The results of Nernst heat theorem let to generalized and called third law of thermodynamics.
The third law of thermodynamics may be stated as follows –
“Entropy of perfectly crystalline solid may be taken as zero at absolute zero temperature.”
In other words it may be stated as –
“Every substance has finite positive entropy but at absolute zero temperature the entropy may become zero and in fact it does become zero in case of perfectly crystalline solids.”
Like first and second law of thermodynamics, the third law is also based upon experiences and there is no direct proof for this theorem.
Importance of third law-
Third law of thermodynamics is used in evaluation of absolute entropies, the importance of third law lies in fact that it helps in calculation of absolute entropy of elements or chemical composition at any desired temperature.