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The Chemistry of Casts Mark as Favorite (3 Favorites)

LESSON PLAN in Chemical Change, History, Chemical Change, Exothermic & Endothermic, Error Analysis. Last updated September 11, 2023.


In this lesson, students will learn about the history of the chemistry involved in creating orthopedic casts. Students will then work in small groups to conduct a hands-on experiment in which they will determine the optimum plaster of Paris to water ratio for model casts.

Grade Level

Middle School

NGSS Alignment

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

  • MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
  • MS-ETS1-1: Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.
  • MS-ETS1-2: Evaluate competing design solutions using a systematic process to determine how well the meet the criteria and constraints of the problem.
  • 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:
    • Using Mathematics and Computational Thinking
    • Analyzing and Interpreting Data
    • Constructing Explanations and Designing Solutions
    • Engaging in Argument from Evidence
    • Obtaining, Evaluating, and Communicating Information


By the end of this lesson, students should be able to:

  • Identify that exothermic reactions give off heat.
  • Explain the role of chemistry in developing orthopedic casts.
  • Experimentally determine which ratio of materials results in the strongest model cast.

Chemistry Topics

This lesson supports students’ understanding of:

  • Exothermic Reactions
  • Chemical Change
  • States of Matter


Teacher Preparation: 15 minutes
Lesson: 90 minutes (Part 1: 40 minutes, Part 2: 30 minutes, Part 3: 20 minutes)


  • Plaster of Paris - CaSO4•1/2H2O – 100g per student group
  • Balance – for measuring plaster of Paris
  • Water – pitchers of water, or pre-measured amounts could be used
  • Graduated cylinders – for measuring water
  • Paper towels – five paper towels per group cut into two-inch strips to use as the fabric for casts; additional paper towels as needed for clean up
  • Plastic water bottles or empty paper towel tubes, 1 per group – to serve as the model limbs that students will test their plaster casts on
  • Disposable cups, 2 per group – for mixing the plaster of Paris
  • Spoons, popsicle sticks, or stirring rods – for mixing the plaster of Paris and water
  • Thermometer, 1 per group
  • Plastic wrap – to protect the thermometer
  • Goggles
  • Gloves
  • Aprons


  • 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.
  • Do not dispose of plaster of Paris in the sink. It may be helpful to set out pitchers of water and block off sinks or complete this activity in an area where students do not have access to sinks.

Teacher Notes

Part 1: History of Casts

  • Begin the lesson by asking students to individually answer the first two questions on “The History of Casts” handout: What is the purpose of putting a cast on a broken or fractured arm or leg? What do you think casts are made of?
  • Walk around the room to review student responses as they write.
  • When students have written a couple of sentences, ask for volunteers to share their responses and write a list of possible cast ingredients on the board.
    • Students should conclude or be led to the idea that casts help bones heal by protecting them and preventing them from moving during the healing process. Answers may vary regarding the materials used in casts.
    • You could also consider asking students about their own experiences with casts. This option should be used with caution as some students may have had traumatic medical experiences.
  • Explain that throughout history, casts have been made from a variety of materials and that the class will work in groups to explore some of them.
  • Divide the class into six small groups and give each group two notecards listing some possible materials used in casts (also available for download in the sidebar):
  • Sticks/bamboo
  • Palm fiber bandages
  • Linen
  • Wax
  • Resin
  • Clay
  • Flour
  • Egg whites
  • Vinegar
  • Camphor
  • Lead acetate
  • Plaster
  • You could include pictures on the notecards or bring samples/containers representing each item.
  • After allowing time for the groups to discuss, call on each group, asking them whether they think their ingredients were used to make casts and to explain their thinking. A student from the group will place their list of ingredients on either the “Cast Materials” or “Not Cast Materials” side of the board.
  • Once all the cards have been placed, read the excerpts from research papers in the “Research Excerpts” handout (available for download in the sidebar) and ask the students to listen for the materials they had been assigned.
  • Alternatively, you could print out the handout, cut out each excerpt, and give one to each group to help them make sure the notecards are all in the correct spot. Each group could read their clue aloud and move notecards as needed.
  • As each research excerpt is read, ask the students if they need to make any changes to the card placement. They may be surprised to realize that all the items on the cards have been used to make casts!
  • The following articles contain the research excerpts used in the “History of Casts” activity:
  • Remind the class that when they began the lesson, they noted that casts were designed to help bones heal by protecting them and preventing them from moving. Then ask what state of matter (solid, liquid, gas) might provide the best means for protection and preventing movement. The students should identify that solids would provide the best option, though some students might bring up the idea of an air cast. To address this, the teacher can mention that air casts still require the use of solid materials.
  • Explain that the class will review some of the chemistry aspects of the materials used to make casts by reviewing their states of matter. Ask students to identify the state of matter for the following items:
  • Sticks/bamboo - solid
  • Palm fiber bandages - solid
  • Linen- solid
  • Wax- solid
  • Resin- solid
  • Clay- solid
  • Flour - liquid
  • Egg whites - liquid
  • Vinegar - solid
  • Camphor - solid
  • Lead acetate - solid
  • Plaster - solid
  • This could also be done by asking each group to quickly conduct internet research on the two materials their group had been assigned and then write a brief description and label the material as solid, liquid, or gas.
  • Explain that the items listed as solids make sense because they can help provide stability for bones and draw students’ attention to the two items on the list that are liquids – egg whites and vinegar.
  • Ask the students why eggs and vinegar might be used to make a cast. Students might identify that the egg white can harden when it is mixed with other items.
  • After listening to student ideas, explain that egg whites contain proteins that can solidify and serve as an adhesive. Also explain that at one time vinegar was believed to have an antiseptic quality that could help in the healing process.
    • This article explains the property of adhesion that is caused by the proteins in egg whites, and this infographic provides images of the different types of proteins found in eggs.
  • Explain that while casts have been made in several different ways, one method for making casts that continues to be used today is the plaster of Paris type.
  • Ask students to read the brief excerpt on their “The History of Casts” handout regarding the use of plaster of Paris in casts. Emphasize the final sentence in the reading, “The strength of the plaster cast is determined by the quality of plaster, water to [plaster] ratio, product age and storage conditions,” explaining that the strength of a cast is based on various factors and that the students will be experimenting to determine what water to plaster of Paris ratio will create the strongest cast (question 3a on the handout).
  • Explain that before they can begin experimenting, they need to review some chemistry terms. On their handout (question 4), have students write the difference between a physical change and a chemical change – if needed, remind them that in a physical change, no new substances are created while a chemical change results in a new substance.
  • Ask the students to list the indicators of a chemical change on their handout (question 5) – students should identify the production of a gas, production of a precipitate, production of light, and/or an unexpected change in temperature or color.
  • Ask the class if combining plaster of Paris and water is an example of a physical change or a chemical change (question 6 on the handout). You may show the students what plaster of Paris looks like before it is combined with water to help provide students with a visual clue, and have them revisit the research excerpt to look for clues as well. They should note from the reading that combining water and plaster of Paris releases heat and identify that combining water and plaster of Paris results in a chemical change.
  • Remind students that when a reaction releases heat, it is called an exothermic reaction. You may also explain that combining water and plaster of Paris is known as a hydration reaction, in which water molecules become part of the crystal structure when the chemical reaction occurs.
  • Explain that the product of a reaction depends on the ratio of reactants in the reaction, and that the class will determine the most effective ratio for creating a cast out of plaster of Paris. To do that, they must first determine the criteria and constraints of the project. On the back side of the handout, have students respond to the prompts about criteria and constraints:
    • Explain that the criteria are the requirements that the project should meet. For instance, the area covered by the cast may be listed as a criteria. The primary criterion will be strength. This can be measured by laying the cast down and placing books on it until the cast breaks. The class could also consider using a strength to weight ratio of the casts since one of the challenges of early casts was their considerable mass.
    • Explain that the constraints will be the limiting factors for the project. For instance, using the same amount of plaster of Paris in each reaction while only changing the amount of water is one example of a constraint. The class should work together to make a list of constraints.

