IB Physics SL B.3.8 Ideal gas approximation limits Question Bank

Practise IB Physics SL B.3.8 by identifying when real gases follow ideal-gas behaviour and why extreme compression breaks the model.

Exam points

Select low-density, low-pressure or high-temperature conditions where real gases approximate ideal behaviour.
Explain why very small volumes and high pressures invalidate point-particle and no-force assumptions.

[Maximum number: 2]

The equipment shown in the diagram was used by a student to investigate the variation with volume, of the pressure p of air, at constant temperature. The air was trapped in a tube of constant cross-sectional area above a column of oil.

Figure for Question B.3.8—Ideal gas approximation limits question 1 — IB Physics SL

The pump forces oil to move up the tube decreasing the volume of the trapped air.

The equation in (b) may be used to predict the pressure of the air at extremely large values of $\frac{1}{H}$ . Suggest why this will be an unreliable estimate of the pressure.

very large $\frac{1}{H}$ means very small volumes / very high pressures

at very small volumes the ideal gas does not apply

OR

at very small volumes some of the assumptions of the kinetic theory of gases do not hold

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B.3.8—Ideal gas approximation limits question 1