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Deviation from Ideal Gas Behaviour

Our theoretical model of gases corresponds very well with the experimental observations. Difficulty arises when we try to test how far the relation pV = nRT reproduce actual pressure - volume - temperature relationship of gases. To test this point, we plot pV vs p plot of gases because at constant temperature, pV will be constant (Boyle's law) and pV vs p graph at all pressures will be a straight line parallel to X-axis.


  1. It can be seen easily that at constant temperature pV vs p plot for real gases is not a straight line.
  2. There is a significant deviation from ideal behavior. Two types of curves are seen.
  3. In the curves for dihydrogen and helium, as the pressure increases the value of pV also increases.
  4. The second type of plot is seen in the case of other gases like carbon monoxide and methane.
  5. In these plots first there is a negative deviation from ideal behavior, the pV value decreases with increase in pressure and reaches to a minimum value characteristic of a gas.
  6. After that pV value starts increasing, the curve then crosses the line for ideal gas and after that shows positiv4e deviation continuously.
  7. It is thus found that real gases do not follow ideal gas equation perfectly under all conditions.

Deviation from ideal behaviour

Boyle's law
Real gases do not obey the ideal gas law exactly. The magnitude and the nature of deviations from Boyle's law may be seen from the plots of PV/RT (compressibility factor) versus pressure of different gases at constant temperature.

Compressibility factor, Z is the ratio of observed molar volume (V) to the ideal molar volume (Videal) = RT/P.


For an ideal gas obeying Boyle's law Z=1 and is independent of T and P.
The curve is a horizontal dotted straight line for a real gas Z depends upon both T and P.

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