In thermodynamics, a system is a spatial domain bounded for the purpose of describing a problem; while the surroundings are the entire spatial domain outside of the system. The communication between them is established through the boundaries of the system. The system and the surroundings make up the universe.
The system and the surroundings interact with each other. As we discussed above, one type of thermodynamic problem is that of predicting changes in a system due to interactions with its surroundings. A system is open if it can exchange mass with the surroundings, and closed if it does not exchange mass with the surroundings. A system is adiabatic if it does not exchange heat energy with the surroundings. We called a system isolated if there is neither heat nor mass crossing its boundaries.
Thermodynamic properties can be divided into two general classes: intensive and extensive properties. An intensive property is one whose value is independent of the size, extent, or mass of the system, and includes pressure, temperature, and density. By contrast, the value of an extensive property changes directly with the mass. Mass and volume are examples of extensive properties. Extensive properties per unit mass, such as specific volume, are intensive properties.
A system is homogeneous if it is has uniform properties throughout, i.e. a property such as density has the same value from point to point in a macroscopic sense. A phase is defined as a quantity of matter that is homogeneous throughout. Hence, a homogeneous system is, essentially, a single-phase system. A heterogeneous system is one with non-uniform properties, and hence, is made up of phases which can be distinguished from one another by the presence of interfaces.
Thestate is the thermodynamic coordinate of the system, specified by a number of intensive variables. The degree of freedom is the number of intensive variables needed to define the state of the system. State functions are those whose changes depend on their end states only and are independent of the path between them.
A process is the series of successive, intermediate states that the system goes through in order to go from an initial to a final state. A process is isothermal or isobaric if the temperatures or pressures of all successive steps are the same, respectively. A reversible process is one for which the exchange of energy between the system and its surroundings takes place under vanishing gradients or driving forces; that is, the properties of the system and surroundings are balanced. In a reversible process, each step of the process can be “reversed” and the original states of the system and surroundings can be restored. Any process that does not take place under infinitesimal gradients is irreversible. Strictly speaking, a reversible process is an idealization.