Third Law Of Thermodynamics

The third law of thermodynamics states that the entropy of a perfect crystal is zero at a zero Kelvin temperature. (Zero Kelvin temperature is also called absolute zero).

Explanation: 

As the temperature of any system decreases, the atom and molecules in the system lose their energy and approach their lowest energy points, by doing so atom & molecule’s kinetic energy decreases and hence their random motion.

Since Entropy is the measurement of the randomness of any closed system. So, when absolute zero temperature is achieved, the atom and molecules of the crystal have the lowest energy and hence no randomness in the particles. So, zero entropy of the system was observed.

Mathematical Application:

Let for any solid S⁰₀ is entropy at zero kelvin and S is entropy at T kelvin.

So, ∆S = S – S₀

Also, we know 

Since S₀ = 0 at by third law.

So, 

By this we can calculate the absolute entropy of any solid at temperature T.