Hydrogen is collected over water by:

**The reasoning** When we collect hydrogen gas in the lab, we use the **water displacement method**. Here's how it works: You produce hydrogen (say, by reacting zinc with dilute acid), and the gas travels through a tube into an inverted container filled with water. As hydrogen bubbles in, it pushes the water down and out, taking its place at the top. This is called "displacement" because the gas literally *displaces* (pushes away) the water. We use this method because hydrogen is virtually insoluble in water and less dense than water, so it rises and collects at the top of the container without dissolving. **Why the wrong options tempt you** - **Upward/Downward** sound right because hydrogen does rise upward (it's lighter than air), but these describe the *direction* hydrogen moves, not the *collection method*. - **Diffusion** is how gases mix and spread out naturally — that's a process, not a collection technique. **Quick takeaway** "Displacement" means the gas pushes water out of the way to take its place — that's how we trap and collect gases like hydrogen over water.