2.2.2 (m) Anomalous Properties of Water

Syllabus

(m) explanation of anomalous properties of H₂O resulting from hydrogen bonding, e.g.

(i) the density of ice compared with water

(ii) its relatively high melting and boiling points

What does this mean?

Melting/Boiling Point

These have been two of the examiners absolute favourite questions over many years.

Make sure you know how to answer them.

How do we even know that Hydrogen bonds are different to other intermolecular forces and how do we know that it only affects N-O-F compounds?

You should be able to sketch something like this (don't learn numbers).

Look at the melting points of Hydrides of Group 4 - CH₄, SiH₄, GeH₄, SnH_4.

_{They increase with molecular weight in the way we would expect since heavier molecules have more electrons and stronger van der Waals.}

Groups 5,6 & 7 follow the same pattern except that NH₃, H₂O and HF are in completely the wrong places.

This shows that only compounds containing NH, OH or FH bonds are affected by some much stronger intermolecular force.

Why does ice float?

If you cool olive oil until it starts to solidify, the "ice" sinks. This seems weird because we're used to water-ice. But this is what happens with most substances when they freeze.

And this makes sense when you apply your Year 7/8 knowledge about particles moving further apart when they are heated.

It means that the colder solid should have particles closer together than the warmer liquid - so it should be more dense and it should sink.

So what is different about water-ice?

Not surprisingly the answer is Hydrogen-bonding.

In solid ice each water molecule is held securely exactly one Hydrogen bond length apart in a fairly open lattice structure.

Given just enough energy to overcome these Hydrogen bonds and begin to move the water molecules can actually get closer to each other.

This makes water more dense than solid ice.

So ice will float on water until it sinks.

Very cold liquid water (4^oC) is the densest.

After this the particles begin moving further apart again - as you'd expect.

So warm water floats on top of cold water.

Surface tension

You can float a steel paper-clip on water providing you don't break the surface.

But steel is much denser than water.

So why doesn't it sink?

Hydrogen bonding - obviously.

Water molecules at the surface have fewer neighbours to stick to.

So, the few Hydrogen bonds they do form are unusually strong.

Back to 2.2.2 Bonding and Structure?

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