one color, the ink is actually
made from a mixture of different colored pigments.
To perform ink chromatography, you put a small dot of ink to be separated at one end of a
strip of paper. This end of the paper strip is then placed in a solvent. The solvent moves up
the paper strip; and, as it travels upward it dissolves the mixture of chemicals and pulls
them up the paper. The chemicals that dissolve best in the solvent will move up the paper
strip further than chemicals that do not dissolve as well. What is produced from this
method is a chromatogram.
Forensic scientists are able to use ink chromatography to solve crimes by matching
documents or stains found at a crime scene to the marker or pen that belongs to a suspect.
Forensic scientists analyze the unknown ink and compare it to writing utensils collected
from possible suspects.
Before the activity, make strips using the marker used on the ransom note. Label
them “mystery ink.” Make 1 per group. Don’t forget what brand you used!
Directions:
1. Divide students groups of 4. Tell them that they are now a team of forensics
detectives with a mystery to solve. Someone stole your dog! You have four possible
suspects: your neighbor (who hates how much your dog barks), your mailman, your
landlord, and your old roommate. The dog thief left a ransom note demanding that
you leave $500 in an envelope in your mailbox or you will never see your dog again.
Before you pay the money, you decide to do some detective work.
2. Have one person from each group come up and get the supplies. It might work best
to pre-fill the cups with about 10 mL of water each.
3. Hand out the “mystery ink” strips and tell the students that this is a sample of the
marker the thief used to write the note. They have obtained markers from all of the
suspects, and will test each marker to see if it matches the note.
4. As a class, decide which marker belongs to which suspect. Write this on the board
and have one person from each group record it on their paper.
*See the sample table at the end of this packet.
5. Instruct the students to cut four strips of the filter
paper, forming a point at one end. Make a large dot
(about the size of a grain of rice) about 1.5 cm up
from the bottom of the pointed end. Label each paper
at the top so you know which marker is which.
6. Use a pencil to make a mark on the paper right next to
the ink dot.
7. Lower the pointed end of the paper into the water, making
sure the dot stays above the water level.
8. Carefully push a toothpick through the top of the paper so
it rests across the top of the cup, holding the strip.
9. Repeat this step for all four markers.
10. When the water has stopped moving up the strip, remove
the paper from the cup and immediately mark the highest
point that the water has traveled up the paper strip with a
pencil.
*It will take 3-5 minutes for the water to stop moving up each strip.
11. Lay the strip down to dry. Continue testing until all strips are done.
12. Point out to the students that each brand of marker uses a different combination of
colors to produce their black ink. Have the students look closely at each strip and
record the colors they can see. Each color represents a different pigment used to
make the ink.
13. To prove that the sample from the note is a certain brand, students will also need to
calculate the Rf (retention factor) value. Rf compares the distance a pigment
traveled up a strip to the distance that the water traveled. To calculate the Rf value:
a. Measure the distance in millimeters from the original color dot to the final
point that the water traveled (between the two pencil marks).
b. Measure the distance in millimeters from the original dot to the highest point
of the first colored pigment.
c. Measure the distances that any
𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑 𝑏𝑦 𝑝𝑖𝑔𝑚𝑒𝑛𝑡
𝑅𝑓 =
other colored pigments traveled.
𝐷𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑒𝑑 𝑏𝑦 𝑤𝑎𝑡𝑒𝑟
d. Calculate the Rf value.
14. Students should calculate and record the Rf values for all of the strips.
15. Have the students decide who they think the dog thief is. Once all the groups have
decided, get a show of hands to show who they think it is. Then reveal the true thief.
Sample table—have each group of students create a similar table on their paper.
Suspect
Sample from
note
Brand
Colors
Rf-1
Rf-2
Rf-3