Upward displacement of air

What is upward displacement of air?

Direct answer: Upward displacement of air collects gases that are denser than air. The gas enters near the bottom of an upright jar and pushes the air upward and out. This method is also called downward delivery: the name describes the incoming gas, not the displaced air.

How to choose and set up the method

1. Compare the gas with air under similar conditions; air has an average molar mass of about 29 g/mol, so a substantially larger molar mass usually indicates a denser gas.
2. Check that the gas is suitable for collection in air and follow the experiment's safety controls, especially for corrosive or toxic gases.
3. Keep the gas jar upright and place the delivery tube near the bottom without sealing the outlet for the displaced air.
4. Let the denser gas enter slowly from below, displacing air upward while minimizing mixing and turbulence.
5. Use the stated collection or identification procedure for that experiment; density alone does not prove that the collected sample is pure.

Examples: choose the direction from relative density

For gases that behave approximately ideally, density is proportional to molar mass at the same temperature and pressure. Compare each gas with air, whose average molar mass is about 29 g/mol.

CO₂ and HCl are denser than air and can displace it upward in an upright jar. NH₃ and H₂ are less dense, so this direction is not appropriate for them.

Common gas-collection mistakes

• Reversing the rule and using upward displacement of air for a gas that is less dense than air.
• Placing the delivery tube near the top, which encourages the incoming denser gas to mix with air instead of filling from below.
• Choosing the method from density alone without considering solubility, reactivity, toxicity, or the experiment's safety instructions.
• Assuming the collected gas is pure without checking for remaining air or carrying out the required identification test.

Sources and review

• OpenStax Chemistry 2e — Gas Density and Molar Mass
• Royal Society of Chemistry — Generating, collecting and testing gases
• Official CSCA exam introduction

Last reviewed: 2026-09-08. Density comparisons use the ideal-gas relation in OpenStax; the RSC resource distinguishes gas-collection methods. These notes explain the principle, not a complete laboratory procedure. Follow the safety instructions for the specific experiment.