Are all spontaneous reactions irreversible?

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Vadiraj K T

yes, example is melting of ice into water.

Raghu Ram Achar

cant it be reversed?

Yes but only with the expenditure of energy, which is not spontaneous. as in II law of TD the entropy denotes the degree of disorderness. where as the spontaneous reaction are always leads to disorderness.

Shufeng Zhang

Entropy: A concept that is not a physical quantity

https://www.researchgate.net/publication/230554936_Entropy_A_concept_that_is_not_a_physical_quantity