Within thermodynamics, a physical property is any property that is measurable, and whose value describes a state of a physical system. Some constants, such as the ideal gas constant, R, do not describe the state of a system, and so are not properties. On the other hand, some so-called constants, such as K_f (the freezing point depression constant, or cryoscopic constant), depend on the identity of a substance, and so may be considered to describe the state of a system, and so may be considered physical properties.