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:
  • Bath Bomb Science
  • Make Fizzy Bath Bombs – Planet Science
  • Sudsy Science: Creating Homemade Bath Bombs