[Kaleyrvt]

This is my question:
Consider a stoppered flask containing benzene, iodine, and air, which has reached equillibrium at 298 K. Which one of the following statements concerning the system at equillibrium is true?
a) The pressure of the vapour phaseis not constant
b)Evaporation of the liquid continues
c)The colour of the liquid phase will change
The mass of the solid iodine is not constant

I don't know if this question is missing a part but it makes absolutely no sense to me.....I feel that there is not enough info to allow me to make a final answer-can someone please *delete me*! I know I am supposed to post my work but I have no clue!!
Thank you!

[Yggdrasil]

Well, consider this question first what does it mean for a system to be at equilibrium?

[Kaleyrvt]

equillibrium  is a state that is reached when the original substances react at the same rateas the products react with each other to reform the original substances. Otherwords, reactions are reversable and a balance is reached bewtween both the forward and reverse reaction.

[Yggdrasil]

That's a good description of equilibrium.  But, perhaps we should focus on what happens to the system over time.  How do the properties of a systems at equilibrium vary over time?

[Kaleyrvt]

I thought they remained constant over time. Thus, a and d are probably incorrect, so I would assume that c is the correct answer-this is becasue equillibriumhas been reached, therefore physical properties can now be observed???

[Yggdrasil]

But with (c), the properties of the system will not be constant over time, since the color of the liquid (which represents the concentration of iodine in the benzene) changes over time.

The correct answer is (b) and the question relies on your recognition that equilibrium is a dynamic process (as you mentioned in your first definition of equilibrium).  Remember that at equilibrium your forward and backward reactions do not stop.  Rather "original substances react at the same rate as the products react with each other to reform the original substances."  So in terms of the equilibrium between liquid and gaseous benzene, the liquid benzene is still evaporating.  However, the gaseous benzene is condensing at the same rate, so overall there is not change over time.