This article describes a two-part

34

Students Learn Exothermic and Endothermic Processes BY EVELYN GRAY This article describes a two-part investigation exploring endothermic and exothermic processes. During the first part of the investigation, students determine whether a chemical combination is endothermic or exothermic. During the second part of the lesson, students engineer a device that uses an endothermic combination of chemicals to make homemade ice cream. This investigation ties into the Matter and Its Interactions standard found in the Next Generation Science Standards (MS- PS1-6; see NGSS box, p. 43). The performance task related to this concept requires students to design, test, and modify a device that uses either an endothermic or exothermic process. What are exothermic and endothermic processes? Endothermic and exothermic processes are the result of the breaking and forming of bonds between atoms. During many reactions, bonds break, atoms rearrange, and new bonds form. To break bonds, energy is taken into the chemical reaction, known as the system, from the surrounding environment. When bonds form, energy is released into the surrounding environment. Consequently, during a reaction, there is an exchange of energy between the system and the surroundings. When more energy is taken in than released, an endothermic process occurs, causing a decrease in temperature. Evaporating liquid water is one example of an endothermic process. An exothermic process occurs when more energy is released than taken in, causing an increase in temperature. Combustion is an exothermic process. Part 1: Teaching exothermic and endothermic processes All of our investigations begin with research, a process known as knowledge probing in my classroom. There are two parts of a knowledge probe: personal and secondary. First identified as an assessment strategy in 1993, personal knowledge probes allow students to reflect on their knowledge of a topic (Angelo and Cross 1993). In a secondary knowledge probe, students gain knowledge of the topic from multiple sources, including textbooks, articles, and videos. Students do not often have prior knowledge about the subject. So, during this knowledge probe, students learn the definition of endothermic and exothermic processes and see examples from video clips and an article (see Resources). We also have group discussions about students understanding of the two types of processes. Students also fill out a worksheet that asks questions about the video clips and article (see Online Supplemental Materials). CONTENT AREA Physical science GRADE LEVEL 6 8 BIG IDEA/UNIT Chemical processes. Some chemical processes release energy; others store (or take in) energy. ESSENTIAL PRE-EXISTING KNOWLEDGE Properties of matter TIME REQUIRED Five one-hour class periods COST $20 SAFETY Safety glasses and gloves (see Safety Guidelines) July 2018 35

Materials Endothermic versus exothermic investigation beakers graduated cylinders measuring spoons thermometers powder laundry detergent calcium chloride Epsom salt 1 mol citric acid solution baking soda vinegar salt ice water Subsequently, students are asked to make a prediction about which combination of chemicals would be the most endothermic. To find out, students test six different combinations of chemicals: detergent and water, calcium chloride and water, Epsom salt and water, citric acid solution and baking soda, vinegar and baking soda, and salt and ice water (see Safety Guidelines; Materials). Students predictions are based on their prior knowledge of the combination of chemicals because they are not expected to understand why or how an energy transfer is taking place. Thus, many students predict ice water and salt to be the most endothermic process based on their prior knowledge of having made ice cream using this combination. Testing the chemical combinations Next, I set up 12 stations around the room, two of each chemical combination, and split my students into groups of two or three. The groups will rotate through each station, spending about six minutes at each one. At each station, students will find all of the required materials, including chemicals, measuring devices, and directions (see Figure 1). The instructions inform students of the amount of chemicals FIGURE 1: Endothermic and exothermic procedures Powdered detergent and water procedure 1. Put 50 ml of water in a beaker. 2. Determine the initial temperature of the water. 3. Add 10 ml of detergent and start the timer. 4. Use the thermometer to stir the solution. 5. Record the temperature every 30 seconds. Calcium chloride and water procedure 1. Put 50 ml of water in a beaker. 2. Determine the initial temperature of the water. 3. Add 10 ml of calcium chloride to the water and the start timer. 4. Use the thermometer to stir the solution. 5. Record the temperature every 30 seconds. Epsom salt and water procedure 1. Put 50 ml of water in a beaker. 2. Determine the initial temperature of the water. 3. Add 10 ml of Epsom salt to the water and start the timer. 4. Use the thermometer to stir the solution. 5. Record the temperature every 30 seconds. Citric acid solution and baking soda procedure 1. Put 50 ml of 1 mol citric acid solution in a beaker. 2. Determine the initial temperature of the citric acid solution. 3. Carefully add 5 ml of baking soda and start the timer. 4. Use the thermometer to stir the solution. 5. Record the temperature every 30 seconds. Vinegar and baking soda procedure 1. Put 50 ml of vinegar in a beaker. 2. Determine the initial temperature of the vinegar. 3. Carefully add 5 ml of baking soda and start the timer. 4. Use the thermometer to stir the solution. 5. Record the temperature every 30 seconds. Salt and ice water procedure 1. Place seven ice cubes in a beaker. 2. Add water up to 100 ml. 3. Use the thermometer to stir for 30 seconds. Then record the initial temperature. 4. Add 10 ml of salt and start timer. 5. Use the thermometer to stir the solution. 6. Record the temperature every 30 seconds. 36

