tartaric acid, HO₂CCHOHCHOHCO₂H, white crystalline dicarboxylic acid. It occurs as three distinct isomers, the dextro-, levo-, and meso- forms. The dextro- and levo- forms are optically active; the meso- form is optically inactive, as is racemic acid, a mixture of equal parts of the dextro- and levo- forms. Tartaric acid is found in many plants, e.g., grapes; this natural acid is chiefly the dextrorotatory d-tartaric acid, called also d-2,3-dihydroxysuccinic acid or l-2,3-dihydroxybutanedioic acid. This form can be partially converted to the others by heating it with an aqueous alkali, e.g., potassium hydroxide. Tartaric acids can be synthesized from maleic acids or fumaric acids by reaction with aqueous potassium permanganate. The various isomeric forms differ in such physical properties as boiling point. Tartaric acid is used chiefly in the form of its salts, e.g., cream of tartar and Rochelle salt.