Re: Why a saturated solution doesn't become a solid?

Good question. And since you brought up the example of "sugar" in water, let's use that to discuss this.

Essentially, a saturated solution can co-exist with the solid. That is, if I am trying to make a saturated solution of sugar, I add enough sugar until it won't dissolve any more. The solution is then saturated - and the solid is still present. In chemical terms, the solute in the solution (the sugar) and the solute in the solid (again, the sugar) have the same chemical potential and have come to equilibrium.

What distinguishes the two solutes is that one is mixed with water. Yes, there are a lot of sugar molecules but there is also a lot of water molecules as well. We measure the number of molecules of a substance using a concept called "the mole". It is a very, very large number (6.022x10²³) but in essence, it is simply a number like a dozen or a decade or a century. It allows us to count individual molecules.

In the case of sugar, at room temperature, it has a density of 4.0 g per 1 mL or 4000 g of sugar per 1000 mL of water. But each molecule of sugar weighs way more than each molecule of water. The consequence is that 4000 grams of sugar is 11.7 moles whereas 1000 mL of water is 55.5 moles. That is, for each sugar, there are actually 5 water molecules.

It is these five molecules that hold onto the sugar and, with the five water molecules around each neighbouring sugar, collectively keep all of the sugar molecules in solution. Think of the water molecules like an entourage (or body guards) that move with the sugar and keep other sugars from getting too close. They lubricate the interaction of the sugar molecules and prevent them from getting together.

It is the latter that prevents the sugar from solidifying. After all, to become solid sugar, you have to "squeeze" all of the water molecules out and, even with just five, there are too many to do that. The water molecules stop the sugar from getting together with other sugar molecules and forming a solid..

I hope this answers your question!