When do real gases depart from the ideal model?
You will be able to: Explain how attractions and finite particle volume affect real-gas pressure.
When do real gases depart from the ideal model?
The ideal model treats gas particles as volume-free and nonattracting. Those approximations become less reliable when particles are crowded or when attraction is significant compared with their kinetic energy.
A useful starting point: How do we read a gas-speed distribution? →
Words and symbols before equations
- Idealization
- A simplifying assumption in a model.
- Finite particle volume
- The real space occupied by particles themselves.
- Attractive effect
- Neighboring particles reduce outward momentum transfer compared with a nonattracting model.
- Excluded volume
- Space unavailable to other particle centers because particles have size.
What this picture assumes
Qualitative comparison at fixed n,V,T, not an equation of state or real pressure calculation. The two effects coexist; the selected label identifies the dominant contribution. No phase boundary is modeled.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- No attractions; negligible volume. Ideal prediction: P = nRT/V. Useful approximation when neglected effects are small.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
At moderate density, attractions can reduce the pressure below nRT/V because particles near a wall are pulled toward their neighbors. Lower temperature often makes these attractions relatively more important.
At high density, particle size reduces available free volume. Short-range repulsion and crowding can push pressure above an ideal prediction. Attraction and size effects compete; “real pressure is always lower” is false.
Higher temperature and lower density often improve ideal behavior. The qualitative picture cannot determine an exact real pressure or phase boundary without a suitable equation or measured data.
A worked example, step by step
For the same n,V,T, the ideal prediction is 10.0 atm but the measured gas pressure is 9.2 atm. Which neglected effect is consistent with the sign of the difference?
- The measured pressure is 0.8 atm below the ideal prediction.
- Attractions can reduce momentum transfer to container walls.
- That effect is consistent with the negative deviation in this case.
- Finite volume can oppose it. This comparison supports a net attraction-dominated deviation, not the absence of particle size.
Do not memorize “high pressure always gives a lower measured pressure.” Attractions and crowding have different effects.
Why can lower temperature increase nonideality?
Compare with an explanation
Lower typical kinetic energy makes attractions relatively more significant and can eventually permit condensation.
Predict. Change one thing. Explain.
Select ideal assumptions, attraction-dominated behavior and crowding-dominated behavior. Use the diagram to explain the direction of the pressure deviation; it does not calculate real material data.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
No attractions; negligible volume. Ideal prediction: P = nRT/V. Useful approximation when neglected effects are small.
Qualitative comparison at fixed n,V,T, not an equation of state or real pressure calculation. The two effects coexist; the selected label identifies the dominant contribution. No phase boundary is modeled.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using particle interactions, concentration, gas behavior or energy transfer. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA gas has Pmeasured/Pideal = 0.90 in one condition and 1.10 in another. Explain each sign and why neither condition invalidates using the ideal model elsewhere.
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Compare with the answer and four-point rubric
- 1 point: A ratio below one is consistent with net attraction-dominated effects.
- 1 point: A ratio above one is consistent with net crowding/repulsion effects.
- 1 point: The balance changes with density, temperature and species.
- 1 point: An approximation can be useful where its neglected effects are small, even if it fails in other conditions.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What two effects does the ideal model neglect?
Interparticle attractions and finite particle volume.
RECALL 2Do real gases always exert less pressure than ideal gases?
No; crowding can produce a positive deviation.
RECALL 3Can this schematic predict a condensation point?
No; it contains no fitted phase-equilibrium data.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
When do real gases depart from the ideal model?
- Attraction-dominated deviation: P may be below nRT/V.
- Crowding-dominated deviation: P may be above nRT/V.
Remember: Do not memorize “high pressure always gives a lower measured pressure.” Attractions and crowding have different effects.
Conditions: Qualitative comparison at fixed n,V,T, not an equation of state or real pressure calculation. The two effects coexist; the selected label identifies the dominant contribution. No phase boundary is modeled.
Refresh Kid · AP Chemistry Unit 3 · Objectives 3.6.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 3.6, objective 3.6.A. CED effective Fall 2024 and June 2026 clarifications checked September 16, 2026. Unit 3: Properties of Substances and Mixtures, Topics 3.1–3.13. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Colligative-property calculations and solution molality/mass-percent/volume-percent calculations are not required here. The optional speed-density model illustrates distributions; it does not require memorizing its mathematical derivation.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
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