HOW WILL WE FREEZE THE ICE CREAM? FIGURE 2: Taking accurate measurements Infrared Thermometer. Observe the thermometer from eye level and record the temperature in degrees Celsius. Measuring Spoons. Use measuring spoons to measure any solids. Fill spoons and then gently shake excess material off or use a piece of paper to level the spoon. Graduated Cylinder. Set the graduated cylinder on a flat surface to take measurement. Pour small amounts of liquid at a time. Measure at eye level looking at the meniscus. Pour any excess liquid into the sink (or waste beaker). to add and when to take the initial temperature of the chemical combinations. Because students will be sharing their data, they must take timely and accurate measurements (Figure 2) using the same methods and units of temperature (Figure 3). Likewise, it is important for students to note any errors or mistakes in the procedures, such as adding too much of a chemical, taking the initial temperature at the wrong time, or forgetting to take the initial temperature. Mistakes like these may render a data point unusable. Students record the results of each station in a lab investigation worksheet (see Online Supplemental Materials). FIGURE 3: Data table Combination #1: Qualitative: Quantitative: Initial :30 1:00 1:30 2:00 2:30 3:00 Temp Analyzing data In this step, student groups will begin the data analysis process. Emphasize to students that analyzing data should make the data easier to comprehend and to display the results. First, students analyze their own group s data, examining the results from each combination and determining the temperature change and whether the combination was endothermic or exothermic. Students summarize their results in a data table (Figure 4). Secondly, students input their data into a shared spreadsheet to consider multiple trials (see Online Supplemental Materials). We stress the understanding that multiple trials allow us to be more confident in our data set. There are 12 trials total (one from each group) for this investigation. FIGURE 4: Data analysis table Combination Initial temperature Highest or lowest temperature recorded Overall temperature change Increase or decrease in temperature? Exothermic or endothermic 1 2 3 4 5 6 July 2018 37

Next, facilitate a class discussion about the data: Are there any data points that don t follow the trend? Was there an error that caused that outlier? Should that data point be included when determining the answer to our question? Following the class discussion, students make a claim and provide evidence about which chemical combination produces the most endothermic reaction. When making their claim, students are encouraged to consider the greatest temperature change, not the coldest temperature. Students usually make the claim for citric acid solution and baking soda because it has the greatest decrease in temperature. However, in some of my classes, ice water and salt had the greatest decrease in temperature. Part 2: Frozen cream During this phase, students engineer a device designed to decrease the temperature of homemade ice cream as much as possible in five minutes. Students devices should create a larger temperature drop than the baseline temperature of the ice cream in order to freeze it. To create their devices, students follow steps of the engineering process: a knowledge probe (research), brainstorm, sketch, build, test, redesign, and a final test. During the knowledge probe, students answer the three following questions in their ice cream challenge design journal (see Online Supplemental Materials). 1. What did you learn in the endothermic versus exothermic processes investigation? 2. What is the basic method for making homemade ice cream? 3. What is the endothermic process that will freeze your ice cream? How will the thermal energy (heat) travel? Note: Students should create a sketch of the thermal energy transfer in their ice cream challenge design journals (see Online Supplemental Materials). In the first question, students reflect on what they discovered during the investigation. What was the Materials Ice cream design gallon freezer bags quart-size freezer bags salt ice milk vanilla sugar measuring spoons and cups thermometers (preferably infrared) adhesive material (duct tape and hot glue) scissors scrap materials (cardboard, construction paper, Styrofoam, bubble wrap, aluminum foil) Students should bring in materials for building their device. The materials do not need to be new. They can be things that would otherwise be thrown away or recycled. Students may not use anything originally designed to insulate food (e.g., cooler, thermos). most endothermic process? Did that endothermic combination have the coldest temperature? What does it mean for a process to be endothermic? Would we be able to freeze ice cream with an exothermic process? We then discuss that ice water and salt is the chemical combination we will be using to freeze the ice cream. Even though citric acid solution and baking soda caused the biggest decrease in temperature, the citric acid solution and baking soda never go below freezing. 38

