# what would happen to water in case of instantaneous pressure reduction to 0? and can we say this process is free expansion?

so here's a thought experiment let's say that I have water in a cylinder and above it a massless, frictionless piston. if I were to pull the piston so that the pressure would go from p1 to 0, instantly.

what would happen to the water? what would the final phase be? what will the phase transition be like? and is it accurate to say that this process is a free expansion of a liquid?

edit: originally, the problem I was reading about was water hammering which is a sudden reduction/increase in pressure due sudden shutting of a valve. which got me to think about what happens when the water pressure is suddenly reduced to zero. so I know that the initial state is p1 and t1, and the final phase depends on thermodynamic state which is p2=0 and t2. the phase transition depends on the thermodynamic process. so I assumed different scenarios,

1)closed system/isothermal process,

2) isolated system/isentropic process,

3) and closed system/sudden pressure reduction(free expansion) according to the phase diagram, if we assume that we start at standard pressure and temperature http://www1.lsbu.ac.uk/water/water_phase_diagram.html

1) the water boils, the final phase is vapor.

2) the isentropic process causes temperature drop, and depending on t2 the final state would be vapor or crystals. the phase transition could be liquid -> vapor -> liquid -> solid, liquid -> solid, or liquid -> vapor -> solid.

3) the last scenario is water free expansion, in this scenario the water would behave as 2) at first, then the surround would heat it, so the final phase will always be vapor.

the problems are: a) I am not sure if that's what really happens because I couldn't find any references or experiments

b) I am not sure if the term "free expansion" applies to liquids and was wondering if there is an accurate descrition for this process

• The usual convention is for you tell other users what your thoughts are, no offence but you must have thought through the process, otherwise it's a bit "homeworky", sorry :) – user140606 Jan 19 '17 at 20:00
• I don't see how could that possibly be a homework but anyway, I'll edit the post to add my thoughts. – Kareem Jan 19 '17 at 20:42
• Hi Kareem, thanks very much for that, the policy is for homework type questions, not just homework. If you saw the number of questions this site receives regarding questions very similar to your original post, you might not be as surprised at my caution. No offence meant, the best of luck getting an answer. In partial apology, I have upvoted your post and if there is a phase transition tag, you could add that to TD to attract more users. – user140606 Jan 19 '17 at 21:36
• thermodynamics (i.e. the water phase diagram) is used for mixture in equilibrium state. The process you described is highly dynamic. However, if you pull the piston slowly, the liquid will vaporize when the pressure reaches the vapor pressure at that temperature. If you can keep the temperature but keep pulling the piston, more vapor will form until the whole container is full of vapor. Then it will follow the gas equation of state. – user115350 Jan 19 '17 at 21:51
• You may enjoy the XCKD WhatIf: Glass Half Empty. It explores the kind of situation you are looking at, and does so with Munroe's witty humor! – Cort Ammon Jan 19 '17 at 23:54