**The third law of thermodynamics states that:**

“The entropy of all the perfect crystalline solids is zeros at absolute zero temperature”. The third law of thermodynamics is also referred to as Nernst law. It provides the basis for the calculation of absolute entropies of the substances.

Mathematically,

**Lim _{T –>0 } S = 0**

If the entropy is zero at temperature T = o, then this law states that the absolute entropy S_{ab} of a substance at temperature T and pressure P is expressed by the following expression:

S_{ab}T∫_{0} (δQ) _{rev} / T

**Importance of third law of thermodynamics is given below:**

1) It helps in calculating the thermodynamic properties.

2) It is helpful in measuring chemical affinity. Because of this it is known as Nernst theorem.

3) It explains the behavior of solids at very low temperature.

4) It helps in analyzing chemical and phase equilibrium.

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