4. Air, a Gas

Double balloon on the pan balance.

Our first breath is of air - we’ve lived with it all our lives, yet few people think about air as a gaseous form of matter that has weight and volume. Gases are another form of matter, just like liquids and solids. They have weight, take up space, and have properties that students can explore.

Students discover that gases are highly compressible: a sample of gas can be squished down to a significantly smaller size. Liquids and solids are essentially incompressible. Students observe that a sample of gas visibly expands when heated and contracts when cooled. What can account for this? While the individual particles that make up each type of gas (e.g., oxygen, nitrogen) are unique, one thing all gases have in common is the arrangement of those individual particles. Compared with solids and liquids, gas particles are on average significantly spread out. They are in constant motion, continuously colliding with one another and taking up much more space than the volume of the individual particles. Again, students see a computer model that demonstrates how a gas responds to increasing or decreasing amounts of thermal energy.


The Child and the Scientist