Place bottle on table, and observe the balloon. The balloon expands when heated and contracts when cooled. A balloon rises because the heated air inside the envelope becomes less dense than the cooler air outside the envelope. Rinse funnel after each use. Inflate and tie each balloon.
Students smell each balloon and identify smell.
Smells could be identified. The balloon is porous. Even though you cannot see them, there are very tiny holes all over the balloon. These holes allow the odor of the liquid inside the balloon to escape. The fabric of the envelope of the hot-air balloon is also porous.