Heavy Gas Challenge
Collect each gas the right way up.
Setting up the bench…
What this lesson covers
Why it matters
Hydrogen chloride is heavier than air; ammonia is lighter. That one fact decides which way up you hold the gas jar.
Predict first
Hydrogen chloride is denser than air, so it is collected…
A gas denser than air is collected by upward displacement of air (jar mouth up); a gas lighter than air by downward displacement of air (jar mouth down). HCl (36.5) and CO₂ (44) sink; NH₃ (17) and H₂ (2) rise. (Air ≈ 29.)
- in an upright jar, by upward displacement of air — correct
- in an inverted jar, by downward displacement of air
- over water
What you do
Sort each gas by how it should be collected.
Check yourself
Ammonia (17 u) is collected by downward displacement of air because it is…
Lighter-than-air gases rise, so the jar is held mouth-down.
Why isn't HCl collected over water?
Very soluble gases like HCl would dissolve, so they're not collected over water.
- lighter than air — correct
- denser than air
- the same density as air
- It is extremely soluble in water — correct
- It floats on top of water
- It reacts violently with glass