Grade Levels: Middle through High School
Content Areas: Physics, Chemistry
Theme: Relationship between temperature and pressure (gas laws).
· Empty aluminum soda can
· Tongs or oven mitt
· Cold ice water
· Large bowl
· Fill the bowl with ice water. Make sure it’s very cold.
· Put about 10-15 ml room temperature water into the empty can.
· Heat the can on the hotplate until the water boils.
· Allow to boil for 1 minute.
· Using the tongs, grasp the can, quickly invert it and place the mouth of the can into the beaker full of water.
· The can should collapse.
Ideas for Lesson:
Start by instructing students to do a quick-write in their journals in response to the following question: What is the relationship between temperature and pressure? As teacher performs DE, allow students to be involved and observe what is happening. Instruct students to record observations in their journals. Then, ask students to share their observations with the class. Teacher should write student responses on the board. Probe students to expand their observations by asking, “Why do you think that happens?” and challenging them to develop a hypothesis. After students observe the crushing of the can, instruct them to write a reflection in their journal, allowing them to modify or add to their initial predictions. Challenge them to provide an explanation for the results. Have them share with the class. Meanwhile, allow student to repeat the DE in small teams at their lab stations (have materials set up and ready to go).
1. What happens when water boils?
2. What is water vapor?
3. What is happening to the temperature?
4. What is happening to the pressure inside the can? Outside the can?
5. What do you think will happen when I plunge the can into cold water?
6. What will happen to the temperature?
7. What will happen to the water vapor inside the can?
8. What will happen to the pressure?
9. What is the relationship between pressure and temperature?
Boiling water inside the can filled the can with hot water vapor. The pressure inside the can at this point is very high since temperature and pressure are directly correlated. Plunging the inverted can into cold water caused the vapor to cool suddenly. When the vapor cools, it condenses, creating a vacuum and causing the pressure outside of the can to be much greater than inside the can. Therefore, the pressure differential crushes the can.
Make sure water is boiling adequately to cause the pressure increase. Invert and submerge the can into ice water quickly for best results. Have a few cans ready to go so that the demonstration can be repeated several times. Have some cans set up a lab stations so that students can try for themselves.
Upon doing this discrepant event with a group of students at lunch, several of them astutely predicted the can would be crushed because of the pressure change. Some students were afraid it would explode on the hotplate. Others had seen it before and risked “spilling the beans” to the other students. I had to ask students “in the know” to keep the surprise secret. I was impressed with how hard the students tried to answer my open-ended inquiry questions (see above), especially since in class, students are sometimes reluctant to participate. However, when performing the discrepant events, students were much more engaged and wanted to see what was going to happen. They were eager to participate. The final event, the crushing of the can, is very dramatic and impresses the students, making it an event they will remember for a long time. This is a fantastic way to teach gas laws to students, particularly since they do not have to memorize an abstract mathematical formula (PV=nRT).
Reflection on Pre-Event Discrepant Event
I performed this discrepant event several times before presenting it to the class. The first time I tried, it worked dramatically and easily, although I was alone. The second time, I performed the discrepant event in front of three students and one teacher during lunch. The can was not crushed at the end, unfortunately, and I learned there are some critical variables that must be controlled in order to have the difference in pressure crush the can upon submerging it into the ice water. Luckily, I had time to repeat the discrepant event once more before presenting it to the class, and my modifications worked; the can was successfully crushed.
Tips for Success:
To make sure the can will be crushed, there are a few important factors. First, it works best with only 10 ml of water, even though the original directions called for 20 ml. This may be because it takes less time to heat the water, allowing for a more rapid increase in pressure inside the can. When performing in front of an audience, it is best to have little wait time. I want just enough to generate discussion and curiosity with the students through open-ended questioning, no more than 5 minutes. This is just about perfect to make the water boil for 1 minute on a hot plate, which is ideal for creating the increased pressure. Once students hear the water boiling inside the can, and they can see the steam escaping from the top, it is time to plunge it into the ice bath. This is the most critical step! The can must be quickly and swiftly be inverted and immediately plunged face-down into the cold water. This prevents the steam from escaping and also helps form a vacuum inside the can, even though the top is open. Then, the hot water inside the can will quickly condense, causing the pressure outside to be greater than inside, and the can will be crushed.
When I told students I was going to demonstrate a science trick, they were instantly curious. I tried to involve the students as much as possible to engage them. I pretended I was performing a magic trick and called a volunteer student from the audience to verify what I was saying was true. I showed them the materials I was using: the hot plate, bowl of ice water, oven mitt, and soda can. I asked them to check and verify that the can was an empty, aluminum, soda can. I asked them to verify that the bowl of ice water was cold by dipping their fingers in it. Once, the water started heating up in the can, I asked how students could safely identify whether the water was heating. They responded with answers like, “Look for the steam at the top of the can,” and, “Listen to the boiling water hitting the sides of the can.” I asked them to make predictions of what was going to happen when I put the can into the cold water. Several students astutely predicted the can would be crushed because of the pressure change. Some students were afraid it would explode on the hotplate, or explode in the water. (One student even backed up to the other side of the classroom). Others had seen it before and risked “spilling the beans” to the other students. I had to ask students “in the know” to keep the surprise secret.
I was impressed with how hard the students tried to answer my open-ended inquiry questions (see above), especially since in class, these same students are reluctant to participate. However, when performing the discrepant event, they were much more excited and wanted to see what was going to happen. They were eager to participate. In addition, they were able to demonstrate their knowledge through oral explanation, as opposed to a high-stakes multiple-choice test, which these students struggle with. (The students observing the discrepant event are in my case study and are currently earning a grade of “D” or below in biology). The students very quickly predicted that the pressure change inside the can would cause it to be crushed. Discrepant events are a great way to teach new concepts to all students and may be able to be modified to allow students who struggle on tests to demonstrate their knowledge in different ways (e.g. oral explanation). The final event, the crushing of the can, is very dramatic and impresses the students, making it an event they will remember for a long time. This is a fantastic way to teach gas laws to students, particularly since they do not have to memorize an abstract mathematical formula (PV=nRT).
Modifications for Using this Discrepant Event for Teaching:
If I was doing this with my class, I would start by asking students to record notes in their journals. This is a great discrepant event to lead-in to gas laws, or the relationship between gas, temperature, and pressure. First, I would want to assess what students already knew about gases. I would show a picture of a mountain climber ascending Mt. Everest, projected onto the board for the entire class to see. The climber has an oxygen mask. I would ask them discussion questions, such as, “What happens to air pressure when a mountain climber climbs a mountain?” “Why is this man wearing an oxygen mask?” I would also ask them “How is boiling water on top of a mountain different than at sea level?” Some students may have camping experience at altitude and may be able to share their experiences of how water boils faster at higher altitudes. I would begin with a quick-write in response to the question, “What is the relationship between temperature, pressure, and volume?” Students would be instructed to think of the mountain climber on the mountain when crafting their answers. Then, they would share their answers with the class.
Students would be asked to record their observations in their journals as I began the discrepant event. I would show them the materials and ask them to come up and use their senses to make accurate observations. Afterwards, students would be instructed to make predictions about what they expect to happen. After I submerge the can into the cold water and it becomes crushed, students would first record the results. They would be instructed to do a second quick-write to come up with an explanation in their science journals. Then, students would share their answers with the class. I would paraphrase their answers on the board for the class to brainstorm ideas. I would help guide students to understand the positive correlation between temperature and pressure. Students would then modify their explanations, and I would then easily be able to introduce the mathematical formulas for the gas laws.