The properties of the system which arise from the bulk behavior of matter are called macroscopic properties. For examples, pressure, temperature, surface tension, viscosity, density, refractive index, etc. In thermodynamics the macroscopic properties cab be sub-divided into two types:
- Extensive Properties
- Intensive Properties.
1) Extensive Properties:
The properties of the system whose value depend upon the amount of substance present in the system are called extensive properties. For examples, mass, volume, surface area, internal energy, enthalpy, entropy, free energy, heat capacity, etc.
2) Intensive Properties:
The properties of the system whose value is independent of the amount or size of the substance present in the system are called intensive properties. For examples, temperature, pressure, viscosity, surface tension, vapour pressure, refractive index, specific heat capacity, etc.
State of a System:
The state of a system means the condition of existence of the system when the macroscopic properties have definite values. If any of the macroscopic properties of the system changes, the state of the system is also said to change. Thus, the state of the system is fixed by its macroscopic properties.
State Variables:
The measurable properties required to describe the state of the system are called state variables. e.g. temperature, pressure, volume, composition, etc are the state variables. It may be noted that for describing a system, it is not necessary to specify the values of all state variables. For any system, a certain minimum number of variables is sufficient to define its state because the other variables became automatically fixed and have definite values. e.g. consider a system consisting of an ideal gas. The state of the system may be defined by only three variables such as temperature (T), pressure (P) and volume (V). The values of other variables such as amount of gas, density, etc will be definite and can be easily calculated.
State Functions:
A state function is a property of the system whose value depends only upon the state of the system and is independent of the path or manner by which the state is reached. The change in the value of these properties (state functions) depends only upon the initial and final states of the system and not on the path by the change from initial to final state is brought about. Some common state functions are pressure (P), volume (V), temperature (T), internal energy (E), enthalpy (H), entropy (S), etc.
Note:
A system in a state of thermodynamic equilibrium is one in which the macroscopic properties do not change with time. The state function gives the difference in the property of the initial state and final state.