Water dissolves salt but not oil. The difference is polarity. A polar molecule has an uneven distribution of electrons, creating partial positive and negative charges. Water is polar because oxygen is more electronegative than hydrogen. The shared electrons spend more time near oxygen, giving it a partial negative charge and leaving hydrogen with a partial positive charge. The molecule has a bent shape, so the charges do not cancel. Oil is nonpolar because its electrons are shared evenly.

Polarity determines solubility. Like dissolves like. Polar solvents dissolve polar solutes. Nonpolar solvents dissolve nonpolar solutes. Water is a polar solvent. Hexane is nonpolar. Salt is ionic and dissolves in water because the polar water molecules surround the ions and pull them apart. Oil does not dissolve in water because the nonpolar molecules are not attracted to the polar water molecules.

Polarity affects boiling point, surface tension, and biological function. Polar molecules have higher boiling points because of dipole-dipole interactions and hydrogen bonding. Water's polarity makes it a universal solvent and gives it a high surface tension. Cell membranes are made of phospholipids, which have a polar head and a nonpolar tail. The heads face water, and the tails face each other, forming a bilayer that separates the inside of the cell from the outside.

Polarity is a consequence of electronegativity and molecular geometry. A molecule with polar bonds can be nonpolar overall if the geometry cancels the dipoles. Carbon dioxide has polar bonds but is linear, so the dipoles cancel and the molecule is nonpolar.