A 1.5 cubic meter, stationary vessel initially contains one metric ton of pure liquid water at temperature T1. The top of the vessel is open to the atmosphere such that water can evaporate. At the end of a length of time, the vessel contains 750 kg of pure water at temperature T1. The pressure at the surface of the water is 1 atm throughout. You define “the liquid water in the vessel” as the system. Which of the following statements is true?

Question

A. This is not a closed system because water leaves, but it is a steady state process because the temperature and pressure of the system are unchanged.
B. The accumulation term of the energy balance can be eliminated because a stationary system can have no accumulation of kinetic or potential energy, and the internal energy has to be the same at the beginning and end, since T and P are the same.
C. The energy balance should include a Q term, since there is no given information that allows us to say that heat transfer is 0 or negligible.
D. The WEC term of the energy balance can be eliminated because this vessel has a constant volume.
E. None of the above are true.

Answer 1

A. Incorrect. The amount (mass) of the water is changing with time, so the system is not at steady state.
B. Incorrect. Some of your system (the water) evaporates, which changes the internal energy. The specific internal energy is the same but it is multiplied by a different M at the end than the beginning.
C. Correct. Although the temperature is the same, a partial phase change occurs, requiring energy. We include the Q term because heat may have been supplied to cause the evaporation.
D. Incorrect. Remember that expansion and contraction refers to the SYSTEM. In this case, the system is “the water,” which is not rigid.
E. Incorrect. One of them is appropriate.