Density Curriculum

Section 2—Lesson 7: Do Gases Have Density?

Background Information

Gases Have Density, Too!

Students often think of density as having to do with solids. However, gases and liquids also have density. Gases and liquids are both fluids. The main difference between them is the distance between their molecules. In a gas, the molecules are far apart, so they move about more freely and aren't affected as much by the forces between molecules. In a liquid, the molecules are close together, and so the forces of the molecules affect each other more. Both liquids and gases take the shape of the container that they are in.

Difficulties Students Have in Realizing that Air is Matter

Students tend to have two kinds of conceptual difficulties in realizing that gases have density. Both relate to uncertainties about air as matter. First, they do not always realize that air takes up space. This is in part because air typically moves out of our way when we move ourselves or other matter into the space that it is in. Second, students often do not believe that air has mass. If they don't realize that air takes up space and has mass, they certainly won't think of it as having density!

In this lesson, we substitute the word "air" for the word "gases" in some instances since students are likely to think of gases as air. We also use the term "air molecules" in quotations since there really is no such thing as an air molecule. Rather, there are molecules of a number of different substances that make up the air. If you use the term "air molecule," we suggest telling the students that it is just a shortened way of referring to the molecules in the air (things like oxygen, nitrogen, hydrogen, etc.) but that there's actually no such thing as a molecule of air. There is a molecule of particular gases that make up the air so another way to handle this issue is to just talk about gases and not substitute "air molecules" for "gases."

Transferring the Understandings

Realizing that gases have density also helps students begin to think about mixed density. In Lesson 1, students may have said that one of the cylinders is hollow. This is an especially common answer when students have not had much experience with the particulate nature of matter. Once students understand that gases have density, they can begin to reason about the density of hollow objects as composed of the density of the surrounding material plus the density of the gas inside. The reasons why the gas contributes so little mass compared to its volume (and therefore makes the overall object less dense rather than more dense) will be elaborated in Section 3.

Realizing that we can find the density of fluids (liquids and gases) is important for transferring understandings about density, such as understanding weather systems, why colder water sinks in warmer water, why icebergs float, and so on. It is a fundamental understanding that will allow students to build many other understandings.