# Example Problem with Complete Solution

6D-1 : Determine Whether Water Condensing is a Reversible Process 2 pts
A piston-and-cylinder device with a free-floating piston contains saturated water vapor at 225oC. Heat is lost from the water to the surroundings which are at 25oC.
Eventually, all of the water has condensed and the cylinder contains saturated liquid water at 225oC. Is this process internally reversible? Reversible? Irreversible? Explain your answer.

Read : What can be said about this case of heat transfer process for (a) isothermal cooling of the water and (b) contact between 225oC and 25oC surroundings?
Given: Tw 225 oC Find: Internally Reversible ? Reversible ?
Tsurr 25 oC Explain.
Diagram:
Assumptions: 1 - The system is the water in the piston-and-cylinder assembly.
2 - The system undergoes a constant-temperature process from saturated vapor to saturated liquid.
Equations / Data / Solve:
No equations are needed to resolve this problem.
There are no equations to solve.
Verify: None of the assumptions made can be verified with the given information.
Answers : Since the temperature is constant during the process, the pressure also remains constant. As shown by the
T-V Diagram, the process is a sequence of equilibrium states, and thus is internally reversible.
 The process is not reversible because there is a significant irreversibility in the surroundings.  The spontaneous heat transfer taking place between the water at 225oC and the surroundings at 25oC is irreversible.