Summary

In this lab, students use chromatography to discover that black ink is a mixture composed of several different pigments.

Grade Level

Elementary school

Objectives

By the end of this lab, students should be able to:

• Explain what makes something a mixture.
• Understand that mixtures can be separated.
• Use the process of chromatography to separate the pigments in black ink.

Chemistry Topics

• Mixtures
• Separating Mixtures

Time

Teacher Preparation: 15 minutes

Lesson: 45 minutes

Materials

• Black washable markers (1 for every group)
• Filter paper (White cone coffee filters - you can cut 4-6 pieces from each one)
• Clear wide mouth cups filled with ~1/2 inch of water (1 for each group)
• Tape
• Large paper clip straightened out (one for each group)
• Chart paper & markers for recording student observations

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wash their hands thoroughly before leaving the lab.
• When students complete the lab, instruct them how to clean up their materials and dispose of any chemicals.

Teacher Notes

• Here is a summary of why the black ink to separate during chromatography:
  • The black ink in washable markers is made from several different pigments.
  • By applying a sample of ink on a very porous piece of paper, like a coffee filter and placing the edge in water the water will travels up the paper.
  • When the water touches the ink it will carry or “pull” the pigments from the black ink along with it.
  • The different colored pigments will move with the water at different rates; some travel farther and faster than others.
  • The distance traveled by each pigment depends upon the chemical makeup of the different pigment molecules and how strongly each of the pigments is attracted to the water molecules and to the paper.
  • This difference in attraction will allow for the ink to separate and reveal the different color pigments that it is composed of.
• As an extension for younger students, teachers could read aloud “Mouse Paint” by Ellen Stoll Walsh or watch the YouTube video of Mouse Paint by Didi Dolan.
• As an extension for older students, teachers could actually do a more advanced chromatography investigation of candy or leaves
• • Engage students by showing a black washable marker.
  • Question them by asking “Black ink is made of black ink, right?” Elicit and discuss student responses.
  • Ask, “How can we find out what it is made of?”
  • Show students the materials for the experiment: cups, water, filter papers, black markers, paperclips and tape.
  • Lead the discussion with students about how these materials could be used to explore and conduct an investigation to see if black is made of only black ink or composed of something else.
  • Have students follow these procedures to conduct the chromatography separation process –see photographs for clarification. (Use a projector or chart paper display the procedures if desired):
    

1. Cut the rectangular strips from the filter paper (about 3-4 inches in length, by 1-2 inches in width). Each group or student will need one.

1. Make a mark using the black marker about a 1 inch from the bottom of the paper. Make sure to make the dot dense with ink.
2. Using tape, attach the end of the paper that is farthest from the ink dot to a straightened paperclip.
3. Add a small amount of water to the cup, about ½ an inch in height.

1. Place the filter inside of the cup. Be sure the black marker circle is toward the bottom and use the paperclip to keep the paper in the center of the cup (avoid having the paper touch the inside of the cup)
2. The paper should dangle into the water, and the edge of the paper should be in the water, but the ink dot should not touch the water.



1. Wait, observe, and discuss what is happening to the black ink with the group.

• During the lab ask for students’ predictions about what they think is happening.
• Walk, talk, share, and discuss what you see with individuals, small groups and the whole class.
• Ask students and groups to share with the rest of the class about what they are observing. Record the student observations on a chart.

• Typical Results (After 1 minute; 3 minutes; 5 minutes- final):