Irreversible process
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An irreversible process is a process for which it is impossible to return both the system and its surroundings to its original state by reversing the process. For instance, a combustion engine transforms gasoline into engine exhaust and energy; however, no process can turn the engine exhaust and the energy transfered to the car cannot be transformed back into gasoline. Irreversible processes generally involve an increase in entropy.
Causes of irreversible processes include friction, unrestrained fluid expansion, heat transfer, and mixing.