When the gas collecting tube is inverted during collection, which property is true of the gas?

• A. It is more dense than air.
• B. It is less dense than air.
• C. It dissolves in water.
• D. It reacts with air.

Answer: (B)

When you collect gas with an inverted tube over water, the gas inside the tube has to rise up and push the air out as it fills the space. A gas that is lighter than air will do exactly that: it is buoyant, so it rises into the top of the inverted tube and remains there, displacing the air. If the gas were heavier (denser) than air, it would not rise into the upper part of the tube in the same way, so you wouldn’t observe it filling the space at the top. So the property you can deduce is that the gas is less dense than air. Dissolving in water or reacting with air aren’t determined by this setup.