A system that is not undergoing any change possesses a unique condition defined by its measurable properties, such as temperature, pressure, and volume. This specific condition is known as the state of the system. A complete set of these properties fully describes the state. If the value of any single property changes, the system shifts to a new, distinct state.
Thermodynamics primarily studies systems in equilibrium states. Equilibrium means a state of balance where no internal potentials or “driving forces” exist to cause change. When isolated from its surroundings, a system in equilibrium remains unchanged.
For a system to be in full thermodynamic equilibrium, several specific balance conditions must be met simultaneously:
- Thermal Equilibrium: The temperature is uniform throughout the entire system. There are no temperature differences to drive heat flow.
- Mechanical Equilibrium: There is no unbalanced change in pressure within the system over time. Pressure can vary with height due to gravity (e.g., a fluid is under greater pressure at deeper layers), but this variation is balanced by the fluid’s weight and is typically negligible in thermodynamic analysis.
- Phase Equilibrium: If the system contains multiple phases (like liquid and vapor), the mass distribution between these phases remains constant.
- Chemical Equilibrium: The chemical composition of the system is stable, meaning no net chemical reactions are occurring.
A system is only in a true thermodynamic equilibrium state when all of the above criteria are satisfied.