Q159
General SciencePhysics
A real gas behaves like an ideal gas at
- a.high temperature and high pressure.
- b.high pressure and low temperature.
- c.low pressure and high temperature.
- d.low pressure and low temperature.
Answer: (C) low pressure and high temperature.
An ideal gas is assumed to have molecules of negligible volume with no forces between them. A real gas comes closest to this at low pressure, when the molecules are far apart, and at high temperature, when they move too fast for intermolecular attraction to matter, so option C is correct. The opposite conditions, high pressure and low temperature (B), are exactly those used to liquefy gases, where deviation from ideal behaviour is greatest. The ideal gas equation is $PV = nRT$; real gases are described by the van der Waals equation $\left(P + \frac{an^2}{V^2}\right)(V - nb) = nRT$, in which $a$ corrects for intermolecular attraction and $b$ for molecular volume.