The van der Waals equation, named for its originator, the Dutch physicist Johannes Diderik van der Waals, is an equation of state that extends the ideal gas law to include the non-zero size of gas molecules and the interactions between them (both of which depend on the specific substance). As a result the equation is able to model the phase change, liquid ↔ ↔ vapor. It also produces simple analytic expressions for the properties of real substances that shed light on their behavior. One way to write this equation is: p = R T v − − b − − a v 2 where p is pressure, T is temperature, and v = V N A / N is molar volume, N A is the Avogadro constant, V is the volume, and N is the number of molecules (the ratio N / N A is a physical quantity with base unit mole in the SI). In addition R = N A k is the universal gas constant, k is the Boltzmann constant, and a and b are experimentally determinable, substance-specific constants.