Why do real gases deviate from ideal behavior, and under what conditions is the deviation greatest?
Topic 3.6 Deviation from Ideal Gas Law: explain why real gases deviate from the ideal gas law at high pressure and low temperature in terms of molecular volume and intermolecular forces.
A focused answer to AP Chemistry Topic 3.6, covering why real gases depart from PV equals nRT, the roles of finite molecular volume and intermolecular attractions, and the conditions (high pressure, low temperature) where deviations matter, with full worked examples.
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What this topic is asking
The College Board (Topic 3.6) wants you to explain why real gases deviate from the ideal gas law, identify the two idealizing assumptions that break down, and state the conditions, high pressure and low temperature, under which deviations are largest. This closes the loop on the gas unit: the ideal gas law and kinetic molecular theory are an approximation, and you should know when and why it fails.
The assumptions that break down
These two assumptions are exactly the ones from kinetic molecular theory (Topic 3.5). They are excellent approximations when particles are far apart and fast-moving, but they fail when particles are crowded or slow.
Why high pressure causes deviation
At high pressure the gas is compressed into a small volume, so the particles are close together and the space taken up by the particles themselves becomes a significant fraction of the container. The free volume available to the gas is then less than the measured volume, which the ideal law does not account for. This effect tends to make the real volume (or pressure) larger than the ideal prediction at very high compression.
Why low temperature causes deviation
This is why a gas eventually condenses to a liquid when cooled enough: the attractions that the ideal model ignores take over once the particles are slow enough. A gas with strong intermolecular forces, such as ammonia with its hydrogen bonding, deviates more than a nonpolar gas of similar size, such as methane.
Which gases deviate most, and when
Putting it together, a real gas departs most from ideal behavior when it is at high pressure and low temperature, and when its particles are large (more volume, stronger dispersion forces) or strongly attracting (polar or hydrogen bonding). It behaves most ideally at high temperature and low pressure, where both corrections vanish. This lets you rank gases or conditions for ideality, a common exam task.
Try this
Q1. State the two assumptions of the ideal gas model that real gases break. [2 points]
- Cue. Negligible particle volume, and no intermolecular forces between particles.
Q2. Explain why raising the temperature makes a real gas behave more ideally. [1 point]
- Cue. Higher kinetic energy makes the intermolecular attractions negligible compared with the particles' motion.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2023 (style)3 marksSection II (short FRQ). (a) State the two assumptions of the ideal gas model that real gases violate. (b) Explain why a real gas behaves most ideally at high temperature and low pressure. (c) Predict whether or deviates more from ideal behavior at the same conditions, and justify.Show worked answer →
A 3-point FRQ on real-gas behavior.
(a) Assumptions (1 point): the ideal model assumes the particles have negligible volume and that there are no intermolecular forces between them; real gas particles have a finite volume and do attract one another.
(b) Ideal conditions (1 point): at high temperature the particles have high kinetic energy, so intermolecular attractions are negligible compared with their motion; at low pressure the particles are far apart, so their own volume is negligible compared with the container. Both effects are minimized, so the gas behaves ideally.
(c) Prediction (1 point): deviates more, because it has hydrogen bonding (stronger intermolecular forces) than nonpolar , so its attractions matter more and pull it further from ideal behavior.
Markers reward naming both violated assumptions, linking high temperature and low pressure to minimizing forces and particle volume, and using intermolecular force strength to compare deviation.
AP 2022 (style)1 marksSection I (multiple choice). A real gas is most likely to behave ideally under which conditions? (A) high pressure and low temperature (B) low pressure and high temperature (C) high pressure and high temperature (D) low pressure and low temperature. Justify your reasoning.Show worked answer →
A 1-point conceptual MCQ. The answer is (B).
A real gas behaves most ideally at low pressure (particles far apart, so their own volume is negligible) and high temperature (high kinetic energy, so intermolecular attractions are negligible). The conditions that cause the largest deviations, in (A), are the opposite: high pressure and low temperature.
Related dot points
- Topic 3.4 Ideal Gas Law: use the ideal gas law and its partial-pressure and gas-density forms to relate the pressure, volume, temperature and amount of a gas in calculations.
A focused answer to AP Chemistry Topic 3.4, covering the ideal gas law PV equals nRT, the combined gas law, partial pressures and Dalton's law, mole fractions and gas density, with full worked examples.
- Topic 3.5 Kinetic Molecular Theory: state the postulates of kinetic molecular theory and use them to explain gas pressure, temperature, and the Maxwell-Boltzmann distribution of molecular speeds.
A focused answer to AP Chemistry Topic 3.5, covering the postulates of kinetic molecular theory, how they explain pressure and temperature, the link between average kinetic energy and temperature, and the Maxwell-Boltzmann speed distribution, with full worked examples.
- Topic 3.1 Intermolecular Forces: identify and rank the intermolecular forces (London dispersion, dipole-dipole, hydrogen bonding, ion-dipole) present in a substance and relate their strength to properties such as boiling point and vapor pressure.
A focused answer to AP Chemistry Topic 3.1, covering London dispersion, dipole-dipole, hydrogen bonding and ion-dipole forces, how to rank their strength, and how intermolecular forces set boiling point, viscosity and vapor pressure, with full worked examples.
- Topic 3.3 Solids, Liquids, and Gases: describe the particle-level differences between the three states and explain how intermolecular forces and temperature determine which state a substance is in.
A focused answer to AP Chemistry Topic 3.3, covering the particulate model of the three states, how intermolecular forces and kinetic energy compete to set the state, and how to read particulate diagrams and heating curves, with full worked examples.
- Topic 3.2 Properties of Solids: relate the macroscopic properties of a solid (melting point, hardness, conductivity) to its type (ionic, metallic, covalent network, molecular) and the forces holding its particles together.
A focused answer to AP Chemistry Topic 3.2, covering the four types of solid (ionic, metallic, covalent network, molecular), the forces in each, and how those forces explain melting point, hardness, brittleness and conductivity, with full worked examples.
Sources & how we know this
- AP Chemistry Course and Exam Description — College Board (2020)