Thermodynamics means the flow of heat and it deals with quantitative relationships existing between heat and other forms of energy in physico-chemical transformations.

We have already studied about the relationship between the heat and the work done in first law of thermodynamics, and about energy and work done in second law of thermodynamics.

Let us study about change in entropy in third law of thermodynamics where we will discuss the concept of entropy.

We have already studied about the relationship between the heat and the work done in first law of thermodynamics, and about energy and work done in second law of thermodynamics.

Let us study about change in entropy in third law of thermodynamics where we will discuss the concept of entropy.

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We know that entropy is the rate of change of disorder occurring in a system. **Walther Nernst** introduced the concept of entropy in the third law of thermodynamics which states that:

For a perfect crystal at the absolute zero temperature, the entropy would be exactly equal to zero.When only one minimum energy state is possessed by a perfect crystal the law would hold true.

If we consider systems such as glasses which are not perfect crystal then a generalized form of 3rd law would be:

When the temperature approaches zero, the randomness or entropy of a system would approach a constant value.If we consider systems such as glasses which are not perfect crystal then a generalized form of 3rd law would be:

The Constant value of entropy is called **Residual Entropy** and it should be noted that it is not necessarily zero.

The Third law can be expressed as:S = entropy which is expressed as J s

T = absolute temperature which is expressed in K.

It can also be written as:

Another application of third law of thermodynamics:

Here C

- Even at 0 K Glassy solids have entropy which is greater than zero.
- Solids that have mixtures of isotopes do not possess zero entropy at 0 K.

For example: Solid chlorine does not have zero entropy at 0 K. - Crystals of CO, N
_{2}O, NO, H_{2}O, etc., do not possess perfect order even at 0 K, thus their entropy is not equal to zero.

We can consider H

Water exists in three different states:

- Gaseous state
- Liquid state
- Solid state

In Gaseous state

The entropy or randomness is very high. Here we are talking about the randomness in motion of the molecules of which the water is made up of. They move with very high entropy.

In Liquid state

Now the randomness is reduced. It is not as free as the gaseous state and hence we can say that entropy of the molecules is reduced. This is because the movement between the molecules is reduced.

In Solid state

In this state the moment between molecules is almost zero. The entropy approaches almost zero value. This is because the molecules are packed very tightly in the solid state and hence the randomness is very low. This is when it is cooled at very low temperature or at an absolute zero temperature.

Now if it cooled further then all the motion between the molecules would stop. This is because these are no free spaces for the motion of the particles. And hence the entropy becomes almost zero.

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