Magnesium chloride is a chloride compound composed of 74.54% chlorine and 25.48% magnesium; it has the chemical formula MgCl₂ and a molecular weight of 95.21. It appears as colorless, odorless monoclinic crystals in the form of flakes, granules, or lumps. It has a bitter taste, is highly hygroscopic, and is readily soluble in water and soluble in ethanol. Its relative density ranges from 2.316 to 2.33 (at 25°C), with a melting point of 714°C and a boiling point of 1412°C. It exists in various hydrated forms, such as MgCl₂·12H₂O and MgCl₂·6H₂O, with magnesium chloride hexahydrate (MgCl₂·6H₂O) being the most stable form under standard temperature and pressure. When dissolved in lower alcohols, it can form crystalline alcohol adducts; in a sealed container, it reacts with gaseous ammonia to form corresponding adducts; and in its molten anhydrous state, it undergoes electrolysis under an electric current to produce magnesium metal and chlorine gas.
Magnesium chloride deposits are primarily found in the salt lakes of the Qaidam Basin, with the Qarhan Salt Lake National Mine Park in Golmud, Qinghai, serving as a prime example. It is produced by first subjecting magnesium chloride hexahydrate to gradual dehydration to form the dihydrate, followed by further dehydration using hydrogen chloride gas. It serves as a vital inorganic raw material in the chemical industry for producing magnesium products such as magnesium carbonate, magnesium hydroxide, and magnesium oxide, as well as serving as a feedstock for antifreeze agents. In the metallurgical industry, it is used to produce magnesium metal (via molten salt electrolysis), liquid chlorine, and high-purity magnesia. In the field of magnesium-based cementitious materials, a magnesium chloride solution acts as the mixing agent for magnesium oxychloride cement (magnesia cement) used to manufacture various products. Additionally, it finds applications in other sectors as a food additive (protein coagulant), snow-melting agent, refrigerant, dust suppressant, and raw material for refractory materials. Magnesium chloride is a substance of low toxicity; the oral LD50 in rats is 2,800 mg/kg, and oral ingestion of 4–5 g by humans can cause diarrhea. According to FAO/WHO (1994) guidelines, no specific limit is set for its Acceptable Daily Intake (ADI). Solid magnesium chloride should be stored in a dry warehouse to prevent deliquescence and loss caused by moisture or rain; in the event of a fire, water may be used for extinguishing.