Gas laws can be a challenge for many high school level students and teachers, for many reasons. The availability of gas canisters is, at least in my state of Colorado, mostly banned or restricted due to safety concerns, leaving teachers with the need to produce their own for use in the lab. Students can sometimes struggle to grasp gas behavior, as it’s much harder to visualize gases rather than readily available solutions, solids, or mixtures. Indeed, for many labs, if gas is a product, we’re often relegated to using balloons or gas columns to capture and measure the gases produced by reactions, which can be tricky or expensive. So this year, I tried something a little different when starting my intro chemistry students’ gas laws unit, and was very happy with the results – start with an inquiry approach, allowing students to discover gas laws on their own, thanks to easily producible gas phenomena in the lab.
Phenomena as a Conversation Starter
As many states are slowly moving to adopting or adapting the Next Generation Science Standards, you may already be steeped in the use of phenomena as an anchor for a unit or lesson; using phenomena or discrepant events is at the heart of posing a problem to students and allowing them to discover the innate laws found in nature as a result. An excellent resource to consult regarding the use of phenomena in a three-dimensional classroom is available on the NGSS website. With gas laws, it can be tempting to jump right in with the laws themselves, teaching students the group of Boyle’s Law, Charles’ Law, and the rest, filling their minds with equations such as P1V1 = P2V2 which can be easily manipulated with basic algebra skills. However, allowing students to derive these laws from their own experiences is even more powerful, leading to deeper understanding of how gases behave, rather than rote memorization of of set of relationships. I found that this led to much deeper conversations in class, including discussion of what gases might be doing on the particulate level, rather than just “because PV=nRT!”
This activity was set up in a stations format, with some stations repeating the sorts of gas investigation, and others entirely different. Students were faced with situations such as heating/cooling a sealed syringe to investigate volume change, heating a aluminum can (containing a small amount of water) over a Bunsen burner and then inverting into ice water, and placing an inflated balloon into a vacuum chamber. Simple examples, to be sure, but surprising or dramatic enough (especially the “crushed can”, a perennial student favorite) to capture student attention and engage them in discussion. At each station, students were tasked with making detailed observations before, during, and after, and then discussing in a roundtable format with small groups of classmates with regards to their observations and what could explain the phenomena observed. This connectivity allowed students to mull their questions out loud, and address other students’ thoughts; I found it intriguing to hear students correcting one another’s thoughts on gases, and the debate that arose.
Experience to Inform Instruction
Having seen how gases behave in a variety of situations, students were more readily able to build their “own” gas laws so that when the named laws of Guy-Lussac, Dalton, and others were revealed, students had already formed those relationships, and the remainder of the unit became more exploration and application, rather than top-down instruction. This was extremely effective in aiding their ability to manipulate gas laws and apply them in novel situations, as well as to take their knowledge of gas behavior and apply it to drawing particulate diagrams and scientific debate. On formative and summative assessment for this unit, I found many students were recalling their experience from the beginning lab activity to help them explain gas behavior as it pertained to new experiences or tasks. While this was not my first time using a phenomenon-based engaging activity to begin a unit, it was certainly the most poignantly effective that I’ve experienced in the classroom, and will influence the way I approach many other units, not just gas behavior.