In principle, all types of liquids can behave as solvents: liquid noble gases , molten metals, molten salts, molten covalent networks, and molecular liquids. In the practice of chemistry and biochemistry, most solvents are molecular liquids. They can be classified into polar and non-polar , according to whether their molecules possess a permanent electric dipole moment . Another distinction is whether their molecules can form hydrogen bonds ( protic and aprotic solvents). Water , the most commonly used solvent, is both polar and sustains hydrogen bonds.