Many substances dissolve in water, but others are quite insoluble—water and many of the Earth’s rocks and minerals have coexisted for billions of years and the rocks are still here. The properties of solvents are often summed up in the phrase, ―like dissolves like,‖ with the implication that ―like does not dissolve unlike.‖ In these generalizations, the property that is like or unlike is molecular polarity. A solvent with polar molecules like water tends to dissolve other substances having polar molecules, as well as substances that form ions when dissolved. This is the case because the charges or partial charges of the solvent molecules and solute molecules attract one another. The molecules of the solvent surround molecules or ions of the solute, an arrangement called solvation. This solvation holds the solute in solution. On the other hand, substances with nonpolar molecules, such as hydrocarbons, fats, and oils, are not appreciably soluble in water. However, they are relatively soluble in solvents that have nonpolar molecules, solvents like those used for dry cleaning clothing..
Why are nonpolar molecules generally insoluble in water and why are some ionic compounds, the minerals in rocks, for example, also so insoluble. The answers can be put in terms of favorable and unfavorable arrangements of molecules and ions in the solvent-solute system and are essentially the same in each case. In a system that consists of two or more different kinds of molecules (or molecules and ions) mixing them together so they are dispersed among one another is always a favorable arrangement. Thus, dissolution, mixing one substance into another, is always favorable. (This qualitative idea can be quantified by introducing the concept of entropy. Favorable arrangements have higher entropy than unfavorable arrangements and systems always change toward higher entropy.) The qualitative idea of favorable and unfavorable arrangements is sufficient to understand solubility.