Entropy: the definition and its units
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
Entropy is a measure of the disorder of a system, that is, the number of ways in which the particles and energy can be arranged. Its units are J per K per mole.
Words that must appear
- Disorder
- Particles and energy
The sign
The entropy change is positive when disorder increases: solid to liquid to gas, or an increase in the number of moles of gas. It is negative when the number of moles of gas decreases.
Do not confuse with
Enthalpy, which is heat transferred. Entropy is disorder.
Common questions
What is entropy?
A measure of the disorder of a system, the number of ways the particles and energy can be arranged.
What are the units of entropy?
J per K per mole.
How do you predict the sign of the entropy change?
Positive if disorder increases, for example more moles of gas. Negative if it decreases.
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