A thermodynamic property is a measurable or computable characteristic of a system, such as pressure, volume and temperature. Two systems with all thermodynamic properties being quantitatively equivalent are said to be in the same thermodynamic state. The state of a system is therefore defined by the values of the system’s thermodynamic properties.
A system is in a state of thermodynamic equilibrium when there is no net transfer of energy or matter with its surroundings, resulting in its thermodynamic properties being constant over time.
We are mainly concerned with systems in thermodynamic equilibrium in our discussion of chemical thermodynamics. This is because the properties of the system is only well defined and no longer time-dependent when the system is at equilibrium. For example, when we subject a system to a change (e.g. by increasing the temperature of the system), we are interested in analysing the initial state of the system before the change when the system is in a state of thermodynamic equilibrium at an initial temperature, and the final state of the system a certain time after the change when all regions of the system have the same final temperature (i.e. the system is again in a state of thermodynamic equilibrium).
Finally, the path taken by the system from its initial state to its final state when a change is initiated is known as a thermodynamic process. A process can also be viewed as an integrated sequence of thermodynamic states at equilibrium, starting with an initial state and ending with a final state.
Question
Consider two systems that are initially at different temperatures. They are brought in contact via an immovable thermally conducting boundary. Do the two systems necessarily have the same state when they are in thermal equilibrium?
Answer
No. Even though the two systems have the same temperature over time, they may have different volumes and pressures. Hence, two systems at thermal equilibrium may not be in the same thermodynamic state.