irreversibility

thermodynamics

Also known as: irreversible process

Written by

The Information Architects of Encyclopaedia Britannica

Last updated

June 29, 2026

Quick Summary

In thermodynamics, irreversibility refers to processes that cannot be perfectly reversed to restore the system and its surroundings to their original states without leaving any net effect. This concept is closely tied to the second law of thermodynamics, which states that in any spontaneous process, the total entropy of an isolated system will either increase or remain constant in the limit of a reversible process.123

Real-world processes, such as friction causing a pendulum to lose energy, are irreversible because some energy is always dissipated and cannot be perfectly recovered. This increase in entropy signifies a loss of the system's ability to perform useful work and represents a move towards greater disorder or randomness.13

Summary created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

3 Britannica Sources

Ask Britannica AI about this topic

Britannica doesn't have a full article on this topic, so we've curated a list of our relevant coverage.

colloid
…into two systems, reversible and irreversible. In a reversible system the products of a physical or chemical reaction may be induced to interact so as to reproduce the original components. In a system of this kind, the colloidal material may have a high molecular weight, with single molecules of colloidal…
Read More
…into two systems, reversible and irreversible. In a reversible system the products of a physical or chemical reaction may be induced to interact so as to reproduce the original components. In a system of this kind, the colloidal material may have a high molecular weight, with single molecules of colloidal…
Read More

entropy
Such a process is called irreversible because no slight change will cause the melted water to turn back into ice while the stove grows hotter. In contrast, a block of ice placed in an ice-water bath will either thaw a little more or freeze a little more, depending on whether…
Read More
Such a process is called irreversible because no slight change will cause the melted water to turn back into ice while the stove grows hotter. In contrast, a block of ice placed in an ice-water bath will either thaw a little more or freeze a little more, depending on whether…
Read More

reversibility
…of a real pendulum is irreversible because a small amount of the mechanical energy of the pendulum would be expended in performing work against frictional forces, and restoration of the pendulum to its exact starting position would require the supply of an equivalent amount of energy from a second system,…
Read More
…of a real pendulum is irreversible because a small amount of the mechanical energy of the pendulum would be expended in performing work against frictional forces, and restoration of the pendulum to its exact starting position would require the supply of an equivalent amount of energy from a second system,…
Read More

thermodynamics
Thermodynamic equilibrium
The first is irreversible (the balloon bursts), and the second is reversible. The concept of reversible processes is something like motion without friction in mechanics. It represents an idealized limiting case that is very useful in discussing the properties of real systems. Many of the results of thermodynamics…
Read More
The first is irreversible (the balloon bursts), and the second is reversible. The concept of reversible processes is something like motion without friction in mechanics. It represents an idealized limiting case that is very useful in discussing the properties of real systems. Many of the results of thermodynamics…
Read More