One mole of an ideal gas is taken from state 1 to state 2. Which statements are correct?

AIn an adiabatic free expansion into vacuum in an insulated container, $Δ S_{g a s} > 0$ and $Δ S_{s u rr} = 0$ .
BIn a reversible adiabatic expansion, $Δ S_{g a s} = 0$ .
CFor an isothermal expansion against a constant external pressure, $Δ S_{g a s}$ equals that of the reversible isothermal path between the same states.
DIn a reversible isothermal expansion, $Δ S_{u ni v erse} > 0$ .

Answer: A, B, C

(A) Volume increases at constant

T

Δ S_{g a s} = R ln (V_{2} / V_{1}) > 0

; no heat reaches the surroundings: true. (B)

q_{re v} = 0

Δ S = 0

: true. (C)

S

is a state function: true. (D) A reversible process has

Δ S_{u ni v erse} = 0

: false.