# What is a State ?

## Gibbs Phase Rule

oF = C - P + 2
Initial State
At a given state all the properties of a system are fixed and can be measured or calculated.
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### Ch 3, Lesson D, Page 1 - What is a State ?

• I hope you recall from chapter 1 that in any given state, all of the properties of the matter inside the system boundary are fixed.
• In the previous lesson, we introduced the Gibbs Phase Rule.
• The Gibbs Phase Rule allows us to determine exactly how many intensive properties we must specify in order to fix, or completely specify the state of a system.
• In order to use Gibbs Phase Rule, we must know what chemical species are in the system and what phases exist in the system as well.
• Once we have specified a number of intensive properties equal to the number of degrees of freedom, then ALL OTHER intensive properties of the system can be determined.
• If we have pure liquid water in our system, then we have 1 – 1 + 2 = 2 oFreedom.
• We can specify 2 intensive variables.  We usually specify T & P because they are useful and easy to measure.  But we might just as well have specified the density and the specific enthalpy because these are also intensive properties.