Pure caffeine occurs either as an anhydrous solid or as the monohydrate, which contains one molecule of water per molecule of caffeine. Anhydrous caffeine has a melting point of 441 to 442 degrees Fahrenheit, whereas caffeine monohydrate has a melting point of 453 to 455 degrees Fahrenheit.
Both anhydrous caffeine and caffeine monohydrate appear as odorless crystalline solids or powders and are completely soluble in water. Caffeine monohydrate is efflorescent in air, meaning it slowly loses its water of hydration. Upon drying in an oven, anhydrous caffeine loses no more than 0.5 percent of its weight, whereas caffeine monohydrate loses 5 to 9 percent of its weight.