A new simple model is introduced for a liquid and an equation of state is derived
based on this model and the statistical mechanical calculations. This equation of
state works well for the non-polar and slightly polar liquids. The important
conclusion that may be deduced from this equation of state is that if the reduced
variables of state are chosen and defined appropriately, then, the principle of
corresponding states works well for liquids, and it may even be unnecessary to use
an additional parameter like the acentric factor