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:
F = C – Ø + 2
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 pure liquid water we have
C = 1
Ø = 1
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.