Why don’t you try it?
Add two cups of sugar to one cup of water in a saucepan and stir while heating slightly. You’ll see that all the sugar will dissolve.
One of the reasons is very simple: Sugar molecules can squeeze into empty spaces between the water molecules, so they are not really taking up much new space.
When you get right down to the submicroscopic level, water isn’t a densely packed pile of molecules. It’s a somewhat open latticework, with the molecules connected to one another in tangled strings. The holes in this latticework can accommodate a surprising number of dissolved particles.
This is especially true of sugar, because sugar molecules are built in such a way that they just love to associate with (Techspeak: hydrogen-bond to) water molecules, and that makes sugar very mixable with water. As a matter of fact, with heating, you can coax more than two pounds (5 cups!) of sugar to dissolve in a single cup of water.
Of course, by the time you get that far, it’s not clear whether you’re dealing with a boiling solution of sugar in water or with bubbling melted sugar containing a little water.
And that’s how candy was born.
Yet another reason is that two cups of sugar is considerably less sugar than it seems. Sugar molecules are both heavier and bulkier than water molecules, so there won’t be as many of them in a pound or in a cup.
Also, the sugar is in granulated form, rather than in the form of a liquid, and the grains don’t settle down into the cup as tightly as you might think. The surprising result is that a cup of sugar contains only about one twenty-fifth as many molecules as there are in a cup of water.
That means that in your two-cups-of-sugar-in-one-cup-of-water solution, there is only one molecule of sugar for every twelve molecules of water.
Not such a big deal, after all.