HOW WILL WE FREEZE THE ICE CREAM? FIGURE 5: Directions for testing the device Testing 1. Place 1/2 cup of water into a quart-size bag. 2. Add four cups of ice and four tablespoons of salt into a gallon-size bag. 3. Take the temperature of the water. 4. You now have one minute to place the bag of water into the device. 5. Start the timer for five minutes. 6. After five minutes, check the temperature of the water. FIGURE 6: Directions for making ice cream Add 1/2 cup milk, 1/2 teaspoon vanilla, and 1 tablespoon sugar to a quart-sized bag. Zip shut and mix. Add 4 cups of ice and 4 tablespoons of salt to a gallon-sized bag. Take temperature of ice cream mixture and place ice cream mixture into gallon-sized bag and zip. Start timer. After five minutes, check the temperature of the ice cream mixture. For the second part of the knowledge probe, I demonstrate the basic device and method for making homemade ice cream (see Materials). As I am demonstrating, I project the directions and ingredients on the board. Students record these directions to answer question 2 of their design journals. Before and after temperatures of the ice cream are taken and used as the baseline. Later, students will compare their results to this baseline data. Finally, students gain some insight into how to improve upon the device. I present this information in a slideshow (see Online Supplemental Materials), and they record the information in the knowledge probe section of their design journal. They learn that salt water has different properties than water and that salt water s lower Salt water has different properties than water, and salt water s lower freezing point results in the melting of some of the ice, which is the endothermic process that freezes the ice cream. freezing point results in the melting of some of the ice, which is the endothermic process that freezes the ice cream. For ice to melt, it must take in heat from its surroundings. In this case, we want the ice to take the heat from the ice cream mixture, causing it to decrease in temperature as much as possible. Therefore, students want to limit the other areas where heat transfer may occur. Students next begin brainstorming. For this part of the lesson, students may choose their partners, work individually, or work in groups of two or three. One important question students must answer during brainstorming is, What materials are available? I provide a few items, including cardboard, aluminum foil, paper, and adhesive materials (tape July 2018 39

