Gases deviate from ideal behaviour at high pressure and low temperature due to molecular volume and attractions.
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Real gases deviate from ideal behaviour because their molecules have finite volume and experience intermolecular attractions. At high pressure the molecules are packed closely and their own volume becomes a significant fraction of the container volume, so the gas behaves differently from the ideal prediction. At low temperature, attractions between molecules pull them together and reduce the pressure below the ideal value. Both effects make the ratio PV/nRT differ from 1. Real gases approach ideal behaviour at low pressure and high temperature, where the molecules are far apart and move fast enough for the attractions to be negligible.
Key ideas: Deviation is judged by Z = PV/nRT. Attractions dominate at moderate pressures and low T and give Z below 1; molecular volume dominates at high P and gives Z above 1. Small, nonpolar gases like He and H2 behave ideally over wide ranges, while polar or easily liquefied gases like NH3 and SO2 deviate more.
Common mistakes and connections: A frequent error is thinking real gases always have lower pressure than ideal; at high pressure it is higher. Also, student sometimes believe gases only deviate near condensation, but deviations begin well before. Corrections lead to the van der Waals equation, virial equation and compressibility charts. Also linked to critical temperature and liquefaction.
Real gases deviate from ideal behaviour (Z = 1): attractions lower Z at moderate pressure, finite molecular size raises it at high pressure. The interactive lesson shows this as a live animation or chart.
Carbon dioxide at 50 atm deviates greatly from ideal gas behaviour, while at 1 atm the deviation is very small.
Gases deviate from ideal behaviour at high pressure and low temperature due to molecular volume and attractions.
Real Gases is a intermediate topic in States of Matter & Solutions. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in States of Matter & Solutions first: Molar Volume, Gas Density, Gas Stoichiometry.
The next topics in States of Matter & Solutions are Van der Waals Equation and Compressibility Factor.
Yes. The interactive lesson includes a self-check question on Real Gases, and the States of Matter & Solutions quiz draws on every topic in this area.
Real Gases is topic 28 of 109 in States of Matter & Solutions.