I've had a bit of confusion on whether static equilibrium can exist in an open system.

My textbook states (on modelling static equilibrium through water in a flask):

"Static equilibrium is shown by flask X as it is open, and the liquid can evaporate and leave the flask. This process of the liquid turning into a gas and leaving the flask shows a static equilibrium because eventually the flask will be empty."

My chemistry teacher states:

"Static equilibrium is defined as when the forward and reverse rates of a process are zero, and concentrations of species stay constant. Since in an open system there will constantly be species entering and leaving the system, the concentrations of species will not stay constant and hence will not reach equilibrium."

Can someone please clarify?


1 Answer 1


The flask would be in equilibrium only after most of the water evaporates -- there would always be an exchange between air (assuming it has some humidity above zero) and the water adsorbed to the glass surface.

For that matter, does the text state the inital temperature of the flask vs. the dew point of the air? If the flask is colder than the dew point, water would condense, not evaporate, until the flask warms.

Your chemistry teacher seems to have expressed equilibrium far better than the text, and you might mention to her or him that the text is unclear -- you might not be the only student with that question.


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