**Thermodynamic state**

A system will be called in a particular thermodynamic state if it satisfies both the below mentioned criteria.

- All the thermodynamic properties of the system are uniform throughout the system
- All the thermodynamic properties of the system remain constant with respect to the time (at least for that moment)

*Note*: all the properties of the system are called state variables

But we all know there is a very large number of state variables

Here arises a question

If we have to fix all the properties of a system to define a thermodynamic state?

The answer is **NO**

We have to define only a minimum number of properties to define a system.

Those minimum number of properties are obtained from *Gibbs phase rule*.

*Gibbs’ Phase rule:*

*Gibbs’ Phase rule:*

F = C – Ø + 2

*Where*:

F = Number of independent intensive properties required to define the system

C = Number of components in the system

Ø = Number of phases of the system

*For example:*

For pure liquid water we have

C = 1

Ø = 1

*Which gives*

F = 2

Which implies that we need only two properties to define the state of water.

*For more read two property rule for pure substances*

If a thermodynamic state of a system remains invariable with respect to the time then we say that the system is in thermodynamic equilibrium.

These are ways of achieving thermodynamic equilibrium.

- When a system is isolated from its surrounding
- When both system and surrounding are in same thermodynamic state. In this case system is defined as dead system.

Note: if a system is in thermodynamic equilibrium then we can say that there is no thermodynamic process is happening within the system.

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