Which condition causes real gases to behave most like ideal gases?

• A. High pressure and low temperature.
• B. Low pressure and high temperature.
• C. Very high density.
• D. Gases always behave ideally.

Answer: (B)

Real gases behave like ideal gases when intermolecular forces are negligible and the molecules occupy little volume relative to the container. This happens most effectively at low pressure, where molecules are far apart, and at high temperature, where their average kinetic energy dominates over attractions. Under these conditions the ideal gas law PV = nRT accurately describes the system because the corrections for molecular size and attractive forces become insignificant (the a and b terms from the Van der Waals equation are small). If you increase pressure or lower temperature, molecules are closer together and attractions become important, causing deviations from ideal behavior. Very high density emphasizes these effects, moving the gas further from ideal behavior. So the scenario with low pressure and high temperature makes real gases behave most like ideal gases.