Imagine the system Table-drop of water on it and air in a closed room, all at the same temperature T. (0°C<T<100°C). After some time the water drop will evaporate. Shouldn't it make the room pressure increase? If so then I can imagine a system where there's no heat input and the system can do work. (violating the first law of thermodynamics) So I conclude that as it's impossible, the pressure in the room cannot increase. However I don't understand how/why the pressure of the room does not increase. There are more molecules in the air in an almost same volume. Almost because the water drop is not there anymore. Is it because of the water drop disappearing that the room pressure remains constant all along the process? If so, how can I calculate this? (yeah, it does not convince me at all!) Thank you very much.