[stringpaper]

Here is a question from a 2012  Chemistry Olympiad Exam:

Which transformation demonstrates that the bonds between water molecules are weaker than the bonds within a water molecule?

a) freezing water
c) boiling water

Can some one help explain to me why the answer is C?

And on the topic of this question, what happens on a molecular level to water when it freezes? I know that when it boils, IMFs are broken.

Thanks!

[Yggdrasil]

To convert liquid water to gaseous water, all intermolecular bonds between water molecule must be broken.

[stringpaper]

What about when water freezes?

[Yggdrasil]

In ice, all intermolecular bonds (hydrogen bonds) between water molecules are formed and they all have a fairly ideal geometry for the strongest interaction.  In liquid water these bonds still persist, but they do not have ideal geometry and they are much more dynamic (they easily break and re-form).

So in other words, to go from ice to liquid water, you just need to slightly weaken the IMFs whereas to go from liquid water to vapor you need to completely break all of the IMFs.