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## Homework Statement

I know that the equation ΔS

_{system}= Q/T is only valid for a reversible process but whenever i see problems involving a irreversible isothermal expansion of ideal gas, the ΔS

_{surroundings}is taken as -Q

_{irr}/T. Why is that equation valid for surroundings, is it because the surroundings being a reservoir/sink, any heat exchange is done reversibly?

## Homework Equations

ΔS

_{system}= Q/T