State the third law of thermodynamics.

Explanation

Third law of Thermodynamics : The entropy of a pure and perfectly crystalline substance is zero at the absolute zero temperature `(-273^(@)C)`.
`S_(lim T to 0)=0`
Third law of thermodynamics is also known as Nernst heat theorem.
Third law of thermodynamics imposes a limitation on the value of entropy.
`S_(T)=underset(0)overset(T)int(C_(P))/(T)dt`
Third law of thermodynamics is useful for calculating the entropy (S) of a substance at any temperature if temperature dependence `C_(p)` is known in evaluating the absolute value of entropy.
Basing on the third law of thermodynamics,
standard molar entropy of a substance can be calculated at any specified temperature. `C_(P)` cannot be measured at absolute zero `(-273^@C)` or around absolute zero. The heat capacity at constant volume `C_(v)`is measured at various temperatures upto as low temperatures as possible . `C_(V)` value at absolute zero is obtained and the Debye equation.
`C_(V)=aT^(3)` (a is constant for a substance)
Near to absolute zero `C_(p)-C_(v)` is negligible . So `C_(p)=C_(V)`. Hence absolute entropy `S^(@)` can be calculated using `C_(V)` value.