Demonstration: Simple Kinetics FOR THE TEACHER Summary In this demonstration, students will see that different food dyes react with bleach at different rates.
Submitted by: Tom Kuntzleman Spring Arbor University in Spring Harbor, Michigan
Grade Level Elementary, Middle, and High school Objectives By the end of this demonstration, students should be able to • Understand that chemical reactions occur at different rates. Chemistry Topics This demonstration supports students’ understanding of • Kinetics • Reaction rate Time Teacher Preparation: 10 minutes Lesson: 5 minutes Materials For each group: • Concentrated (8.25% sodium hypochlorite) household bleach • 6 Droppers or pipets • Food dyes (red, yellow and blue) • Water • Five 600-mL beakers • Five 250-mL beakers • Stirring rod (optional) Safety • Safety goggles should always be worn when working in a chemistry lab. • When working with base, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water. • 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 • The results of the presentation should look like this: yellow fades to colorless, orange fades to red, green fades to blue, and the red and blue dyes persist. American Association of Chemistry Teachers
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For the extension results, the presentation should look like this: red fades to colorless, purple fades to blue, and blue persists. This demonstration is simple to prepare and carry out. Challenging students to explain their observations challenges them to use critical thinking skills.
FOR THE STUDENT Lesson
Explore Set up 1. It is important to create solutions following these directions so that the food coloring is not highly concentrated in the solution. 2. Measure ~500 mL of water into five separate 600 mL beakers. 3. To make blue water, add two drops of blue food coloring to the first beaker. 4. To make green water, add one drop of blue food coloring and one drop of yellow food coloring to the second beaker (this will work better than using green food coloring). 5. To make yellow water, add two drops of yellow food coloring to the third beaker. 6. To make orange water, add one drop of yellow and one drop of red food coloring to the fourth beaker. 7. To make red water, add two drops of red food coloring to the fifth beaker. 8. Pour 100 mL of each colored water solution into its own 250-mL beaker.
Presentation 1. Add 10 drops of household bleach to each of the 250 mL beakers. Swirl the contents of each beaker or use a stirring rod to mix thoroughly. 2. Observe the contents of each beaker. 3. Ask students to explain the color changes observed. Results after 3 minutes:
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Results after 1 hour:
Explain •
Yellow food dye reacts very quickly with sodium hypochlorite. You can tell this because the yellow color disappears very quickly when bleach is added to yellow food dye. On the other hand, blue and red food dyes react very slowly with sodium hypochlorite. You can tell this because the blue and red colors take a long time to disappear when bleach is added to these colors. Green food dye happens to be made of a combination of yellow food dye and blue food dyes: Green = Yellow + Blue
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When bleach is added to green food dye, the yellow dye reacts quickly to form colorless product. However, the blue color in the green dye does not react quickly, so you can still see its color. That’s why adding bleach to green food dye turns it blue. Likewise, the contents of the orange water are made of a combination of red and yellow food dyes: Orange = Yellow + Red
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When bleach is added to the orange water, yellow dye reacts quickly to form colorless product. However, the red color in the orange water does not react quickly, so you can still see its color. That’s why adding bleach to the orange water turns it red.
Extend Set up 1. Place 500 mL of water into a container. 2. To make purple water, add one drop of blue food coloring and one drop of red food coloring to the second container. 3. Pour 100 mL of the blue, purple, and red water into a 250-mL beaker. Presentation 1. Add 1–2 droppers full (full droppers, not drops) of household bleach to each of the 250mL beakers. Swirl the contents of each beaker. 2. Observe the contents of each beaker. 3. Ask students to explain the color changes observed.
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Explain •
Red food dye reacts more quickly with sodium hypochlorite than blue food dye. You can tell this because the red color disappears more quickly than the blue color when bleach is added to the red and blue water. The contents of the purple water are made of a combination of red and blue food dyes: Purple = Blue + Red
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When bleach is added to the purple water, red dye reacts quickly to form colorless product. However, the blue color in the purple water does not react quickly, so you can still see its color. That’s why adding bleach to the purple water turns it blue. To get the red dye to react rapidly enough to see quick color changes in this part, full squirts of bleach (rather than just a few drops) were added. Increasing the concentration of the bleach in this manner increased the rate of reaction of red dye with the sodium hypochlorite, allowing for quick color change.
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