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Bath Bomb Chemistry! (5 Favorites)

LAB in Reaction Rate, Reaction Rate, Acid Base Reactions, Chemical Change. Last updated March 5, 2019.


Summary

In this lab, the students will work in cooperative groups to investigate how to make a scented, effervescent bath bomb and explore the chemistry needed to create them. Groups will test a variety of different variables in order to compare the reaction rate of each sample. Students will use the results to determine which variables are optimal for making and using bath bombs.

Grade Level

Middle

NGSS Alignment

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

  • MS-ETS1-3: Analyze data from tests to determine similarities and difference among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.
  • Scientific and Engineering Practices:
    • Analyzing and Interpreting Data

Objectives

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

  • Investigate how evidence of chemical reactions indicates that new substances with different properties are formed.
  • Determine reaction rate for a chemical reaction and analyze how a specific variable maight affect the reaction rate.
  • Demonstrate safe practices during laboratory and field investigations.
  • Plan and implement comparative investigations by making observations, and asking well-defined questions.
  • Organize data, identify patterns and analyze experimental results.

Chemistry Topics

This lab supports students’ understanding of

  • Kinetics
  • Reaction Rate
  • Chemical reactions
  • Chemical Change
  • Indicators of Chemical Reactions
  • Acid-Base Reactions

Time

Teacher Preparation: 1 hour

Lesson: 90 minutes (2 days needed)

Materials (per group)

  • 1 Ice cube tray
  • 2 Mixing bowls
  • 1 Spoon
  • Measuring spoons
  • Measuring cup sets
  • 1 Pipette
  • Thermometer
  • Timer/stopwatch
  • 120 ml or ½ cup Citric acid
  • 240 ml or 1 cup Baking soda
  • 180 ml or 3/4 cup Cornstarch
  • 1/4 cup or 60 ml Epsom salts
  • 3 teaspoons or 15 ml Oil (vegetable, olive, or coconut)
  • 1 teaspoon or 5 ml Water (may need more)
  • Food coloring (several drops)
  • Essential oils (several drops)
  • Tubs of room temperature water to test the bath bombs (one per group)

Other (for specific variables/groups only):

  • Tub of hot water for one group (enough to fill their testing tub)
  • Tub of ice water for one group (enough to fill their testing tub)

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.
  • Exercise caution when using a heat source.
  • When working with acids, if any solution gets on students’ skin, they should immediately alert you and thoroughly flush their skin with water.
  • When working with acids and bases, if any solution gets on your skin, immediately rinse the area with water.

Teacher Notes

  • Bath bombs can be fun to make and use to teach students about chemical reactions.
  • There are many different recipes for making bath bombs, but most require a few key ingredients: baking soda and citric acid. Baking soda, sodium bicarbonate (NaHCO3) is a weak base and citric acid (C6H8O7) is a weak acid. When these two ingredients are mixed together and introduced to water, they undergo a chemical reaction which produces carbon dioxide (CO2) gas. The equation for this reaction is shown below:

3NaHCO3 + C6H8O7 → Na3C6H5O7 + 3H2O + 3CO2

  • This gas produces the bubbles in the bath water. Another key ingredient is cornstarch. This is a dry ingredient that gets mixed with the baking soda and citric acid.
  • The amount of cornstarch used to create the bath bomb will affect the reaction rate when dropped into the water. The temperature of the water may also affect the reaction rate. The reaction rate for a chemical reaction is a measurement of how quickly the reaction happens, or how quickly the reactants (the baking soda and citric acid) create the products (CO2 bubbles). The faster the bath bomb dissolves, observable by more CO2 bubbles and a “fizzier” reaction, signifies that the reaction is happening faster. The students will use a timer to measure how long it takes the bath bomb to react in water in order to determine the reaction rate.
  • I suggest that each group tests a different variable. If there are more groups than variable in your classroom, you can have multiple groups conduct the control recipe, or have multiple groups test a particular variable, or even have groups come up with another variable to test.
  • Variable suggestions: Groups should complete either the control trial, extra cornstarch trial (add 1¼ cup to recipe), less cornstarch trial (add ¼ cup to recipe), hot water trial or cold water trial.
  • Part of the challenge of making homemade bath bombs is adding the right amount of wet ingredients. If you live in a humid environment, you may not need to add all of the wet ingredients. If the bath bombs are very crumbly, the recipes may not have had enough water in them.
  • After the students have created the bath bombs in the ice cube trays, they should be stored the ice cube trays in a dry area without humidity overnight.
  • On day 2, prepare the testing tubs of water for the students. Most groups will need tubs of room temperature water, while the groups testing the effect of hot or cold water will need hot water or ice.
  • Citric acid can be purchased at the grocery store near the canning supplies. It can also be purchased in bulk from Amazon.
  • Essential oils can be purchased from any craft supply store.
  • There are many websites and videos to refer to for recipe modifications, instructions, and tips on making homemade bath bombs:

For the Student

Lesson

Background

Why would you buy expensive bath bombs when you can easily make them by using your knowledge of chemistry? Bath bombs become fizzy when dropped in water due to a chemical reaction between the ingredients. This can make your bath relaxing, bubbly, and invigorating and depending on what ingredients you use, it can also smell great!

