[‚med·ə′hī‚drāt ¦sōd·ē·əm ′kär·bə‚nāt] (inorganic chemistry) Na2CO3·H2O Water-soluble, white crystals with an alkaline taste, loses water at 109°C, melts at 851°C; used in medicine, photography, and water pH control, and as a food additive. Also known as crystal carbonate; soda crystals.