High School • Gas Laws • Legacy Series
This is a high school Chemistry class and Bethany is teaching a unit on the Gas Laws.
Traditionally in Chemistry students learn about the behavior of gasses through investigating gas laws. But a law is a description of a phenomenon and if taught through a “lens of laws” students focus their attention on correlations among variables. For example, as pressure increases volume decreases. Theories provide explanations for laws. Explaining gas laws requires a keen understanding of molecular movement and how energy influences this movement. One of the challenges is that many of the gases we interact with on a daily basis are not visible. For this unit of instruction Bethany chose to have students reason with familiar gases first. She posed a puzzling phenomenon about a railroad tanker car that collapsed after being steam-cleaned and sealed.
Students developed initial models about what could cause the tanker to collapse and then did experiments with pop cans crushing with the aid of steam to add to the initial models. The pop can experiments helped students link an observable phenomenon with theoretical components such as the role of phase changes and the speed of gas molecules— but to reason with the role of pressure inside and outside of a system students did additional experiments and read about pressure. Over time students constructed a rich explanation for the oil tanker collapsing, while also considering similar phenomena. Students were then ready to apply what they learned to less similar observable phenomena. Bethany chose a set of relevant phenomena to students’ lives and had students explain modifications made to race cars based on their understandings of the gas laws. Students could choose to think about modifications to tires or engines.
Bethany’s practice is characterized by a constant press for why explanations. Each activity aims to add to an explanation and each interaction with students either starts with a press for why or help students first explain what, and how, then why. Students constantly transition back and forth between observable and unobservable features of an explanation.
1) Elicitation of student ideas.
2) Explaining an oil tanker crushing phenomenon. Students watch video. Unprompted they start thinking about why this happened. Students then record initial ideas. Students then work in groups to draw before, middle and after pictures.
3) Whole‐class debrief. Students share posters of their initial ideas.
4) Homework: Students did a reading about the behavior of gases.
Discussion questions for teachers: 1) What general ideas do most students seem to have about the behavior of gasses? Which of these ideas can be built on? 2) How does Bethany uncover students’ ideas? What kinds of questions does she ask when students give incomplete responses or reveal an alternative conception? What does she do when she first enters a group/ exits a group?
1) Warm-up Yesterday you heard different groups present their ideas about why the tanker crushed. You also read about the behavior of gases as homework. What are 3 new ideas you have about why the tanker is crushing? Teacher reviews student ideas from previous day.
2) Making an initial consensus model. Teacher reviews student posters using 2 guiding questions: A) Make a list of what’s causing the tanker to crush. B) Does anything seem to be linked together? Does one thing happening seem to cause another thing to happen?
3) Pop Can Activity: Back pocket questions are used to a) help students link together parts of the causal story b) identify gaps in the students’ causal stories and c) help students begin to think about how they could further study the phenomenon.
Discussion question for teachers: In the video Bethany works with two groups as they consider the relationship between phase changes and pressure changes. How does she tailor her back‐pocket questions in response to each group’s line of thinking?
2) Examining variables. Teacher helps students hypothesize about 5 experiments inspired by student ideas from the previous day.
3) Pop can Activity II. As students conduct experiments the teacher poses back pocket questions to a) help students link together parts of the causal story b) help students identify gaps in their causal stories.
- Experiment #1: Amount of water in the can
- Experiment #2: Temperature of the water bath
- Experiment #3: Amount of time on the hot plate
- Experiment #4: Volume of the can
- Experiment #5: Amount of seal
4) Warm‐up, Day 4. Teacher prompts students to consider 4 key ideas in their explanations. The warm‐up asks students to “Choose one of the following ideas and explain how it relates to the experiment you did yesterday.
- Temperature change‐ speed of molecules
- Temperature change‐ phase of matter
- Pressure inside
- Pressure outside
5) Complete pop can Activity II. As students finish experiments the teacher poses back pocket questions to a) help students link together 4 parts of the causal story.
Day 5. Think back to your experiment with the cans.State your results as a rule. How did changing the manipulated variable affect the amount of crushing? When _________________the can crushed more because ____________________.
Reporting out and connecting experimental findings. Teacher coordinates discussion of findings across groups. Teacher describes what happens and presses student to say why.
Discussion questions for teachers: 1) Record the questions Bethany asks, group them into conceptual categories, and then consider her reasoning for asking these rounds of questions in this particular order. 2) Which questions seemed to be most productive for helping students move their thinking forward?
1) Increasing content understanding about air pressure. Students struggled to reason with internal and external air pressure. Teacher reviews homework assignment about pressure. Teacher has students show work to class as a way to “work on student ideas.” Then teacher helps students reason with forces and pressure.
2) Balancing Act. Teacher asks students to reason with number of molecules, temperature, volume, and pressure (in terms of pulling and balancing internal and external pressure) throughout 4 experiments.
- Station 1 Balloon Blow up
- Station 2 Marshmallow vs. Pebble
- Station 3 Expanding Balloon
- Station 4 Balloon in Flask
3) Warm‐up, day 6. Draw a diagram to explain why this tire would inflate. Draw the air molecules outside the tire, draw the air molecules inside the tire. Use arrows to indicate where pressure is higher or lower .
