Deviation from Ideal Gas Law
Use matched conditions to distinguish attraction and excluded-volume effects from ideal behavior.
AP Chemistry Topic 3.6 · CED printed p. 65 / PDF p. 72. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- KMT ideal assumptions
- Interparticle attractions
- PV=nRT
Real particles attract
The ideal law neglects attractions and particle volumes. At a given n,V,T, intermolecular attraction can reduce momentum transfer to the walls relative to the ideal prediction, making measured pressure lower. Attractions matter more as particles are closer and thermal motion is weaker, particularly near condensation at lower temperature. More strongly interacting gases often show greater attraction-related deviation than similarly sized weakly interacting gases under the same conditions.
Attraction does not mean the gas particles stop moving. It changes how they interact and distribute momentum. If actual condensation occurs, not all originally added material remains in the gas phase, so using the original total amount as gas n also fails. State the phase assumption when comparing a real sample to the ideal calculation.
Particle volume matters under strong compression
At very high pressure, the intrinsic volumes of particles occupy a meaningful fraction of the container. The free volume available for center motion is smaller than the container volume. This excluded-volume effect can make pressure larger than the ideal prediction. It competes with attractions, so real-gas pressure is not always lower than ideal. Use actual conditions or data to decide which effect dominates.
Ideal behavior is approached at low density and sufficiently high temperature relative to condensation, where separations are large and attraction effects are small. “High temperature” is relative to the species and conditions. A real-gas measurement can be summarized by PV/(nRT): below 1 suggests attraction-dominated pressure reduction, above 1 suggests excluded-volume-dominated increase in this simple model. No van der Waals calculation is needed for the AP objective.
Let's work through it
Work it through
At fixed n,V,T the ideal calculation predicts 10.0 atm. Real sample A measures 9.20 atm near condensation; sample B under much stronger compression measures a pressure above its own matched-condition ideal prediction.
- For A, measured/ideal pressure = 0.920. A lower pressure is consistent with attraction reducing wall momentum transfer; verify that all n is still gas.
- For B, a larger-than-ideal pressure is consistent with significant excluded particle volume under strong compression.
- These are competing effects, not two different “ideal laws.” Comparing each real measurement with its matched n,V,T ideal reference makes the signs meaningful.
- Reducing density and moving sufficiently away from condensation generally reduces both concerns.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Why are low temperature and high density often poor ideal-gas conditions?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Why are low temperature and high density often poor ideal-gas conditions?
Worked answer and reason
Attractions matter more relative to thermal motion and particle separations become smaller.
The neglected interactions become significant.
Retrieval 2
Must measured real-gas pressure always be below ideal pressure?
Worked answer and reason
No. Excluded particle volume can raise it above ideal under strong compression.
Different neglected effects have opposing pressure tendencies.
Catch the tempting mistake
- All nonideal behavior comes from attraction.
- Finite particle volume also matters, particularly at very high pressure.
- High temperature alone guarantees ideality at any density.
- Extremely high density can still make finite particle volume important.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 3 problem 14 · ap application · data
- Unit 3 problem 15 · ap synthesis · multiple
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For the teacher
- Have students state the matched comparison: same n,V,T.
- Use both signs of deviation and require the gas-phase amount assumption.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 3.6.A
- Explain the relationship among non-ideal behaviors of gases, interparticle forces, and/or volumes.
- 3.6.A.1
- The ideal gas law does not explain the actual behavior of real gases. Deviations from the ideal gas law may result from interparticle attractions among gas molecules, particularly at conditions that are close to those resulting in condensation. Deviations may also arise from particle volumes, particularly at extremely high pressures.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 (supplied source) — Exact topic-numbered learning objectives, essential knowledge, science practices and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §9.6, Non-Ideal Gas Behavior — Attraction and excluded-volume deviations from ideal-gas pressure.
Glossary and deeper reading · 2 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Kinetic molecular theory
Kinetic molecular theory connects gas pressure and temperature to particle motion and collisions. In its ideal form, particles move randomly, occupy negligible volume, and undergo elastic collisions without intermolecular attractions. For example, hotter ideal gas particles have a greater average translational kinetic energy.
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Condensation
Condensation is a phase change from gas to liquid. It releases energy to the surroundings as particles form more closely associated liquid arrangements. For example, water vapor condensing on a cooler surface forms liquid droplets.