* Hydrogen is less dense than water: This means it will rise above the water in a container.

* Hydrogen is practically insoluble in water: This means it won't significantly dissolve in the water, allowing it to be collected in a pure form.

Here's how it works:

1. Generation: Hydrogen is typically generated through a chemical reaction, often between a metal and an acid.

2. Collection: The hydrogen gas produced is passed through a delivery tube into an inverted container filled with water.

3. Displacement: As the hydrogen gas bubbles up through the water, it displaces the water, pushing it downwards.

4. Collection: The container is filled with hydrogen gas, which is then collected by sealing the container.

Why not collect it by upward displacement of air?

While hydrogen is lighter than air, it isn't significantly lighter to make this method practical. Here's why:

* Diffusion: Hydrogen gas diffuses quickly through air, making it difficult to collect in a pure form.

* Flammability: Hydrogen is highly flammable, and collecting it in air poses a significant fire hazard.

Therefore, downward displacement of water is a safer and more efficient method for collecting hydrogen gas.