Upward displacement of air
Chinese equivalent: 向上排空气法（用于密度比空气大的气体）
Upward displacement of air collects gases denser than air, such as CO₂ and HCl: gas enters at the bottom and pushes air upward. See the method and examples.
Syllabus path: Chemical Experimentation and Application › Preparation and Testing of Common Gases
CSCA Exam Prep
This term is aligned to the CSCA undergraduate admissions exam syllabus. Continue with the linked tutorial and topic practice.
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CSCA Chemistry Exam Glossary
Definition
What is upward displacement of air?
Direct answer: Upward displacement of air is a gas-collection method for gases denser than air. The gas is delivered near the bottom of an upright gas jar, so it settles lower and pushes the less-dense air upward and out; common school-laboratory examples include carbon dioxide and hydrogen chloride.
How to choose and set up the method
- 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.
- Check that the gas is suitable for collection in air and follow the experiment's safety controls, especially for corrosive or toxic gases.
- Keep the gas jar upright and place the delivery tube near the bottom without sealing the outlet for the displaced air.
- Let the denser gas enter slowly from below, displacing air upward while minimizing mixing and turbulence.
- 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
At the same temperature and pressure, gas density is proportional to molar mass. Compare each gas with air at about 29 g/mol before choosing the collection direction.
| Gas | Approx. molar mass | Compared with air | Suitable direction |
|---|---|---|---|
| CO₂ | 44 g/mol | Denser | Upward displacement of air |
| HCl | 36.5 g/mol | Denser | Upward displacement of air |
| NH₃ | 17 g/mol | Less dense | Not upward displacement of air |
| H₂ | 2 g/mol | Much less dense | Not upward displacement of air |
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.
- Treating a filled-looking jar as proof of gas identity or purity without using the required test.
Sources and review
Last reviewed: . The density rule, setup direction, and example comparisons were checked against the cited gas-law reference. Follow laboratory safety guidance for the specific gas; use the official CSCA source for current exam scope and rules.
Related tutorial and examples
Preparation and Testing of Common Gases
Preparation and Testing of Common Gases Core Concepts and Logical Framework The preparation and testing of common gases is a core skill in chemical experimentation. It follows a clear logical process: Selecting the reaction principle → Designing the generation apparatus → Perform…
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