I want to demonstrate different states of matter and need activities for third graders for gases.
— D., Georgia

It’s hard to teach about something we can’t see!
Here are a few ideas:

Perfumes: Open a bottle of cologne in the room. Students can observe evaporation if you pour some on a dark counter.

Solid room air fresheners: This is a scented material in solid form and, over time, you will see the solid disappear as it sublimates.

“Ghost” in a Bottle: (Have theatrical fun with this.) Refrigerate a large, empty pop bottle before class. Bring it to class, open it and place a coin over the opening. In a few moments you will hear the coin rattling as the gas inside the bottle heats and expands.

Crushing a can: (Practice this demo for safety.) Boil a few mL of water in an aluminum can until steam comes out. Grasp the can with tongs and quickly turn it upside down into a pan of ice water. The steam condenses, emptying the can of a lot of gas. Atmospheric pressure outside is now greater than the pressure inside the can, causing it to crush.

Break a ruler using air: place a wooden ruler half way over the edge of a table. Flatten a full sheet of newspaper across the ruler. A forceful karate chop should break the ruler. The large surface area of the newspaper has substantial atmospheric mass pushing down on it, holding the ruler in place.

Hope this helps!

Image credit: Ken Boyd via Pixabay