Author: Jon Valasek
Students will measure both qualitatively and quantitatively the heat of physical changes and a chemical reaction.
What should students know as a result of this lesson?
What should the students be able to do as a result of this lesson?
Divide the students into groups of two, three, or four. Introduce this investigation by posing the following question, "If I dissolve sugar in water is it a physical or chemical change?" Accept answers. This might lead to the students in defining physical and chemical change. Allow them to explain. The definitions should parallel those found in the explanation of science section. Then ask, "Do you think that if I dissolve sugar in room temperature water that the temperature of the water will increase, decrease, or remain the same?" Take a poll. Without answering the question ask, "What if I added common table salt to room temperature water, would the temperature of that water increase, decrease, or remain the same?" Take a poll. Still without answering the questions ask, "Does it matter what I put in the water?"
Assessment: Have students discuss the questions in their groups and the reasons why there might be a difference in temperature with different substances. Without answering the rhetorical questions state, "Let's find out!" (Before the exploration phase you might want to group the students and assign jobs: material gatherer, manager, experimenter, and recorder. Review rules for safely conducting lab activities with the students.)
Please note: The next part of this lesson is a series of exploration/explanation episodes in which discussion is interspersed with students' lab activities.
Set out the dry chemicals, plastic bags, cups of water, 10 mL graduated cylinders, and tablespoons. Have groups appoint a material gatherer to acquire the materials from a central location.
Instruct material gatherers to place about one-half tablespoon of each dry chemical into separate plastic bags. Then take the filled bags, a cup of distilled water, and a 10 mL graduated cylinder back to their group.
When materials are positioned instruct the students that they will add water to each one of the bags and determine whether the water temperature increases, decreases, or remains the same. State, "Predict what the temperature change will be in each bag." Accept all answers. And. "How will you determine the change?" Although most groups will use their hands, some might suggest using more sensitive lips. After each group has made their prediction have them add 10 mL of water to each bag, one at a time, and sense the temperature difference.
Assessment: Ask the groups to report their findings and state if their predictions matched the results. Ammonium chloride solution should feel cold, the calcium chloride should feel warm, and the sodium bicarbonate should feel cold.
Discuss with the students why some substances cause the water temperature to increase while other substances cause the water temperature to decrease when dissolved in water. Ask for explanations. See the Explanation of the science sections for details.
Assessment: Ask the students to decide if adding table salt to water would cause the water to gain or lose heat. Accept answers.
State that scientists call changes or reactions where heat is lost to the surroundings as endothermic and exothermic respectively. Further explain that if the container feels cool after the addition of the substance, then the change is endothermic. If the container in which the change takes place feels warm on the outside after the addition of the substance, then the change is exothermic. The addition of table salt, sodium chloride, to water is slightly endothermic. State, "Can we determine whether a substance dissolved in water produces an endothermic or exothermic change?" Without consulting a chemical reference and prior to experimentation, we cannot tell what a substance will do.
Assessment: Ask, "So what will table salt do?" The answer is we don't know. State if we consulted a chemical reference we would find that sodium chloride added to water is slightly endothermic. To prove this you could add sodium chloride to water as a demonstration and pass the bag around the room for all students to feel.
Now that the students have warmed up to the task, ask, "How can we quantify the experiment we have just performed?" Answers should include measuring the temperature of the water before and after the addition of a substance with a thermometer and measuring the time during the dilution, and measuring the mass of the substances. Direct the material gatherers to obtain the dry chemicals and place them in the plastic bags as before. To avoid a mess they then can mass each filled bag and subtract the mass of the bag or if the balances are not sensitive to measure the mass of one empty bag determine the mass of 10 bags and divide by 10 to obtain the average mass of one bag. Also they should fill their cup of water and obtain a 50 mL graduated cylinder and a thermometer.
Have students devise procedures to measure the temperature differences in the three dilutions.
Have groups report their deliberations. A possible procedure follows:
Assessment: Have students discuss the results in their groups and summarize their findings for the class. As an additional exercise have the students speculate on the addition of baking soda to vinegar. Ask, "How is this different than what we have previously done?" This is an example of a chemical change; the previous exercises were all physical changes. Ask, "What will happen if we add the baking soda to the vinegar. Will the solution become warmer or colder?" Accept answers.
Have students select other chemicals or suggest other household chemicals that they could test and devise procedures to test the materials.
Assessment: Receive written or oral reports from individuals or groups on the results of their testing.
Physical and chemical properties and changes; collecting data using a thermometer.
Common Core Standards:
These activities can be conducted as discovery or as inquiry if the concepts of physical and chemical properties, physical changes, chemical reactions, density, and reactivity are first introduced or the students have knowledge of these concepts in prior lessons.
The chemicals used in this activity are relatively safe. Go to www.flinnsci.com for Materials Safety Data Sheets. Precautions include not touching the chemicals and wearing eye protection. All chemicals can be washed down the drain with plenty of water.
From cold and hot packs to melting ice on the road, exothermic and endothermic physical changes and chemical reactions are useful in our lives.
Students should be able to determine the heat change of any physical change or chemical reaction by direct measurement. Also students will be able to classify the change or reaction as exothermic or endothermic.
Grouping Suggestions: Students should be placed in groups of two or three. However a novel idea on grouping is found in, "Cooperative Learning in the Science Classroom," by Emily Lin, the Science Teacher Vol 73 No 5 Summer 2006, pp 34-39
Pacing/Suggested Time: Two 45 minutes periods.
N/A - Students should devise their own data sheets based on the discussion in class