Here's why:

* Density: Hydrogen gas has a molecular weight of 2 g/mol, while air has an average molecular weight of approximately 29 g/mol. This means that hydrogen gas is much lighter than air.

* Upward Displacement: When hydrogen gas is produced in a reaction, it rises through the air and displaces the heavier air molecules. This is why it's collected in an inverted container (like a gas jar or a beaker).

* Collection: The hydrogen gas fills the container from the bottom up, pushing the air out of the top. This method ensures that the container is filled with pure hydrogen gas.

Note: This method works best for gases that are less dense than air. If you were collecting a gas denser than air, you would need to use a different method like downward delivery or water displacement.