Aluminum chloride and water react to form aluminum hydroxide and hydrochloric acid. However, the reaction is exothermic when there is much more aluminum chloride than water, thus dropping a small amount of water on to solid aluminum chloride creates hydrogen chloride gas, a toxic substance that corrodes certain metals. When aluminum chloride is placed in enough water, the reaction cools, and the ions dissolve in water.
The balanced equation of this reaction is 2AlCl3 + 3H2O = Al2O3 + 6HCl, or 2 moles of aluminum chloride reacting with 3 moles of water to produce 1 mole of aluminum oxide and 6 moles of hydrogen chloride. In water, hydrogen chloride forms hydrochloric acid. Aluminum oxide is the second-hardest mineral on Earth after diamond, although the substance formed in this reaction is generally dissolved in water with higher concentrations of hydroxonium, or (OH-) ions.
Aluminum chloride is a white when pure but yellow or gray when found in nature. It is also known as aluminum trichloride, trichloroalumane and trichloroaluminum. This substance has been tested on rainbow trout, mice and rabbits to determine toxicity levels in animals. When dissolved in the human stomach, anywhere between 17 and 30 percent of the digested aluminum chloride is excreted through kidneys.