[realdon]

Phenolphthalein is colorless at low pH and pink at high pH is because

a) hydrogen  bonding at low pH
b)reaction at  low pH to form multiple conjugated aromatic rings
c) molecular  rearrangements
d) reaction at high pH  to form multiple conjugated aromatic rings
e) reaction to form high  energy species

can anybody help me on this one?

Thanks

[Arkcon]

So, you want to know which of those responses causes the color change, but you can't figure out which one.  OK, look at each one, can you think of an example where the response occurs and there is no color change?  That might help you iron it down.  Example: A.  hydrogen bonding at low pH -- given all the situations where it can occur, is hydrogen bonding an adequate explanation for color change?

[realdon]

a) not possible because it H bonds with water all the time
b) not sure
c) not sure, maybe
d) not sure
e) not possible because it can't form a high energy species

I still can't find anything on b-d, i looked everyone on the web

[realdon]

nvm, I got it, thanks anyways

[agrobert]

Did you look here?

http://en.wikipedia.org/wiki/Phenolphthalein

Wiki is your friend.

Do you know what conjugated structures look like?

Can you explain why such conjugation occurs at high pH?

Think about the changing concentration of H⁺ to ^-OH.  In acidic (low pH) conditions a tertiary carbocation is reasonably stable.  In basic (high pH) conditions a rearranged molecule is highly conjugated.

[Arkcon]

a) not possible because it H bonds with water all the time
b) not sure
c) not sure, maybe
d) not sure
e) not possible because it can't form a high energy species

I still can't find anything on b-d, i looked everyone on the web

Alright, you've got it.  But, B & D are the opposites of each other, did you stop and think, "Phenolphthalein is clear, base is added and it changes color, what chemical reacts with base ...". 

I'm glad you were able to kick A and E out, you've noticed the question wasn't as tough as you originally thought.  This happens a lot 'round here -- people say, "I just don't know what to do!!!11!! "  When one or two of the multiple choice options is ridiculous.