Why It Won’t Mix with Water

Pour xylol (xylene) into water and nothing blends. The two settle into layers, with the solvent floating on top, and no amount of stirring makes them combine. This is not a quirk of xylol in particular but a clean example of one of chemistry’s oldest rules of thumb: like dissolves like.

The rule turns on how a molecule carries its electric charge. Water is polar: each molecule has a slightly positive and a slightly negative end, and those ends grip one another through hydrogen bonds, which is what makes water such a tight, cohesive liquid. Xylol is the opposite, nonpolar. Its ring of carbon and hydrogen shares charge evenly, with no positive or negative end to offer. Put the two together and the water molecules would rather hold onto each other than make room for a molecule they cannot bond with, so they close ranks and squeeze the xylol out. The solvent, being less dense, ends up on top.

That single property explains much of how xylol behaves. It dissolves oils, greases, waxes, and oil-based paints because those are nonpolar too, and it fails on anything water-based, which is why it will not thin latex paint or rinse away under a tap. It also means a spill floats and spreads across water rather than dissolving into it, a real concern for the environment, and that water is useless for washing xylol off skin or flushing it down a drain. Grasp the polar-versus-nonpolar split and you can predict most of what xylol will and will not touch.