Definition of Third Law of Thermodynamics

The third law of thermodynamics states that as the temperature approaches absolute zero (0 K), the entropy of a system approaches a constant (and minimum) value. The entropy of a perfect crystalline state is zero at 0 K. In this way, the third law provides an absolute reference point for the determination of entropy of any substance. The third law also says that it is not possible for any system to reach absolute zero in a finite number of steps. This effectively makes it impossible to ever attain a temperature of exactly 0 K.