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{{Thermodynamics|cTopic=[[List of thermodynamic properties|System properties]]}}
In [[thermodynamics]], '''entropy''' is a numerical quantity that shows that many physical processes can go in only one direction in time. For example, you can pour cream into coffee and mix it, but you cannot "unmix" it; you can burn a piece of wood, but you cannot "unburn" it. The word 'entropy' has entered popular usage to refer a lack of order or predictability, or of a gradual decline into disorder.<ref name="lexico">{{cite web |title=Definition of entropy in English |url=https://www.lexico.com/en/definition/entropy |archive-url=https://web.archive.org/web/20190711005908/https://www.lexico.com/en/definition/entropy |url-status=dead |archive-date=July 11, 2019 |website=Lexico Powered By Oxford |access-date=18 November 2020}}</ref> A more physical interpretation of thermodynamic entropy refers to spread of energy or matter, or to extent and diversity of microscopic motion.
If you reversed a movie of coffee being mixed or wood being burned, you would see things that are impossible in the real world. Another way of saying that those reverse processes are impossible is to say that mixing coffee and burning wood are "irreversible". Irreversibility is described by an important law of nature known as the [[second law of thermodynamics]], which says that in an isolated system (a system not connected to any other system) which is undergoing change, entropy increases over time.<ref>Theoretically, coffee can be "unmixed" and wood can be "unburned", but for this you would need a "machine" that would generate more entropy than was lost in the original process. This is why the second law only holds for isolated system which means they cannot be connected to some external "machine".</ref>
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