Draw a sketch of a hot air balloon and of a helium balloon. Define the systems by identifying the system boundaries. Show inputs and outputs of matter and energy in each. Use your Evidence Notebook if necessary.

Explanation:

When you heat the air it expand its volume, but its mass remain the same, so the density of the hot air balloon is smaller than the density of the air outside it, so the hot air balloon can float.

In the case of the helium balloon, there's no source of energy, but the helium is naturally lighter than the atmospheric air, so its density is naturally smaller than the air, and the helium balloon have a smaller density than the air too, so it can float.