Copper sulphate pentahydrate is an inorganic compound with the chemical formula CuSO4-5H2O, commonly known as blue alum, bile alum or copper alum . Copper sulfate pentahydrate is very stable at room temperature and pressure, not deliquescent, in dry air will gradually weathering, heated to 45 ºC when the loss of two molecules of water of crystallisation, 110 ºC when the loss of four molecules of water of crystallisation, called copper sulfate monohydrate, 200 ºC when the loss of all the water of crystallisation into anhydrous material. Can also lose five crystal water under the action of concentrated sulfuric acid. Anhydrous material is also easy to absorb water into hydrated copper sulfate. Water absorption reaction to produce copper sulfate pentahydrate (blue), often use this property to test whether some liquid organic matter contains trace amounts of water (such as the identification of whether the alcohol contains water, in the alcohol to be identified by adding a small amount of anhydrous copper sulfate, such as the white anhydrous copper sulfate becomes blue, it means that the alcohol is mixed with water).