To gain experience in asking questions and conducting inquiry by exploring the separation of colors in water and other solvents; to communicate and share findings of student investigations.
This lesson uses a technique called paper chromatography. The water is absorbed by the coffee filter and rises up the filter. When the water reaches a spot of black ink, it carries the components of the spot up the filter. As the water continues to rise, the components do not all travel with it at the same rate. Some are more soluble in water than others, and the more soluble ones travel faster. After a time, the various components are at different distances from the original spot, having been separated from one another. Substances in which other substances dissolve, like water, are called solvents. Scientists use chromatography frequently to separate and identify the component parts of a mixture.
This activity will help children gain experience in conducting simple investigations of their own while working in small groups. Throughout the lesson, encourage children to observe more and more carefully, measure things carefully, record data clearly in logs and journals, and communicate their results in charts and simple graphs as well as in prose. Student investigations should be followed up with presentations to the entire class to emphasize the importance of clear communication in science.
This activity should be done as a teacher demonstration.
- Fill two colorless plastic or glass cups about 3/4 full of water.
- To one of these add a few drops of blue food coloring and to the other a few drops of yellow. Mix well. (The colors should be pretty deep for a good effect.)
- Pour about 1/3 of each colored solution into another empty cup. Mix well.
- Ask students to describe what is observed. (The new color (green) seems to be a mixture of blue and yellow.)
- Add a few drops of green food coloring to a fourth cup of water.
- Can you tell the difference between the color in this fourth container from that in the third?
- Is the green food coloring a mixture of blue and yellow?
- How can we find out?
Try to guide the students to think about how to separate a combination of dyes into its individual components in order to figure out what the combination is.
How Do We Separate Colors?
Each person or small group will need these materials:
- 1, 8-ounce wide-mouth plastic cup filled with a half inch (1 cm) of water
- Melitta coffee filters (#6 size) or similar brand and size
- Black water-soluble marker (nonpermanent markers for overheads work best)
- 2-3 paper towels
What to do:
- Using the black marker, have students decorate both sides of the coffee filter with a few dots, lines, or other markings. The simpler the pattern, the more striking the results will be. Caution students to be careful not to mark the ribbed bottom edge.
- Have students place their filters in the cup of water. Only the ribbed edge should be in the water. Then, allow the filter to sit undisturbed. Every few minutes, students should check to observe what is happening and make drawings of what they see. Is there a color separation? Is it the same for each mark made?
- Even though it takes 10 to 15 minutes for the colors to fully separate, students should be attentive to the separation process as it goes on. You may decide to have students record what they observe in five-minute intervals, or you may simply ask guiding questions, such as those below, to keep students focused on the observation. As soon as the water level has risen to the top of the paper, remove the filter from the cup and gently open the filter. Compare the filters.
Have students answer these questions:
- What colors do you see on the filter after you open it up?
- What happened to the black ink? Where is it on the filter paper?
- What happened in the activity that you didn't expect or that was different from what you expected?
Students' investigations at this level should focus on detecting similarities and differences among the things they investigate. Encourage them to look at other students' filter papers to see if the results are similar or different. They should come to see that in trying to identify and explain likenesses and differences, they are doing what goes on in science all the time. Have them try to explain some of the differences they observe.
Students may find it puzzling when different groups of students get different results doing supposedly the same experiment. That, too, happens to scientists, sometimes because of the methods or materials used, but sometimes because the thing being studied actually varies. In this experiment, the color separations should be similar; the differences will result from the differences in the original student decorations of the filter paper.
What Difference Would It Make?
After students have separated the colors in black ink, they are ready to extend their knowledge with further explorations. Time should be provided to let students run enough trials to be confident of their results.
Use these guiding questions to have students design and test investigations that explore chromatography:
- Do you think the same thing would happen if you used red ink? Green ink? Purple ink? Try it.
- What is the effect of temperature on color separation? How can you find out? Try it.
- Does the shape of the filter make a difference in what you observe? Try cutting the filter into different shapes.
- Does the kind of mark (dot, line, etc.) make a difference? How can you find out? Try it.
- What do you think would happen if you used other kinds of inks, not just water-soluble markers?
To help students develop their investigations, they can visit Art from Science for Kids at the American Chemical Society. Be sure to have students brainstorm original ideas before they visit the website.
Students should present their investigation and results to the rest of the class. As explanations take on more and more importance, teachers must insist that students pay attention to the explanations of others and remain open to new ideas. As students prepare and refine their presentations, encourage them to describe their procedures with enough detail to enable others to replicate them. Explain to them that submitting their work to the criticism of others is part of the scientific way of doing research.
Present the students with these problems:
- Janet's younger brother has asked her to make some grape Kool-Aid for him. The problem is that he is allergic to blue food coloring. If he eats or drinks anything with this food coloring in it, he will break out in a rash. How can Janet decide if it is okay for her brother to drink the grape Kool-Aid?
- Joseph wants to paint a birdhouse that he made for his backyard. Should he paint it with water-soluble paint? Why or why not?
Students should write a one-page paragraph in response to each problem. They should provide evidence from the activities in the lesson to support their answers.
Water is the simplest solvent to use in paper chromatography, but not all components of a sample may dissolve in water. Ask students what they think would happen if they used a different liquid in the cup? Do they think the colors will still separate?
Have students construct an investigation to repeat the test with different types of liquids, such as vinegar, lemon juice, rubbing alcohol, or carbonated water. (If you do this, the students will have to cover the jars with a plastic bag to have the fumes contained. Make sure they do not use volatile liquids like gasoline.) If they are testing different inks in different liquids, make sure that they test each type of ink separately in each type of liquid. Have them record their observations in a science log and compare their results with those of the rest of the class.
Give students an opportunity to develop their own investigation about some questions they may still have.
- How will other kinds of paper work?
- Will different brands of the same color marker separate differently?