There are many different recipes for bath bombs, but all require a few key ingredients: baking soda and citric acid. Baking soda (sodium bicarbonate, NaHCO3) is a weak base and citric acid (C6H8O7) is a weak acid. When these two ingredients are mixed together with water, they undergo a chemical reaction which produces carbon dioxide (CO2) gas. This gas is what makes the fizzy bubbles in the water. The equation for this reaction is shown below:

3NaHCO3 + C6H8O7 → Na3C6H5O7 + 3H2O + 3CO2

Another key ingredient is cornstarch. This is a dry ingredient that gets mixed in with the baking soda and citric acid. The amount of cornstarch you use to create the bath bomb may affect the reaction rate when dropped into the water.

Another variable that may impact the reaction rate of the bath bomb is the temperature of the water. How do you think the temperature of water can increase or decrease the reaction rate?

The reaction rate for a chemical reaction is a measurement of how quickly the reaction happens, or how quickly the reactants (the baking soda and citric acid) create the products (CO2 bubbles). The faster the bath bomb reacts, observable by more CO2 bubbles and a “fizzier” reaction, means that the reaction is happening faster.

Pre-lab Questions

  1. Vocabulary: Define the following words below:
    1. Chemical reaction
    2. Acid
    3. Base
    4. Reaction rate
    5. Reactant
    6. Product
  2. In a bath bomb, what are the reactants and what are the products?
  3. Does the temperature of the water affect the reaction rate of the bath bomb?
  4. Does the amount of cornstarch affect the reaction rate of the bath bomb?

Problem

What variable will affect the reaction rate (fizziness) of a bath bomb?

Materials (per group)

  • 1 Ice cube tray
  • 2 Mixing bowls
  • 1 Spoon
  • Measuring spoons
  • Measuring cup sets
  • 1 Pipette
  • Thermometer
  • Timer/stopwatch
  • 120 ml or ½ cup Citric acid
  • 240 ml or 1 cup Baking soda
  • 180 ml or 3/4 cup Cornstarch
  • 1/4 cup or 60 ml Epsom salts
  • 3 teaspoons or 15 ml Oil (vegetable, olive, or coconut)
  • 1 teaspoon or 5 ml Water
  • Food coloring (several drops)
  • Essential oils (several drops)
  • Tubs of room temperature water to test the bath bombs (one per group)

Other (for specific variables/groups only)

  • Tub of hot water for one group (enough to fill their testing tub)
  • Tub of ice water for one group (enough to fill their testing tub)

Safety

  • Always wear safety goggles when handling chemicals in the lab.
  • When working with acids and bases, if any solution gets on your skin, immediately rinse the area with water.
  • Wash your hands thoroughly before leaving the lab.
  • After the lab, follow your teacher’s directions on how to dispose of any chemicals.

Procedure

  1. Determine which independent variable your group will test during this experiment. You will either complete the control trial, extra cornstarch trial (add 1¼ cup to recipe), less cornstarch trial (add ¼ cup to recipe), hot water trial or cold water trial. Record it in the data table below.
  2. In one bowl, mix together: citric acid, baking soda, cornstarch, and Epsom salts. (If your group is testing the amount of cornstarch as the independent variable being tested, then you will need to use a different amount than one is listed in the materials section.)
  3. In the second mixing bowl, mix together the oil, water, food coloring, and essential oil.
  4. Using a pipette, add a few drops of the wet ingredients to the dry ingredients. You should see it fizz a little as the chemical reaction begins. To stop the chemical reaction, press down on the fizzy spot with the spoon. Mix in the damp spot with the rest of the dry ingredients. Keep adding the wet ingredients a few drops at a time until they are completely mixed in with the dry ingredients.
    1. Part of the challenge of making homemade bath bombs is adding the right amount of wet ingredients. If you live in a humid environment, you may not need to add all of the wet ingredients.
    2. If the bath bombs are very crumbly, you may need more water.
  5. Press the mixture into the ice cube tray and let it sit at room temperature for a day.
  6. After a day, carefully remove the bath bombs from the ice cube tray.
  7. Record the temperature of the water in your testing tub.
  8. Prepare to time your bath bomb reaction rate.
  9. Drop one bath bomb into the testing tub of water and record the time it takes for the bath bomb to completely react.
  10. Repeat with additional bath bombs to have a total of three trials.
  11. Collect data from the other groups, and share your data with the other groups.

Data

My group tested:

Independent Variable

Reaction Rate (Minutes and Seconds)

Trial 1

Trial 2

Trial 3

Average

Control Group
(no changes)

Extra Cornstarch

Less Cornstarch

Hot Water

Ice Water

Calculations

Calculate the average reaction rate for each group’s three trials. Show your work below, and record final averages in the data table.

Conclusion

Discuss your results, and make a conclusion about the variables that can affect bath bombs based on your data.