• Entropy is defined as a thermodynamic parameter representing the state of disorder of a system at the atomic, ionic, or molecular level.
  • Entropy is a thermodynamic property which serves as a measure of how close a system is to equilibrium.
  • Entropy is a measure of disorder in a system; the higher the entropy the greater the disorder. In the context of entropy, "perfect internal disorder" is synonymous with "equilibrium".
  • Entropy is a measure of the unavailability of a system’s energy to do work; Thus, thermodynamic entropy is a measure of the amount of energy in a physical system that cannot be used to do work.
  • Entropy is a measure of the dispersal of energy; how much energy is spread out in a process, or how widely spread out it becomes, at a specific temperature.
  • Entropy is the capacity factor for thermal energy that is hidden with respect to temperature.
  • Entropy is a measure of disorder in the universe.
  • Entropy is the tendency of a system, that is left to itself, to descend into chaos.

Second Law of Thermodynamics

According to the second law of thermodynamics the entropy of an isolated system never decreases. An isolated system will spontaneously evolve toward thermodynamic equilibrium, the configuration with maximum entropy.
Systems that are not isolated may decrease in entropy, provided they increase the entropy of their environment by at least that same amount.
Since entropy is a state function, the change in the entropy of a system is the same for any process that goes from a given initial state to a given final state, whether the process is reversible or irreversible.


The idea of "irreversibility" is central to the understanding of entropy. Most people have an intuitive understanding of irreversibility (a dissipative process): if one watches a movie of everyday life running forward and in reverse, it is easy to distinguish between the two. The movie running in reverse shows impossible things happening: water jumping out of a glass into a pitcher above it, smoke going down a chimney, water "unmelting" to form ice in a warm room, crashed cars reassembling themselves, and so on.
The intuitive meaning of expressions such as "you can't unscramble an egg", "don't cry over spilled milk" or "you can't take the cream out of the coffee" is that these are irreversible processes. There is a direction in time by which spilled milk does not go back into the glass (see: The arrow of time).
In thermodynamics, one says that the "forward" processes – pouring water from a pitcher, smoke going up a chimney, etc. – are "irreversible": they cannot happen in reverse, even though, on a microscopic level, no laws of physics would be violated if they did. This reflects the time-asymmetry of entropy.
All real physical processes involving systems in everyday life, with many atoms or molecules, are irreversible. For an irreversible process in an isolated system, the thermodynamic state variable known as entropy is always increasing.
The reason that the movie in reverse is so easily recognized is because it shows processes for which entropy is decreasing, which is physically impossible.

Entropy as energy dispersal

Entropy can also be described in terms of "energy dispersal" and the "spreading of energy", while avoiding all mention of "disorder", "randomness" and "chaos". In this approach, the second law of thermodynamics is introduced as: "Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so."

This explanation can be used in the context of common experiences such as a rock falling, a hot frying pan cooling down, iron rusting, air leaving a punctured tyre and ice melting in a warm room. Entropy is then depicted as a sophisticated kind of "before and after" yardstick: Measuring how much energy is spread out over time as a result of a process such as heating a system, or how widely spread out the energy is after something happens in comparison with its previous state, in a process such as gas expansion or fluids mixing (at a constant temperature).

The equations are explored with reference to the common experiences, with emphasis that in chemistry the energy that entropy measures as dispersing is the internal energy of molecules.

Chemical reactions

The second law of thermodynamics, states that a closed system has entropy which may increase or otherwise remain constant. Chemical reactions cause changes in entropy and entropy plays an important role in determining in which direction a chemical reaction spontaneously proceeds.

Systems ecology and Negentropy

Nowadays, many biologists use the term 'entropy of an organism', or its antonym 'negentropy', as a measure of the structural order within an organism.

Historical frame

The term entropy was coined in 1865 by the German physicist Rudolf Clausius, who stated that: “The entropy of the universe tends to a maximum.”.


Unlike many other functions of state, entropy cannot be directly observed but must be calculated. Entropy can be calculated for a substance as the standard molar entropy from absolute zero temperature (also known as absolute entropy).
Entropy has the dimension of energy divided by temperature, which has a unit of joules per kelvin (J/K) in the International System of Units.
While these are the same units as heat capacity, the two concepts are distinct. Entropy is not a conserved quantity: for example, in an isolated system with non-uniform temperature, heat might irreversibly flow and the temperature become more uniform such that entropy increases.

The arrow of time

Entropy is the only quantity in the physical sciences that seems to imply a particular direction of progress, sometimes called an arrow of time. As time progresses, the second law of thermodynamics states that the entropy of an isolated system never decreases (but rather will increase). Hence, from this perspective, entropy measurement is thought of as a kind of clock (an isolated system has low entopy in the past, and high entropy in the future).
The Second Law of Thermodynamics allows for the entropy to remain the same regardless of the direction of time. If the entropy is constant in either direction of time, there would be no preferred direction. However, the entropy can only be a constant if the system is in the highest possible state of disorder, such as a gas that always was (and always will be) uniformly spread out in its container.
The existence of a thermodynamic arrow of time implies that the system is highly ordered (i.e. low entropy) in one time direction only, which would by definition be the "past". Thus this law is about the boundary conditions rather than the equations of motion of our world.

Linguistic derivation

The term Entropy is derived from the Ancient Greek word entropía (ἐντροπία) meaning “a turning towards”.
This is a combination of the prefix en- (ἐν) meaning "in", and the word tropḗ (τροπή) meaning "a turning", in analogy with energy.

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