Olivia Pearson

Colby College 2016

January 15, 2016

Teacher’s Guide: Who took LuSeal?

Description: This lesson plan shows the students how crimes can be solved using chemistry. The students will learn about the topic of paper chromatography and how to separate the dyes that make up the ink of a pen through classroom activities. We will discuss various kinds of “invisible ink” during a demonstration and the students can then use this knowledge to decode a secret message.

Learning Goals:

Students will test:

  1. Which of four pens is made of ink that matches the ink found on a note at a crime scene
  2. How to decode a secret “invisible” message written with white wax

Students will observe:

  1. How to decode a secret message using lemon juice and heat

Students will understand:

  1. How scientists use the method of paper chromatography and how we can figure out which of four suspects took LuSeal.
  2. How different kinds of “invisible ink” work.

Recommended Grades:K-8th

Estimated Time Required: ~45 minutes, not including 20 mins prep time before class

Key concepts and Definitions of Terms:

Paper Chromatography – Paper containing a small ink dot is suspended in a container that has a shallow liquid layer at the bottom (such as water). As the water moves up the paper, the different dyes in the ink mixture travel up the paper at different rates and create different patterns. Therefore, we can determine which pen was used to write a specific notes based on the chromatograms each pen produces.

Acids and Oxidation – acids are compounds that can donate a hydrogen ion to another compound and has a pH less than 7. When heated, electrons are lost from the acid and it becomes oxidized.

Solubility – This is the property of a substance that describes its ability to dissolve into a solvent. Polar solutes dissolve in polar solvents and nonpolar solutes dissolve into nonpolar solvents.

Materials Needed:

-4 different pens of the same color

-50-100 coffee filters

-20-25 plastic cups (depending on number of students in class)

-20-25 skewers

-4-5 ransom notes (1 per table)

-1 stuffed animal or object to hide (ex: stuffed toy seal)

-lemon juice

-1 notecard

-source of heat (hairdryer, hair straightener, hot plate, toaster oven, etc.)

-20-25 pieces of paper

-1 white block of wax (or 1 white crayon with white paper)

-watercolor paint

-4-5 weighing boats (one per table for paint)

-20-25 paint brushes

-crayons (for the worksheet)

Estimated Cost: ~$20

Safety Information:

-Safety goggles should be worn at all times

General Outline of Procedures

Paper Chromatography Activity – Who took LuSeal?

K-3rd Grade:

  1. Before class, cut out rectangular strips from the coffee filters. Take each pen and make a dot about an inch from the end of the rectangle. Make sure each table of students has one rectangular piece with a dot from each of the pens (one full set of the different pens) and one that has the ink found on the ransom note—one table should have 5 total samples.
  2. Show the students the stuffed animal/object that will be hidden. Turn off the lights and have them put their heads on the desk. Hide the stuffed animal somewhere in the classroom (suggestion: behind the door), and place 1 copy of the ransom note on each table.
  1. Tell the students that someone stole the animal and hid it somewhere in the room. Four suspects have been detained—each with pens—and they need to figure out who took the object using paper chromatography.
  2. Demonstrate to the students how to perform the experiment. Fill the plastic cups so there is ~1 cm of water at the bottom. Pierce the top of the paper (opposite the end of the ink dot) with a skewer and rest the skewer on the top of the cup, allowing the bottom of the paper to touch the water within the cup—see picture. Observe as the water moves up the paper and separates the pigments in the ink.
  3. Tell the students to perform the experiment with each of the four pieces of paper with the four ink dots from the different suspects. Have them figure out which suspect is innocent.

What Happens: As the water moves up the paper, the dyes in the ink move at different rates and create different patterns. The suspect guilty of stealing the object will have a pen whose ink creates the same pattern on the chromatogram as that of the ink found on the ransom note.

4th-8th Grades

  1. Before class, cut out rectangular strips from the coffee filters. Take each pen and make a dot about an inch from the end of the rectangle. Make sure each pair of students has one rectangular piece with a dot from each of the pens (one full set of the different pens) and one that has the ink found on the ransom note—one pair of students should have 5 total samples. Make sure each student has his or her own cup and skewer.
  2. Tell the students that someone stole an object and hid it somewhere in the room. Four suspects have been detained—each with pens—and they need to figure out who took the object by using paper chromatography.
  3. Tell the students to fill the plastic cups so there is ~1 cm of water at the bottom. They should then pierce the top of the paper (opposite the end of the ink dot) with a skewer and rest the skewer on the top of the cup, allowing the bottom of the paper to touch the water within the cup—see picture. Observe as the water moves up the paper and separates the pigments in the ink.
  1. Tell the students to perform the experiment with each of the four pieces of paper with the four ink dots from the different suspects. Have them figure out which suspect is innocent.

Lemon Juice Demonstration

  1. The night before (or at least an hour before class), use a small paintbrush to write a message on a notecard using lemon juice and allow it to dry.
  2. Ask the students if they are able to read the message.
  3. Apply heat to the notecard with a hair straightener, by placing it in the oven, or by setting it on top of a hot plate and applying pressure.
  4. When removed from the heat, the message will be brown and easy to read.

What Happens: When the heat is applied, the acidic lemon juice is oxidized and turns brown. However, the paper burns at a much higher temperature so it stays white.

Invisible Ink Activity

  1. Before class, use a block of white wax to write a message on a piece of paper that contains the location of the missing object. Make 20-25 copies depending on the number of students.
  2. Tell the students that you questioned the suspect, who said that he/she would tell you the location of the missing object. However, because the suspect was so upset about being caught, he/she wrote the location in invisible ink.
  3. Tell the students that in order to uncover the secret message, they must paint over it with watercolor paint. Make sure that each table has one weighing boat and a tube of watercolor paint and that each student has one paintbrush.
  4. When the students paint over the wax, the message will appear (ex. “Behind the door”).

What Happens: The wax is nonpolar and repels the polar watercolor paint thereby revealing the secret message.

Special Notes:

-It is essential when performing the paper chromatography experiment that the ink dot does not actually touch the water at the bottom

-It is very important that permanent markers are not used in this experiment. Water-based inks must be used.

References

"BCA Chemistry." Web. 14 Jan. 2016. <

"Paper Chromatography." Web. 14 Jan. 2016. <

"Solubility." Web. 14 Jan. 2016. <

Student Worksheet (k-3rd Grade)

  1. Draw each chromatograph.

Ransom Note / Suspect #1 / Suspect #2 / Suspect #3 / Suspect #4

2. Which suspect is guilty?

  1. How did you uncover the secret message?

Student Worksheet (4th-8th Grades)

  1. What are the different colors on the chromatograms?
  1. Which suspect is guilty? How do you know?
  1. Why could you read the message written in lemon juice after heat was applied?
  1. Why were you able to see the secret message after painting over it?

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