Fumaric acid, maleic acid

   FUMARIC ACID, MALEIC ACID, two organic acids having the same molecular composition (C4H4O4) and the same chemical constitution (COOH.CH:CH.COOH), but differing in physical and chemical properties. Fumaric acid occurs in fumitory and some other plants, and may be prepared by heating its stereoisomer maleic acid. It forms very sour, needle-like crystals that are almost insoluble in water, and sublimes at 200° C. At higher temperatures it is converted into anhydrous maleic acid. Maleic acid crystallizes in the form of rhombic prisms which melt at 130° C. and are readily soluble in water.