The Behavior of Gases and Liquids

The principal goal of physical chemistry is to understand the properties and behavior of material systems and to apply this understanding in useful ways. 

The state of a system is specified by giving the values of a certain number of independent variables (state variables). 

In an equilibrium one-phase fluid system of one substance, three macroscopic variables such as temperature, volume, and amount of substance can be independent variables and can be used to specify the macroscopic equilibrium state of the system. At least one of the variables used to specify the state of the system must be proportional to the size of the system (be extensive). Other macroscopic variables are mathematical functions of the independent variables. 

The intensive state, which includes only intensive variables (variables that are independent of the size of the system), is specified by only two variables in the case of an equiUbrium one-phase fluid system of one substance. 

Nonideal gases and Uquids are described mathematically by various equations of state. 

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In fluid mechanics the principles of conservation of mass, conservation of momentum, the first and second laws of thermodynamics, and empirically developed correlations are used to predict the behavior of gases and liquids at rest or in motion. The field is generally divided into fluid statics and fluid dynamics and further subdivided on the basis of compressibility. Liquids can usually be considered as incompressible, while gases are usually assumed to be compressible. 

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