Demonstrate that gases are matter with mass and volume, using balloon and air investigations as evidence.
It is easy to believe that solids and liquids are matter; you can see and touch them. Gases are trickier because most are invisible, but they are matter too: they have mass, and they take up space, called volume. A balloon is one of the best tools for proving this, because it lets you see the shape gas takes up even though you cannot see the gas itself.
An empty, uninflated balloon and an inflated balloon actually have different masses, if you could weigh them precisely enough; the inflated one is a little heavier because it now holds more air particles. A full balloon also takes up real space: try pushing two inflated balloons into a small box, and you will feel the air pushing back.
Mastery looks like: They can describe at least two investigations that show a gas has mass and volume, using balloon or air evidence.
Common stumbles: Thinking air only fills space without actually being physical matter; assuming an inflated balloon has the exact same mass as an empty one.