equilibrium

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equilibrium

thermodynamics

Also known as: equilibrium state

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June 26, 2026

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In thermodynamics, equilibrium is a state where a system's properties do not change over time and require external effects to alter. A system in thermodynamic equilibrium is in mechanical, chemical, and thermal balance, meaning there's no tendency for spontaneous change.12

For a system with a given energy, equilibrium maximizes entropy. If pressure and temperature are constant, the Gibbs free energy is minimized at equilibrium. In simpler terms, equilibrium means no acceleration and a stable internal energy state. For a rigid body, this also means no net torque.13

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thermodynamics
Thermodynamic equilibrium
A particularly important concept is thermodynamic equilibrium, in which there is no tendency for the state of a system to change spontaneously. For example, the gas in a cylinder with a movable piston will be at equilibrium if the temperature and pressure inside are…
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A particularly important concept is thermodynamic equilibrium, in which there is no tendency for the state of a system to change spontaneously. For example, the gas in a cylinder with a movable piston will be at equilibrium if the temperature and pressure inside are…
Read More

liquid
Solutions of electrolytes
The equilibrium properties of electrolyte solutions can be studied experimentally by electrochemical measurements, freezing-point depressions, solubility determinations, osmotic pressures, or measurements of vapour pressure. Most electrolytes, such as salts, are nonvolatile at ordinary temperature, and, in that event, the vapour pressure exerted by the solution is…
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The equilibrium properties of electrolyte solutions can be studied experimentally by electrochemical measurements, freezing-point depressions, solubility determinations, osmotic pressures, or measurements of vapour pressure. Most electrolytes, such as salts, are nonvolatile at ordinary temperature, and, in that event, the vapour pressure exerted by the solution is…
Read More

gas
Behaviour and properties
…is useful to separate the equilibrium properties and the nonequilibrium transport properties. By definition, a system in equilibrium can undergo no net change unless some external action is performed on it (e.g., pushing in a piston or adding heat). Its behaviour is steady with time, and no changes appear to…
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…is useful to separate the equilibrium properties and the nonequilibrium transport properties. By definition, a system in equilibrium can undergo no net change unless some external action is performed on it (e.g., pushing in a piston or adding heat). Its behaviour is steady with time, and no changes appear to…
Read More

liquid
Phase diagram of a pure substance
…pure substance are fixed, the equilibrium state of the substance is also fixed. This is illustrated in Figure 1, which shows the phase diagram for pure argon. In the diagram a single phase is shown as an area, two as a line, and three as the intersection of the lines…
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…pure substance are fixed, the equilibrium state of the substance is also fixed. This is illustrated in Figure 1, which shows the phase diagram for pure argon. In the diagram a single phase is shown as an area, two as a line, and three as the intersection of the lines…
Read More