The problem: A red-hot 2.00 kg piece of iron at temperature t1=880k is thrown into a huge lake whose temperature is t2=280K. Assume the lake is so large that its temperature rise is insignificant. The book says that this process is irreversible. Why? I have another question. In a reversible process, are the initial and final state variables (p,v,T,Eint,S) the same? Or is does that only true for a cycle? I'm really confused haha. If all the processes in a carnot cycle are reversible, how come the temperature changes when going from the isothermal to adiabatic process, even though the process is done very slowly? How come that process is not irreversible? Thanks.