Explain gas compression at constant temperature
Keep average kinetic motion consistent while reducing available space for a fixed sample.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Describe widely separated randomly moving gas particles.
- Relate equal temperature to the same average kinetic-energy comparison for the same gas.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In constant-temperature compression of a fixed sample, what must stay fixed?
What you will learn
- Explain qualitative pressure changes with volume at fixed gas mass and temperature.
- Distinguish reduced separation from increased average speed or particle shrinkage.
Reduce space without changing temperature
For a fixed mass of gas compressed at constant temperature, its average particle kinetic-energy account stays the same. The same particles occupy less space and collide with a given wall area more frequently on average. The increased average force per unit area raises pressure. The individual particles do not become smaller, and a speed increase is not required by this constant-temperature comparison.
Expanding the same fixed gas mass at constant temperature gives more available space and less frequent impacts per wall area on average, so pressure decreases. The relevant quantity is volume, not merely the length or width of one drawn box. A changed container shape alone does not necessarily establish a changed total volume.
Identify the fixed amount and temperature
Real rapid compression can change a gas's temperature. That is a different situation from the stated constant-temperature comparison. Written questions should specify conditions, or provide enough information to establish them. Do not claim that every compression is automatically constant-temperature or that it always leaves particle speeds unchanged.
A sample gaining gas while its volume changes also fails the fixed-mass condition. Qualitative pressure comparisons depend on the stated mass and temperature controls. The Supplement adds pV = constant for the same controlled model; Core learners explain the collision mechanism without needing those numerical calculations.
Sources for this lesson
Pause and explain
Which explains a pressure rise during constant-temperature compression?
Worked example
A fixed gas sample is compressed from 120 to 60 cm³ at constant temperature. Predict pressure qualitatively and describe particle size and average motion.
Show the worked solution
- The same gas mass occupies less space.
- Average kinetic motion stays the same because temperature is fixed; particles do not shrink.
- More frequent impacts per wall area raise pressure.
Answer Pressure increases; particles retain their size and the average kinetic-motion comparison.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain pressure rise for a fixed gas sample compressed at constant temperature.
Give me a hint
Use smaller space and collision frequency without requiring faster particles.
Compare my reasoning
- Mass and temperature are fixed.
- Average separation decreases while particle size and the average kinetic-motion comparison are retained.
- More frequent wall impacts per area increase pressure.
Smaller volume increases wall-impact frequency and pressure at the stated controls.
Look for these in your work
- Retain fixed temperature.
- Distinguish separation from particle size.
Predict pressure changes for a fixed gas mass expanded and compressed at constant temperature, then explain why a longer vessel is insufficient evidence of expansion.
Give me a hint
Compare total volume rather than one geometric dimension.
Compare my reasoning
- Increasing volume lowers pressure through less frequent impacts per equal wall area.
- Decreasing volume raises pressure through more frequent impacts.
- A longer vessel can have a smaller cross-sectional area, so its total volume must be known.
Expansion lowers pressure; compression raises it; length alone does not determine volume.
Look for these in your work
- State both controlled predictions.
- Use the actual three-dimensional volume.
A rapid compression raises gas temperature, and a student says the constant-temperature particle explanation proves that average motion cannot change. Correct the claim.
Give me a hint
Check the supplied temperature condition before choosing the model.
Compare my reasoning
- The actual described temperature rises.
- The constant-temperature condition is therefore absent.
- Greater average kinetic motion is consistent with the higher temperature, so it cannot be ruled out by an inapplicable control.
This is not constant-temperature compression; average kinetic motion can increase.
Look for these in your work
- Identify the failed condition.
- Use the described temperature evidence.
Common mistakes
- Compression is not always constant-temperature.
- Use volume, not one container dimension alone.
- Fixed temperature does not support a required speed increase.
What can you explain now?
What happens to pressure on constant-temperature expansion of a fixed gas mass?
Compare with the explanation
It decreases.
Greater available space reduces impact frequency per equal wall area on average.
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Make it stick
Explain volume changes under the fixed controls and reject particle-shrinkage, dimension-only and uncontrolled-temperature claims.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.