Solubility

Dissolving means pulling particles of one substance apart and surrounding them with particles of another. That only happens easily if the new attractions forming between solute and solvent are about as strong as the attractions being broken within each pure substance. Polar molecules attract each other through dipole-dipole forces and hydrogen bonding; nonpolar molecules attract each other only through weak London dispersion forces. Mix a polar substance with a nonpolar one and you'd have to break strong polar attractions but only replace them with weak ones, which costs more energy than it releases. That mismatch is why oil, built from nonpolar hydrocarbon chains, will not mix into vinegar, which is mostly water held together by hydrogen bonds. The two liquids separate into layers instead.

Ethanol, CH3CH2OH, shows how one molecule can bridge two worlds. Its OH group hydrogen bonds with water, so it mixes with water. Its hydrocarbon tail is nonpolar, so it also mixes with hexane through dispersion forces. Octane, a pure hydrocarbon chain with no polar group, only has dispersion forces available, so it mixes with hexane but not with water. The molecule's structure, not its size or mass, decides which forces it can offer.

This is why matching forces matters more than matching molar mass: a tiny polar molecule like water will not dissolve a huge nonpolar one like wax, no matter how you measure them. As a chain gets longer, for instance adding CH2 groups to an alcohol, the nonpolar tail grows and starts to dominate, so solubility in water drops while solubility in nonpolar solvents rises. This sets up the next idea, how the balance of forces controls not just whether things mix but how much solute a solvent can hold.