4) Returning to air pressure experiments and explanations. Students completed diagrams and questions for the air pressure experiments.
Discussion questions for teachers: 1) What are different strategies Bethany uses to plan for as well as in‐the‐moment work on students’ ideas? 2) What kinds of ideas do students wrestle with during the experiments? Do the conversations move beyond procedural talk? When, how?
Students review air pressure experiments. Specifically the relationships between pressure and the a) number of molecules, b) volume and c) temperature.
2) Adding to the model. Teacher facilitates a whole class discussion about the information that needs to be added to their initial model. Then students work in small groups to add to their models of the can crushing for the experiment they conducted. NOTE: The teacher notices students are trying to apply ideas about kinetic molecular theory to the can crushing experiment. They reason with partial understandings about the speed of molecules, how the temperature influences speed, why the space is important to molecular movement. The teacher helps students link these ideas to the experiment but also directs their focus to a different but related underlying explanation (the number of molecules) to explain why the can crushes.
1) In response to how students were reasoning with molecules, the teacher designed a warm up to help students think about the number of molecules at each stage in the can crushing. “Say I started with 10 water molecules (X’s) in the bottom of the can and 20 air molecules (O’s) in the can and then I heated it up to boiling and let it boil for 2 minutes. Draw a picture of a can and mark where you think the X’s and O’s would be after these 2 minutes.”
2) Adding to the model. Students return to diagrams and continue to add information to their models using a checklist with key concepts for the students to include.
Discussion questions for teachers a) How is the explanation check list developed How is it used? b) Generally speaking, compared to days 1 & 2 how have students built on their original ideas? And what new ideas are they entertaining now?
1) Warm up. The teacher asks a warm up that helps connect ideas about kinetic molecular theory with the gas laws. “When you flip over the can into cold water, describe what happens to the phaseand speed of the gas molecules (both X’s and the O’s) inside.”
2) Pressure and Collisions. Students talk about correlations between pressure and number of collisions on a container and then reason with three ways the pressure/number of collisions can decrease—the amount of space (V), the number of molecules (n) and the temperature (T). Students then look at these relationships in a computer simulation. http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=25
3) Can you compress water? Teacher helps student reason with the differences between air molecules and water molecules. Need to think about pressure of water in can puts on the can and why this is an important part of the explanation for why the can does not crush.
4) Return to posters to add new knowledge. Teacher highlights 2 parts of the explanation checklist that students are generally not attending to in their explanations. She also provides each group with 3 sheets of fill in the blank sentences to make sure students are reasoning with a full explanation for each of the 3 can crushing stages.
Discussion questions for teachers 1) Bethany adds an additional scaffold to the check list by giving students sentence starters. How does this shift the ways students participate in class?
1) Teacher facilitates whole class conversation of a full explanation of the tanker Generalizing to the crushing.
2) Review of learning. Teacher has students refer back to initial explanations. behavior of all gasses.
3) The Behavior of Gases. Teacher reviews general ideas about what influences the behavior of gases and bridges to scientific language. Then the teacher introduces the Ideal Gas Law.
4) Focus on correlation between P & V. Pressure Volume Student do activity with syringe and scale to Pressing students for evidence examine the relationship between P & V. Students graph results and talk about an inverse relationship. based explanations
5) Warm up, Day 11. Explain from data the previous day how we know that pressure and volume are considered inversely proportional.
6) Review lab and introduce Boyle’s Law. Teacher introduces the idea of P1V1=P2V2. Teacher asks questions that help students reason conceptually and mathematically with the equation. Students then record notes on a master table with the ideal gas law broken down in to 3 different relationships.
7) Volume and Temperature. Teacher introduces the idea of Kelvin by having students think back to volume and pressure lab and whether or not 0 pressure or 0 volume is possible. Students then do mathematical calculations to examine the relationship.
8) Warm up, Day 12. Your car tire ahs 10L of air and it’s 0 degrees Celsius outside. Later that day the temperature has increased to 26 degrees Celsius. What will happen to the volume of the tire? Why? 9) Review of Volume and Temperature. Students take notes on Charles’s Law. V1/T1=V2/T2.
10) Pressure and Temperature. Students take notes on P1/T1=P2/T2 and then do practice problems.
Discussion questions for teachers: 1) Thought experiment: Typically in Chemistry students learn each gas law and do calculations. Compare this to Bethany’s “concept first” approach. Also think about the difference between learning about a theory about the behavior of gasses versus laws.
1) Warm up, Day 13. What are the gases in the engine? What gas(es) are present during intake, compression, combustion, and exhaust?
2) Engine Modification Challenge. Students select 3 modifications and describe why each modification helps a car have more power. They also need to describe why a pressure sensor is needed on tires. Students read background information on each modification and complete a fill in the blank form that helps students reason with P,V,T and n. tire pressure monitoring system bore out cylinders cold air intake high performance exhaust system intercooler nitrous oxide system turbocharger supercharger
3) Warm up, Day 14. Why is getting more oxygen into the engine beneficial? (Think about the combustion reaction).
4) Summative assessment. Gas Quiz.