Safety guidelines Endothermic vs. exothermic investigation General safety precautions 1. Do not touch, taste, or smell any chemicals unless specifically instructed to do so. 2. Wear indirectly vented chemical splash safety glasses and gloves at all times and when handling all chemicals and observing and recording temperature. 3. Report any accident or injury to the instructor immediately, no matter how trivial it may appear. 4. Check the label on chemical bottles twice before removing any of the contents. Take only as many chemicals as you need. 5. Never return unused chemicals to their original containers. 6. When transferring reagents from one container to another, hold the containers away from your body. 7. Never remove chemicals or other materials from the laboratory area. 8. Take great care when transporting acids and other chemicals from one part of the laboratory to another. Hold them securely and walk carefully. 9. When using the infrared thermometer, do not point it at eyes or a reflective surface, and wear safety goggles when using it. Specific safety precautions for chemicals Powdered laundry detergent. In the case of ingestion, dilute the chemical by drinking one to two glasses of water. Seek medical attention if nausea and vomiting persists. For skin exposure, rinse exposed area with water. In the case of eye contact, rinse with lukewarm water for 15 minutes. In case of inhalation, leave the dusty area for an area with fresh air. Calcium chloride. If the calcium chloride comes in contact with skin, immediately flush skin with plenty of water. Cover the irritated skin with an emollient cream. If the chemical comes in contact with the eye, check for and remove any contact lenses, immediately flush eyes with plenty of water for at least 15 minutes, and seek medical attention. If inhaled, go outside for fresh air. If not breathing, give artificial respiration. If breathing is difficult, give oxygen. Get medical attention. Epsom salt. If contact with the eye occurs, immediately flush eyes with running water for at least 15 minutes, keeping eyelids open. In the event that the Epsom salt comes in contact with the skin, wash immediately with plenty of water. Gently and thoroughly wash the contaminated skin with running water and nonabrasive soap. Cover the irritated skin with an emollient cream. If irritation persists, seek medical attention. 1 mol citric acid solution. If the citric acid solution comes in contact with skin, it may cause irritation. Remove affected clothing and rinse skin with a mild soap and warm water. If it comes in contact with the eye, it can cause serious irritation. Affected students should carefully rinse eyes, remove contacts (if and when possible), and continue to rinse. Baking soda may cause skin and eye irritation. In the case of skin irritation, the affected skin should be washed with warm water and soap. If the eye is affected, contacts should be removed and the eye should be rinsed for 15 minutes. Vinegar. If the student inhales the vapors, irritation can occur in the respiratory tract. Move student to an area with fresh air. If the vinegar comes in contact with the skin, it may cause irritation. Flush immediately and thoroughly with water. The vinegar will cause irritation to the eye. Flush immediately and thoroughly with water for 15 minutes. Salt. If salt comes in contact with the eye, check for and remove any contact lenses and immediately flush eyes with plenty of water for at least 15 minutes. 40

HOW WILL WE FREEZE THE ICE CREAM? Safety guidelines Ice cream design General safety precautions 1. Do not touch, taste, or smell any chemicals unless specifically instructed to do so. 2. Wear indirectly vented chemical splash goggles. 3. Students may be allowed to eat the ice cream upon completion of this project. There are several things to consider in this case. The making and consuming of ice cream needs to be done in the cafeteria. Make certain the area and measuring utensils such as measuring cups and spoons have been thoroughly cleaned prior to beginning the ice cream making process. Use infrared thermometers so that there doesn t have to be contact between the thermometer and the ice cream. Have students wash hands as necessary throughout the ice cream-making process. 4. The teacher must consult with the nurse to see if any students are diabetic or are lactose intolerant. If so, you may need to use a lactose-free alternative to milk. To address the needs of students with diabetes, you may need to freeze water instead of making ice cream. 5. Report any accident or injury to the instructor immediately, no matter how trivial it may appear. Specific safety precautions Milk. Check the expiration date and keep it cool in a refrigerator prior to using. Keep in mind that some students may have allergies or intolerances to milk. Salt. If the salt comes in contact with the eye, check for and remove any contact lenses and immediately flush eyes with plenty of water for at least 15 minutes. and glue), but other materials must be brought from home, such as towels, cotton balls, or bubble wrap. Students must answer the following questions to conclude their brainstorming: What is the goal of your device? What are some things you should consider when trying to reach that goal? What changes do you want to make to the basic method? How will those changes increase the efficiency of your device? Next, students sketch their device (see Online Supplemental Materials for examples of student sketches). They are required to draw a detailed sketch of their device, include size estimates and materials, and briefly explain how it will work. After building their devices, students may begin to test them. To test, we simply freeze water in a gallonsize bag (Figure 5). Upon completion of the test, students meet and discuss various successes and failures. On the day ice cream is made, the class meets in the cafeteria instead of the laboratory. In small groups, they share the design of their device as well as how cold they were able to get their water. Students take notes during these discussions and should begin to think about how they could improve their device. After the discussion, students resketch and rebuild their device, making at least one change based on what they learned from the testing phase. Students typically modify their device in two ways. Many groups make a device that resembles an insulated cooler (although we forbid the use of any premade/store-bought coolers or thermoses) because their goal is to limit heat transfer from the outside. The second common modification is creating a handle so that body heat does not transfer directly to the ice. Finally, we make ice cream (see Figure 6 for instructions). On the day ice cream is made, the class meets in the cafeteria instead of the laboratory. No July 2018 41

