Real+Gases

Most of the laws of gases assume that the gases exhibit ideal behavior. Ideal behavior involves two things: no particle volume and no attraction. With these assumptions, the ideal gas law, PV=nRT, can be used. In reality, however, if a gas is compressed enough the particles will attract and will liquefy. Similarly if the gas is cooled to its condensation point, it will liquefy. Therefore, at low temperatures or high pressures, the effect of the attractive forces becomes larger. However, if the gas particles are moving fast enough, attractive forces between the molecules that cause liquidation are not a factor. Gas molecules also have a definite volume, small though it may be, the volume of the molecules play a factor in both large and small container volumes.
 * __Ideal Gases vs. Real Gases__**

The ideal gas law is a very clean, efficient way to deal with gases. Unfortunately, the two basic assumptions of the ideal gas law, that molecules are point masses and that they do not attract, are ideals. In reality, every molecule has a volume and attracts other molecules, to some extent. At low pressure and high temperature, this effect is negligible. As the pressure rises and temperature drops, the behavior of real gases strays from the ideal. At extremes of pressure and temperature, the attractive forces and proximity may even force the gas into a liquid.

When the attractions between its particles are significant, the measured pressure of a real gas is less than the pressure predicted by the ideal gas equation.

Preal = measured pressure Pideal = pressure calculated from the ideal gas equation and measured n, T, and V Preal < Pideal




 * __Van der Waals Forces__**

Joseph Van der Waals studied the behavior of real gases and made comparisons to the ideal gas law. He derived an equation to account for the differences. The equation adds in two constants, a and b, to the ideal gas law. These constants are derived to give the best agreement between the observed behavior and the equation. Therefore, each gas has its own values for the constants.

The Van der Waals equation is stated as:
 * P + (n²a)/V² (V-nb) = nRT**

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