Students will understand after this activity that mixing colors just changes the physical property of color. When there is a color change during a chemical reaction, there is a change at the molecular level and new substances are formed.
For Invitation to Learn
Not all color changes indicate a chemical reaction. Merely mixing colors is a physical change. No new substance is formed. This can be confusing to students when trying to understand the difference between the color change in a chemical reaction and color change when two colors are mixed together (physical change: blue and yellow food coloring mixed together creates green; chemical reaction: colorless vinegar added to purple cabbage juice turns pink).
This activity will help students understand the differences between color mixing and color change due to a chemical reaction as they “get back” the colors they mixed together using chromatography. Chromatography is a method used to separate the different ingredients of a mixture. It was first used by Russian botanist Mikhail Tsvet to separate the pigments that make up plant dyes. It is now used to determine the ingredients that make up flavors or scents, to analyze the components of pollutants, to find traces of drugs in urine, and to separate blood proteins to identify various species of animals. Chromatography is also commonly used in police labs to determine unknown substances found at crime scenes.
There are many different types of chromatography, but all of them involve a gas or a liquid (the water in this activity) flowing through a stationary substance (the paper towel). Because the physical and chemical make-up of the pigments used to make colors vary, the rate and distance at which they travel along the paper towel varies, causing the colors to separate out.
This activity is written as a hands-on activity for cooperative teams. However, the color mixing (steps 1-5) could be done as a teacher demonstration using petri dishes on an overhead projector. Teams should complete the color separation (chromatography).
1-Use science process and thinking skills.
2-Manifest scientific attitudes and interests.
3-Understand science concepts and principles.
4-Communicate effectively using science language and reasoning.
Invitation to Learn:
*Sodium polyacrylate is an essentially harmless substance that was developed by NASA to use in astronaut diapers. Sodium polyacylate can be obtained from most chemical suppliers.
Cooperative teams of 3-5 should complete the following procedures: (See Team Procedures)
Part 1 Color Mixing:
After the teams have completed the above procedures, lead a discussion about the color mixing activity. Have students share what colors each mixture made. Record their responses on the board. Ask: "Is color mixing a physical change or a chemical reaction? What evidence supports your answer?" You may have students supporting both choices. Some students may feel, since there was a color change, that there was a chemical reaction. Accept all answers and record the responses on the board. Tell the students that the next part of the activity will help answer the question. Ask: "Can the substances in a chemical reaction be returned to their original states?" (No)" Can the substances combined in a physical change (mixture) be separated out?" (Yes)
Have the teams complete the following procedures:
Part 2 Chromatography:
At this point lead a discussion about what the students have observed. Share the information from the "Background" section above. Make sure students understand that mixing colors just changes the physical property of color and that usually the colors can be separated out. This shows that only a physical change has occurred. When there is a color change during a chemical reaction, there is a change at the molecular level and new substances are formed.
Chromatography can be used to separate the colors from the inks in felt tipped markers. Have students use the procedures in step 2 using a variety of water color markers.
Use this rubric to assess your students ' performances:
Student 's Name ____________________________________
||Descriptions and data clear and accurate. All observations
||Descriptions and data mostly clear and accurate. All observations
||Descriptions and data somewhat clear and accurate. All obervations
|Descriptions and data unclear and inaccurate. All observations incomplete.|
|Used time well and focused attention on the activity.||Used time fairly well. Stayed focused on the activity most
of the time.
||Did the activity but did not appear very interested. Focus
was lost on several occasions.
||Participation was minimal OR student seemed negative about
This lesson is part of the Fifth Grade Science Teacher Resource Book (TRB3) http://www.usoe.org/curr/science/core/5th/TRB5/. The TRB3 is designed to be your textbook in teaching science curriculum to your students. This book covers all the objectives of each standard and benchmark. If taught efficiently, a student should do well on the End-of-Level (CRT) tests. The TRB3 is designed for teachers who know very little about science, as well as for teachers who have a broad understanding of science.