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Author: Jon Valasek
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Participants will perform a qualitative activity to observe the temperature of three substances when added to water then they will perform a qualitative activity measuring the temperature change versus time for the three substances. From this experience the participants will develop a lesson plan to teach heat loss and gain in physical changes and chemical reactions.
Divide the Participants 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 participants 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 participants 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!"
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 participants 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 participants 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 participants 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 have the participants add sodium chloride to water.
Now that the participants 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 the empty bag determine with the consent of the participants that the mass of the bags are negligible. Also they should fill their cup of water and obtain a 50 mL graduated cylinder and a thermometer.
Have the groups devise procedures to measure the temperature differences in the three dilutions.
Assessment: Have groups report their deliberations. A possible procedure follows:
Assessment: Have the participants discuss the results in their groups.
As an additional exercise have the participants 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.
Assessment: Have the participants perform the addition of baking soda to vinegar in a similar fashion as they preformed the previous exercise.
Have participants develop lesson plans to incorporate this activity into their classroom.
Assessment: Review lesson plans for completeness.
Heat loss or gain in physical changes and chemical reactions is a fundamental concept that has a number of applications from hot or cold packs to melting ice on roads. An understanding of these endothermic and exothermic changes and reactions will cause students to become more scientifically literate.
Science as Inquiry: as a result of activities, in grades 5-8, all students should develop
Physical Science: as a result of activities, in grades 5-8, all students should develop an understanding of
Standard A: Professional development for teachers of science requires learning essential science content through the perspectives and methods of inquiry. Science learning experiences for teachers must:
Standard B: Professional development for teachers of science requires integrating knowledge of science, learning, pedagogy, and students; it also requires applying that knowledge to science teaching. Learning experiences for teachers of science must
Participants should be able to complete this exercise in 60 minutes.
Ammonium chloride and calcium are slightly toxic if ingested. Participants should wear safety goggles. All material can be disposed of by putting down the drain with plenty of water.
The overall assessment would be to have the participants develop lessons plans and conduct this lesson in their classrooms.
Physical change- a change in physical state (solid, liquid, or gas) with no change in chemical composition. Dissolving a solute in a solvent is a physical change.
Chemical change- a change in chemical composition to a different kind of matter.
Heat- energy transferred between two objects
Temperature- measurement of average kinetic energy (motions)
Endothermic- heat is transferred out
Exothermic- heat is absorbed in
Heat of salvation- the sum of the energy it takes a solvent to break apart a substance into individual molecules or ions and keep them apart in solution. This can be endothermic or endothermic. The addition of ammonium chloride, calcium chloride, and sodium bicarbonate to water causes each to be dissolved generating a temperature difference.
Heat of reaction- the energy generated when a chemical reaction takes place. This can be endothermic or endothermic.
Baking soda is sodium bicarbonate that has many household uses including putting out cooking stove fires and deodorizing refrigerators.
Vinegar is a 5 percent acetic acid solution.
When baking soda is added to vinegar a chemical reaction takes place and sodium acetate, water, and carbon dioxide gas are produced.
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Be sensitive to gender, ethnic, and religious backgrounds of the participants.
Additional student activities to include: measuring energy changes, discovering instant ice packs, and measuring calories can be found in the above cited Flinn reference.
Procedures are found in Flinn ChemTopics™ Labs, Experiments and demonstrations in chemistry. Thermochemistry Volume 10 Exploring Energy Changes, Exothermic and Endothermic reactions.