Polar liquids are made up of covalently bonded molecules that each have a partial positive charge on one end and a partial negative charge on the other end. They can dissolve solids that are made of polar molecules but cannot combine with a substance made of nonpolar molecules.
Polar liquids comprise covalent molecules, where atoms share electrons to form bonds. The electrons in a covalent bond are not equally shared between the atoms of different elements. Elements that have a higher electronegativity tend to pull the shared pair of electrons closer to themselves, obtaining a partial negative charge while leaving the other atoms involved in the covalent bond with a partial positive charge. This creates a permanent dipole moment in the molecule, making it polar. Nonpolar molecules do not have partial charges.
Polar liquids, such as water, are held together by the dipole-dipole attraction between the liquid’s molecules. The partial positive charge of one molecule attracts the partial negative charge of another molecule. The dipole interactions can be disrupted by other polar molecules. For example, when a polar solid is added to a polar liquid, the partial positive charge of a liquid molecule attracts the partial negative charge of the molecules from the solid, thus allowing the solid to dissolve in the liquid.
Nonpolar solids cannot dissolve in a polar liquid because they do not possess partial charges and are not strong enough to disrupt the dipole interactions between the liquid molecules.