Simple Kinetics Mark as Favorite (12 Favorites)

In this demonstration, students will see that different food dyes react with bleach at different rates.

Elementary, middle, and high school

This demonstration supports the following units, topics and learning objectives:

Unit 5: Kinetics
- Topic 5.1: Reaction Rates
  - TRA-3.A: Explain the relationship between the rate of a chemical reaction and experimental parameters.
- Topic 5.2: Introduction to Rate Law
  - TRA-3.B: Represent experimental data with a consistent rate law expression.
- Topic 5.3: Concentration Changes Over Time
  - TRA-3.C: Identify the rate law expression of a chemical reaction using data that show how the concentration of reactions species change over time.
Unit 9: Applications of Thermodynamics
- Topic 9.4: Thermodynamic and Kinetic Control
  - ENE-4.D: Explain, in terms of kinetics, why a thermodynamically favored reaction might not occur at a measurable rate.

This demonstration will help prepare your students to meet the performance expectations in the following standards:

HS-PS3-5: Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.
Science and Engineering Practices: Plan and conduct an investigation individually and collaboratively to produce data to serve as the basis for evidence, and in the design: decide on types, how much, and accuracy of data needed to produce reliable measurements and consider limitations on the precision of the data (e.g., number of trials, cost, risk, time), and refine the design accordingly.
Crosscutting Concepts:
- Patterns: Different patterns may be observed at each of the scales at which a system is studied and can provide evidence for causality in explanations of phenomena.

By the end of this demonstration, students should be able to

This demonstration supports students’ understanding of

Teacher Preparation: 10 minutes

Lesson: 5 minutes

For each group:

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.

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.
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.

Set up

It is important to create solutions following these directions so that the food coloring is not highly concentrated in the solution.
Measure ~500 mL of water into five separate 600 mL beakers.
To make blue water, add two drops of blue food coloring to the first beaker.
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).
To make yellow water, add two drops of yellow food coloring to the third beaker.
To make orange water, add one drop of yellow and one drop of red food coloring to the fourth beaker.
To make red water, add two drops of red food coloring to the fifth beaker.
Pour 100 mL of each colored water solution into its own 250-mL beaker.

Presentation

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.
Observe the contents of each beaker.
Ask students to explain the color changes observed.

Results after 3 minutes:

Results after one 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

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

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

Place 500 mL of water into a container.
To make purple water, add one drop of blue food coloring and one drop of red food coloring to the second container.
Pour 100 mL of the blue, purple, and red water into a 250-mL beaker.

Presentation

Add 1–2 droppers full (full droppers, not drops) of household bleach to each of the 250-mL beakers. Swirl the contents of each beaker.
Observe the contents of each beaker.
Ask students to explain the color changes observed.

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

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.