Why is water a good conductor of electricity?

[Archosaur]

Experience tells me it is, but my instinct tells me it shouldn't be.
If you place 2 leads of a dc source into water,
how does the charge get from one end to the other?

I can see how it would work with salt water, with the disassociated ions carrying the charge, but that doesn't really have anything to do with the water, other than its ability to dissolve ionic compounds.

My chem prof said that the electrons simply move through the water, never interfering with the molecules, but that doesn't seem right to me. Again, how does that have anything to do with the water? Also, if this were correct, then wouldn't a vacuum be an equally good conductor, if not better?

Just to be clear, I'm not claiming that it isn't a good conductor. I know it is.
I just need to figure out why my instincts are telling me it's not...

Thanks for any help

 

[pzona]

When you run a current through water, dissociation occurs (H2O -> H+ + OH-), but only to a very small extent. You're right to wonder why water itself conducts electricity; the short answer is that it doesn't. Distilled water is actually a very poor conductor of electricity because of the insignificant dissociation. When you deal with water on an everyday basis (tap water, rain water, etc), there are a ton of impurities in it, such as salts and metal ions. This is why water seems to be a good conductor of electricity, because you normally don't deal with distilled water in everyday situations.