Dalton's Law of partial pressure states that the total pressure of a gas mixture is equal to the sum of the partial pressures of the individual gases. The partial pressure of a gas is the pressure it would exert if it occupied the entire volume alone. Kinetic molecular theory explains gas behavior based on the assumption that gas particles are in continuous, random motion and exhibit elastic collisions. Real gases deviate from ideal behavior at high pressures and low temperatures due to intermolecular forces. The van der Waals equation accounts for these non-idealities.