Text: "Long-range intermolecular forces are often called Van der Waals forces because they lead to the Van der Waals equation of state. In particular, the constant a in that equation is a measure of their strength. For unexcited, non-polar molecules the forces are attractive. These forces give rise to a large variety of physical and chemical phenomena, such as surface tension, friction, changes of phase, adhesion and cohesion of liquids and solids, viscosity, diffusion, the Joule-Thomson effect, and the departures of gases from the ideal gas law (virtual coefficients). Van der Waal forces arise from several different physical causes."

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