Re: Why do sugar cubes dissolve in water?

This is just a particular instance of a much more general question: why do some substances dissolve in a particular solvent and not others? This is one of the very basic questions that chemists have been thinking about and working on for at least the last 200 years!

Modern chemistry is, above all else, about the relationship between what we observe on the everyday scale of things and what is happening on the sub-microsopic scale to the atoms and molecules.

When we mix a solid with a liquid, one of two things will happen (if there is not a chemical reaction, which I guess is a third possibility): either the molecules of the solid will separate from one another and mix in with the molecules of the liquid, or they will continue to cling together in separate lumps. In the first case we observe that the solid dissolves. In the second case we observe that the solid stays intact, and does not dissolve.

Which one happens depends on the forces between the molecules. If the force of attraction holding two molecules of the solid together is much stronger than the force of attraction between a molecule of the liquid and a molecule of the solid, then the solid will stay together. If the force is weaker, the same, or just a tiny bit stronger, the solid will break up and dissolve. The situation is just a little bit biassed in favour of dissolving, because of the extra randomness in the arrangement. Nature loves disorder! (I guess that is why it is easier to mess up your room than to tidy it ;-)

The molecule of sugar is large (45 atoms) while that of water is small (3 atoms). But in one particular way they are very similar: they both have -O-H groups in the molecule. In the case of sugar there are lots of them, and they are on the outside of an irregularly shaped, but roughly spherical molecule. This means that the main force that holds sugar molecules together -- hydrogen bonding -- is exactly the same as the main force between a sugar molecule and a water molecule. So it is very similar in strength. The particular irregular shape of sugar molecules mean that they do not pack together very efficiently in a solid. These are the factors that make sure that sugar will dissolve in water.

There is a rough rule that we use a lot in chemistry: like dissolves like. It should be used with caution, though, because there are many situations where it does not work.