New tweets fly by in my Twitter feed so quickly that I feel like I catch only a small portion of what’s shared. Who’s got time to sit there and read all those tweets? But catching this tweet was a gem. The tweet was simple enough; a posted video about teaching gas laws without having to use equations. Hmmm… interesting idea. Especially with NGSS on my mind. This school year I’ve focused on removing memorization and building science and engineering skills. I’ve also struggled with how to integrate gas laws into my curriculum when gases aren’t mentioned in the NGSS document. This lesson allowed me teach gas relationships and incorporate SEPs.
The tweeted video by Scott Milam demonstrated a method of teaching gas laws using three simple steps:
1. Create simple chart with the variables P, V, T, an n written across the top. The order of the variables doesn’t matter (which I love because there’s no memorization). The purpose of this chart is simply to help the student organize information from the question.
2. Determine how changing one variable ought to change the second variable. For example, if pressure increased then students know volume should decrease based on the inverse relationship. I made my students think through the relationship and use up/down arrows to show what should happen to the dependent variable. (See picture of student test below.)
3. Take the variable that is changing and create a “multiplier”. Let’s use this simple gas law problem as an example:
A gas occupies 12.5 liters at a pressure of 480.0 mm Hg. What is the pressure of the gas when the volume is decreased to 8.4 liters?
Based on the question students should consider the size of the container and how it changes. In this example the volume of the container is shrinking. If there’s less room to move around then the particles of gas will collide with the container more frequently. More collisions equates to a greater pressure. This conceptual understanding leads students to a MULTIPLIER. The pressure needs to increase. We need to create a ratio of the initial pressure and final pressure that gives a multiplier that is greater than one. Due to the inverse relationship the pressure should increase by the same factor that the volume decreases. Here are the two options:
Option 1: P = 480.0 mmHg x (12.5 liters/8.4 liters)
This multiplier gives us a number greater than one. This would make the pressure increase.
Option 2: P = 480.00 mmHg x (8.4 liters/12.5 liters)
This multiplier gives us a number less than one. This would make the pressure decrease.
My students easily understood that Option 1 would make the pressure increase and therefore the calculation should be done that way. An excerpt of student work on our test is shown below:
I loved this method of teaching gas relationships for two reasons:
1. Gas laws are not mentioned in NGSS!
I spent my winter break last December combing through the NGSS manual. My initial thought was “Why are so many things missing???!!” I panicked thinking that I would have to change my entire curriculum. Slowly I realized that I didn’t have to lose my beloved content. NGSS simply gives us guidelines on the skills that need to be taught. I realized I could incorporate core topics of chemistry by adjusting the way I taught them. I didn’t feel like I was sacrificing curriculum by teaching gas relationships in this way. In fact, I felt like the method enhanced their understanding.
2. This method involved Science and Engineering Practices 5 & 6
SEP-5 (Using Mathematics and Computational Thinking) expects high school students to use “algebraic thinking and analysis… to analyze, represent, and model data.” This practice focuses on the use of math and logic to make quantitative predictions. SEP-5 encourages the use of computers in the lab to collect and measure data to look for patterns (my students were analyzing pressure/volume data from a Boyle’s Law Vernier lab). Within this context my students were able to take particle models of gas molecules and build a conceptual understanding of an inverse relationship which ultimately allowed them to build the mathematical relationship.
SEP-6 (Constructing Explanations and Designing Solutions) expects high school students to
“construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations).” Again, in the context of this lab experience, students developed a quantitative claim regarding the inverse relationship between the independent and dependent variables based on evidence.
For the critics of this method I will state that there’s nothing wrong with teaching the gas law equations. If time permits I will discuss PV = nRT later in the year (after I’ve taught my students the definition of a mole). I changed over to the “multiplier” method this year because gas relationships fit perfectly into my Matter Unit, where we studied solids, liquids, and gases. It also seemed like an awesome opportunity to challenge my students to think mathematically.
You can find the gas law video by Scott Milam below:
A Better Way to Calculate P V n and T
You can find a copy of my Boyle’s Law Vernier Lab below. It includes particle modeling but I never have my kids plug into Boyle’s Law to perform calculations:
Boyle’s Law Vernier Lab