Author: Jon Valasek
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Engagement: 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!"
Exploration: 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.
Explanation: 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:
- Record mass of the substance plus bag.
- Measure and record the temperature of the water.
- Measure and record the temperature of the air.
- Place the thermometer in the bag and add 30 mL of water.
- Continuously massage the bag to mix the contents.
- Record the temperature every 10 seconds for the first minute and then every 30 seconds for an additional 2 minutes.
- Graph the temperature and time data.
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.
Elaboration: Have participants develop lesson plans to incorporate this activity into their classroom.
Assessment: Review lesson plans for completeness.