A salt of a polybasic acid (i.e. an acid having two or more acidic hydrogens) in which not all the hydrogen atoms have been replaced by positive ions. For example, the dibasic acid carbonic acid (H2CO3) forms acid salts (hydrogencarbonates) containing the ion HCO3−. Some salts of monobasic acids are also known as acid salts. For instance, the compound potassium hydrogendifluoride, KHF2, contains the ion [F…H-F]−, in which there is hydrogen bonding between the fluoride ion F− and a hydrogen fluoride molecule.