A dipole is any object with a separation of electrostatic charge, positive at one end and negative at the other (right) . It is the electrostatic equivalent of a magnet; and as with magnets, opposite ends of dipoles attract one another. Water is a liquid at room temperature, and ammonia gas is easily liquefied at -33C, because their molecules are dipoles and are attracted to one another. Methane lacks these dipole attractions and therefore must be cooled to -164C before van der Waals forces cause it to liquefy. Water is a good solvent for other molecules with dipoles, and for ionic salts, because the charges at the ends of the water molecules can interact with opposite charges on the molecules or ions being dissolved.