# 2 Major Application of Third law of thermodynamics (Easy)

There are two major applications of Third law of thermodynamics, which are mentioned below.

1. Using the third law of thermodynamics, we can find whether the substance is pure crystalline or not?
2. We can find the absolute entropy of any substance at a given temperature.

### Explanation:

#1 How can we find whether the substance is pure crystalline or not, using the third law of thermodynamics?

It is not always true that the entropy of all the substances will be zero at absolute zero temperature.

Let me explain this to you.

Third law of thermodynamics is all about perfectly crystalline substances.

It states that the entropy of a perfectly Crystalline substance will be zero at 0 Kelvin temperature.

That means if the substance is not perfectly crystalline, then its entropy will not be zero at 0 Kelvin temperature.

And such substances are not pure crystalline substances.

These substances will have some imperfections in their crystal structure.

These substances will show some disorder or randomness.

In this way, we can find whether the substances are pure crystalline or not.

#2 How can we find the absolute entropy of any substance at a given temperature using 3rd law of thermodynamics?

Once you have studied the third law, you can easily find the absolute entropy of any substance at a given temperature.

You just have to compare the entropy of a given substance at T temperature with the entropy of that substance at zero Kelvin temperature.

We have to calculate the change in entropy between these temperatures.

ST – S0 = ∆S……………………….(1)

Now, we know that at 0 K temperature, the entropy will be zero. (So, S0 =0).

We can easily find the change in entropy (∆S ), using the formula.

ΔS = ST – S0 = 0TCpdT/T

Now, after getting this entropy change (∆S), we can easily get the entropy at a given temperature (ST) from the above equation (1).

I hope you have now understood the applications of Third law of thermodynamics.

For more detailed information on Third law of thermodynamics, check the ultimate guide on Third law of thermodynamics.

(This guide has all the important knowledge about Third law of thermodynamics along with examples and lots more. You will love this guide for sure.)

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