[Byrne]

"One drop of water may or may not establish a state of equilibrium when placed in a closed bottle.  Explain."

Equilbrium occurs when there is a balance between forward and reverse process occurring at the same rate, I know this much.  But I'm having troubles relating the concept of equilibrium to this partciular statement.  I don't see how adding water to a closed bottle would have an effect on equilibrium at all.

[AWK]

Vapour-liquid equilibrium ?

[Mitch]

Any closed system will approach equilibrium on a long enough time scale, assuming no forces act on the system to cause it to change.

[Byrne]

Any closed system will approach equilibrium on a long enough time scale, assuming no forces act on the system to cause it to change.

Well, here's what my chemistry teacher said.

Physical changes do not always reach equilibrium.  It depends on the relative quantities involved.  A physical change establishes equilibrium if the system is saturated.

First of all, I do not understand the first statement.  How is it that physical changes cannot always reach equilibrium?  

Also, how does this apply to the original question.  When I asked him, he didn't quite answer the question, but instead provided some information that was intended to probably guide me in the right direction.  It instead has left me more confused.  I was under the impression a closed system will always eventually reach a state of equilibrium... but that's for chemical equilibrium, I guess.  Why would physical equilibrium be any different?

[Borek]

Every system will be in the state of equilibrium, just not neesarilly every equilibrium will be present

Imagine some amount of water in a closed container. If teh container is  small enough, there are both water and vapor - so there is gas/liquid equilibrium. Not take larger container (or heat the syste up) - water disappears, there is no liquid/gas equilibrium. But does it means system is not at equilibrium?

[Byrne]

Every system will be in the state of equilibrium, just not neesarilly every equilibrium will be present

Imagine some amount of water in a closed container. If teh container is  small enough, there are both water and vapor - so there is gas/liquid equilibrium. Not take larger container (or heat the syste up) - water disappears, there is no liquid/gas equilibrium. But does it means system is not at equilibrium?

Equilibrium occurs when the rate of forward process equals the rate of reverse process, correct?  So if only a vapor is present, wouldn't equilibrium not exist because there is no forward or reverse process in the system?  A closed system such as the one in this question would be at equilibrium when the container is saturated with water vapor and the rate of condenstation equals the rate of evaporation.  Basically, a system will only reach equilibrium if it is saturated, because if not, all of the water will evaporate into water vapor, correct?

Now, looking at the original problem, if I were to add a drop of water to a closed system that is saturated with water vapor and has a certain volume of liquid water present, the drop of water would increase the volume of liquid water present in the closed container and therefore decrease the volume of available for water vapor.  Therefore, the system will shift towards condenstation because it is supersaturated and eventually reestablish equilibrium where the rate of condensation equals the rate of evaporation, correct?  Now, if I were to add a drop of water to unsaturated volume of water vapor, the liquid water would simply evaporate and no state of dynamic equilibrium would result since there is no forward/reverse process present.  However, if I were to add a drop of water to a closed container saturated with water vapor, a state of equilibrium will be established as the volume of the drop of water decreases the volume available for the water vapor, resulting in supersaturation and shift in the direction of condensation until the system is saturated (i.e. evaporation equals condensation).

So, am I on the right track?

[Borek]

So if only a vapor is present, wouldn't equilibrium not exist because there is no forward or reverse process in the system?

Just don't forget to add "liquid vapor" to "equilibrium". A system can be described by many equilibria, some of them disappear in some conditions. Others are still obeyed.

So, am I on the right track?

Looks OK to me. You have omitted second most probable situation - after you have added a drop of water it partially evaporated saturating container. Vapor-liquid equilibrium appears.

[Donaldson Tan]

equilibrium is not restricted to vapourisation and condensation processes.

if a system is observed to exhibit time-independent stability, it is said to be in equilibrium.