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 of a temperature `(-273^@C)`:
`S_("lim" T to 0)=0`
This law.imposes a limitation on the value of entropy.
`S_T=underset(0)overset(T)int C_(p)/T dT`
Thus entropy (S) of a substance at any temperature is calculated if the temperature dependence of `.C_p.` is known in evaluating absolute entropy of any substance.
Importance of this law: One of the most important applications of third law is the calculation of entropy changes in chemical reactions.