Salt in which there is only partial replacement of the acidic hydrogen of an acid by metal or other cations. For polybasic acids the formulae are of the type NaHSO₄ (sodium hydrogensulfate) and Na₃H(CO₃)₂.2H₂O (sodium sesquicarbonate). For monobasic acids such as HF the acid salts are of the form KHF2 (potassium hydrogen fluoride). Although the latter were at one time formulated as a normal salt plus excess acid (i.e. KF.HF) it is preferable to treat these as hydrogen-bonded systems of the type K⁺(F–H–F)⁻.