My Science PLC has a few concepts that we have been reviewing with our students in grades 6 – 12 for the past few years. One of those concepts was brought to us by one of our high school teachers because she noticed that her students were having difficulty with conceiving of air as having mass. She found a great probe called Air Ideas Probe (a concept cartoon formative assessment classroom technique (FACT), from Science Formative Assessments by Page Keeley). In the scenario a teacher places two balloons full of air on either side of a suspended dowel and they are perfectly balanced. The probe asks students to think about what would happen to the dowel if the teacher let the air out of one of the balloons. Four fictitious students give four possible answers. The first student thinks that the balloon with the air still in it will be lighter than the other side because it has air in it so it and air floats so it will go up, the next student thinks the balloon that has the air out of it is denser so it will go down (same effect as the first response but for a different reason), the next student thinks the balloon with the air still in it is heavier because the air weighs something so it will sink, and the last student thinks the dowel will remain balanced because there are still just two balloons there (the air makes no difference).
What a marvelous probe because it goes to the heart of what our students believe about air. Does air have mass and therefore have weight? My two 8th grade classes remember taking this probe last year where they chose which student they thought was correct and then explained why they thought so. Once they weighed in on what they thought (sorry for pun) we actually demoed it so that all could see what should have happened (more on this later). So I asked my 8th graders about and most of them or at least many of them remembered having done this. I said, “great, I’ll just have you take the probe again to see what you remember.” I had some balloons handy just in case but I was pretty sure most would remember and choose the correct student and at least many of them would remember why that student is correct. I’m glad I had some balloons handy. Less than half (about 43%) of my 8th graders chose the correct response and (spoiler alert, don’t read on if you want to test your knowledge on this one without first choosing which response from the ones above you think is correct) some students actually argued that air has no weight! NO WEIGHT! I mean, I could understand saying air is a gas so it weighs very little but NO WEIGHT?
That’s not the worse part, not by a long shot. See I was ready with my balloons. I used a meter stick balanced by a string to get the balloons balanced. (I know, there are probably many ways I could have done that better but I really wasn’t that prepared to do this because I thought I wouldn’t have to do it.) You can probably guess what happened. It went wrong, both times! Both times I’m standing there letting the air out of the balloons as students are shouting what they think will happen. (Spoiler alert again because by this time I’m assuming we are expecting the balloon that is still full of air to sink because it has air in it and the other one doesn’t.) But of course when I let all the air out and released the balloon we all stared as the meter stick moved a bit only to look perfectly balanced when it stopped. Ugh! This has happened to me before so I had my Ohaus digital scale ready to go.
The scale didn’t let me down. In my first class the balloon with air weighed a whopping .18g more than the empty one. I tried to explain how that probably wasn’t enough weight to tip the scales. Was my effort in vain? By the second class I remembered to blow the balloons up more so the full one ended up weighing a whole .4g more than the empty one! Some students also noted that not only was there air in the filled balloon but that the air was under pressure, which should weigh more than the air around it. Because I probably convinced the rest of my students that air has no mass, I had students look up what air is composed of then we used The Elements app on the iPads to find the atomic weights of all the elements in air (we used the teeny, tiny numbers in grams to give us perspective).