Part 2: Making Model Casts

  • Begin by reviewing the criteria and constraints for the project that the students discussed in the previous part of the lesson.
  • Ask the students to propose plaster of Paris to water ratios that they can test, with each group being responsible for testing one ratio. You could begin by telling students the suggested ratio from the plaster of Paris label. Alternatively, you could assign one of the following suggested plaster-to-water ratios to each group, with the first number indicating the mass (in grams) of plaster and the second number indicating the mass of water: 100:100, 100:70, 100:60, 100:50, 100:40. Depending on the number of groups, students could also test 100:80 and 100:90 or multiple groups could use the same ratio. The 100:100 ratio will be very runny and the 100:40 will be difficult to stir. As a reminder, 1 cm3 of water has a mass of 1 gram.
  • Review the experiment instructions with the students, making sure they understand that the no plaster (dry or mixed with water) should go into the sink, as it could harden and damage plumbing systems.
  • Show a video (options include Reduction Fracture using Plaster of Paris and Plaster Cast Application) demonstrating how a cast is placed on a limb. Explain that as the students saw in the video, they will use their plaster and paper towels to create a cast on a plastic water bottle (or similar object).
  • Explain that the students will record observations while they mix their assigned ratio and as they make their casts. (They will fill in the rows for the other ratios by exchanging information with their classmates who tested those ratios in Part 3 of this lesson.)
    • To save time in the lesson, you could pre-measure the 100 grams of plaster of Paris for each group.
  • Explain that once students mix their plaster of Paris with water, they should measure out 120 grams of their mixture for use in their model cast. Students should place a thermometer in the remaining wet plaster.
    • The temperature takes approximately twenty minutes to increase and will increase by about 2-3°C (5°F).
  • Once each group finishes their cast, they should clean up their area and make sure they have completed their observations on their lab sheet.

Part 3: Evaluating Model Casts

  • This part of the lesson should take place after the casts have had time to dry. It might work best to conduct this part of the lesson during the following class period, to ensure that the casts have had enough time to dry.
  • Ask each group to share out the findings from their ratio of plaster of Paris and water.
    • To save time on sharing out group data, the groups could use a screencasting tool to explain their findings and the students could be assigned to view the videos from the other groups.
  • The class will test the casts by laying the casts down and placing books on top of them until they crack. The same types of books should be placed on each cast. If plastic bottles are used to make the casts, the bottles can be removed prior to testing. It is not possible to remove the paper towel tubes from the casts prior to testing.
  • Students will individually use the information collected from the activity to draft an argument about which ratio of plaster of Paris to water should be used in casts.
  • Wrap up the lesson by explaining that while plaster of Paris is still used for casts, alternate cast materials are being used. These include thermoplastics, polyurethane, fiberglass, silicone rubber, and wood chips. If time allows, the class could discuss potential benefits and drawbacks of using these types of materials for casts. As an extension activity, students could select one of these alternatives to research and make a Venn diagram (or another type of graphic organizer) to compare/contrast this material with plaster casts.
  • Be sure to remind students that casts used for medical purposes should only be applied by medical professionals.