Entropy of a system decreases .........

One mole of an ideal gas at $300 \ K$ in thermal contact with surroundings expands isothermally from $1.0 \ L$ to $2.0 \ L$ against a constant external pressure of $3.0 \ atm$. In this process,the change in entropy of the surroundings $(\Delta S_{\text{surr}})$ in $J \ K^{-1}$ is $(1 \ L \ atm = 101.3 \ J)$.

Why can't the efficiency of a heat engine ever be $100 \%$?

$A$ solid body of constant heat capacity $1 \ J/^oC$ is being heated by keeping it in contact with reservoirs in two ways :
$(i)$ Sequentially keeping in contact with $2$ reservoirs such that each reservoir supplies same amount of heat.
$(ii)$ Sequentially keeping in contact with $8$ reservoirs such that each reservoir supplies same amount of heat.
In both the cases,the body is brought from initial temperature $100^oC$ to final temperature $200^oC$. The entropy change of the body in the two cases respectively is :

$A$ hot piece of copper at $100^{\circ} C$ is dropped into a pond at $30^{\circ} C$. The copper cools down to $30^{\circ} C$,while the pond remains at its initial temperature due to its large size. Then,

The fundamental limitation to the coefficient of performance of a refrigerator is given by