Introduction (Thermodynamics)
The state (or behaviour) of a system is described by variables or properties which may be classified as: (a) extensive properties such as mass, volume, kinetic energy; and (b) intensive properties which are independent of system size, e.g., pressure, temperature, concentration.
An extensive property can be treated like an intensive property by specifying that it refers to a unit amount of the substance concerned. Thus, mass and volume are extensive properties, but density, which is mass per unit volume, and specific volume, which is volume per unit mass, are intensive properties. In a similar way, specific heat is an intensive property, whereas heat capacity is an extensive property.
The variables whose values determine the thermodynamic state of a system are called its thermodynamic coordinates or its state variables. Pressure and temperature are examples of state variables. When the thermodynamic coordinates of a system change, the system is said to undergo a process. There are two types of processes: reversible and irreversible.
An appreciation of these processes can readily be obtained by drawing reference to any two thermodynamic states of a system. Let a particular path between the two states be considered.
If this path is such that in a process all the changes that take place in any part of the system or its surroundings are exactly reversed when the process is carried out in the reverse direction, then the process is said to be reversible. In a reversible path, when a given process has been carried out and then reversed, both the system and its surroundings come back exactly to their original states and no net change occurs in any part of the system or the surroundings. On the other hand, a process which involves the spontaneous transition of a system from a nonequilibrium state to an equilibrium state is called an irreversible process.
An irreversible process is also called a spontaneous or a natural process. Such processes always leave a net change in the system and the surroundings after the system has been taken from one state to another and then brought back to the original state. The mixing of gases, the passage of heat from a hot body to a cold one etc. are examples of spontaneous processes.