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Some liquid water is present inside a sealed flask at room temperature. Is water evaporating, and is the system at equilibrium? First of all, can water evaporate in a sealed flask at room temperature? If so, is the reaction happening a reversible reaction? Any help would be appreciated. Thanks.

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Even at room temperature, some of the water molecules in the liquid will have sufficient energy to 'escape' and become vapour. Because the flask is sealed however, some of the water vapour molecules will return to the liquid phase.

Eventually, an equilibrium will be reached when the rate at which water molecules which escape the liquid is equal to the rate at which water molecules in the vapour enter the liquid. At this point, the pressure reached by the water vapour is known as the vapour pressure of water (at the given temperature).

The 'reaction' is not completely reversible since some heat is lost to the surrounding environment through the wall of the flask.

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  • $\begingroup$ Thanks, very helpful. So the system will eventually reach equilibrium, despite the fact that it is not completely reversible? $\endgroup$ – McB Nov 25 '14 at 15:53
  • $\begingroup$ If heat can transfer between the flask and environment, equilibrium will be reached when the temperature of the flask reaches the temperature of the environment. At 25C, the vapour pressure of water is 0.0313 atm. $\endgroup$ – theo Nov 25 '14 at 16:04

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