lab materials or ingredients can be used in the cafeteria. Calcium chloride and citric acid should be stored in another room. Assessment Students are assessed on the investigation portion of this lesson using a rubric (see Online Supplemental Materials). This assessment looks at students ability to explain endothermic and exothermic processes, make observations, analyze data, and graph the data. We also expect students to write an accurate claim and provide evidence to support that claim (see Online Supplemental Materials for examples). Finally, students must give a thoughtful reflection on the investigation process as they work through the evaluation questions. We first assess students by having them reflect on their engineering process: 1. What scientific knowledge did you use to design your device? Include information about endothermic and exothermic processes, specifically the endothermic process used to make the ice cream. 2. How was your device designed to work? (Why does it freeze the milk and what makes it efficient?) 3. Select one revision made to your device following the initial testing. Explain why you made that revision. 4. How could you further improve your device? We further assess the core idea on a unit test. Here are some example questions from the test: 1. Complete the data table below by calculating the change in temperature for each combination of chemicals. 2. Make a claim and provide evidence to support your claim to answer the following question: Which of the chemical combinations is the most endothermic? 3. Explain how chemical bonds are related to this phenomenon. Combination Salt and ice water Citric acid, ice, and baking soda Ice water and calcium chloride Conclusion Temperature before Temperature after 2 C -3 C 2 C -5 C 2 C 10 C Change in temperature Middle school students are very excited to mix chemicals together, and the promise of ice cream motivates them to learn. During the inquiry portion of this lesson, students are surprised to see the temperature change for each combination, especially when they discover that salt and ice water do not create the biggest decrease in temperature. In the engineering portion, they use creative thinking, persistence, and self-direction as they work through problems that may occur when they bring their ideas to life. RESOURCES Exothermic and endothermic processes Video 1 www.youtube.com/watch?v=ejxl0irbtqe Video 2 www.youtube.com/watch?v=gqkji-nq3os Video 3 www.youtube.com/watch?v=ntfbxj3zd_4 Article www.khanacademy.org/test-prep/mcat/chemicalprocesses/thermochemistry/a/endothermic-vsexothermic-reactions ONLINE SUPPLEMENTAL MATERIALS Worksheet and answer key, lab investigation worksheet, spreadsheets, ice cream challenge design journal (with answer key), slideshow, student sketches, and evidence statements www.nsta.org/scope1807 Evelyn Gray (gray.evelyn86@gmail.com) is a science teacher at DeWitt Junior High School in DeWitt, Michigan. 42

HOW WILL WE FREEZE THE ICE CREAM? Connecting to the Next Generation Science Standards (NGSS Lead States 2013) The chart below makes one set of connections between the instruction outlined in this article and the NGSS. Other valid connections are likely; however, space restrictions prevent us from listing all possibilities. The materials, lessons, and activities outlined in the article are just one step toward reaching the performance expectations listed below. Standards MS-PS1 Matter and Its Interactions www.nextgenscience.org/dci-arrangement/ms-ps1-matter-and-its-interactions MS-ETS1 Engineering Design www.nextgenscience.org/dci-arrangement/ms-ets1-engineering-design Performance Expectations MS-PS1-6. Undertake a design project to construct, test, and modify a device that either releases or absorbs thermal energy by chemical processes. MS-ETS1-4. Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved. DIMENSIONS CLASSROOM CONNECTIONS Science and Engineering Practices Analyzing and Interpreting Data Constructing Explanations and Designing Solutions Students analyze and interpret temperature data from chemical processes that they collected individually. They also consider the entire class set of data. Students design a device to minimize energy transfer to quickly and efficiently freeze ice cream. They test their devices and redesign. Disciplinary Core Ideas PS1.B: Chemical Reactions Some chemical reactions release energy; others store energy. ETS1.B: Designing Possible Solutions A solution needs to be tested, and then modified on the basis of the test results, in order to improve it. Students observe several reactions (chemical processes) for temperature change. They identify which store energy and which release energy as either endothermic or exothermic. Students design and test their device to see how quickly and efficiently it freezes ice cream. Crosscutting Concept Energy and Matter Students collect temperature data as a way of observing energy being released and stored during several chemical processes. They also attempt to minimize energy transfer while designing a device. July 2018 43