Chemical Forums

Chemistry Forums for Students => High School Chemistry Forum => Chemistry Olympiad and other competitions => Topic started by: stringpaper on February 26, 2013, 09:34:33 PM

Title: Water Phase Changes
Post by: stringpaper on February 26, 2013, 09:34:33 PM

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!

Title: Re: Water Phase Changes
Post by: Yggdrasil on February 26, 2013, 09:58:22 PM

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

Title: Re: Water Phase Changes
Post by: stringpaper on February 26, 2013, 11:50:48 PM

What about when water freezes?

Title: Re: Water Phase Changes
Post by: Yggdrasil on February 27, 2013, 09:06:10 AM

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.