For Day 18, students determined the density of carbon dioxide by collecting the gas produced by Alka-Seltzer over water. Students determined the mass of the gas collected (using what they knew about the Law of Conservation of Mass) and measured the volume of the gas collected. With these measurements, students were about to come up with a density value fairly close to the accepted value of .0019 g/mL. After the class compared their data, we discussed the concept of “order of magnitude” and the idea that a gas is 3 orders of magnitude (or 1000x) less dense than a liquid. Students then drew particle models to account for the density differences of solids, liquids and gases. Initially, every group drew slightly less particles in the gas compared to the liquid but spread them out. The gas in these represenations was less dense but not 1000x less dense. Students came to the conclusion that to truly represent the density of gas, they would have to draw a fraction of a particle. The class decided that fractions of a particle didn’t exist so they agreed that one particle in the box would show the density difference well enough. This lab and discussion was completed over a double-blocked lab period.