Magnesium hydroxide is an inorganic alkaline compound and the primary component of the mineral brucite . Its chemical formula is Mg(OH)₂, and its molecular weight is 58.32. At room temperature, it appears as white hexagonal flaky crystals or powder; it has a density of approximately 2.36 g/cm³, a refractive index of 1.58, and a Mohs hardness of 2.5. It decomposes at 280°C under vacuum. It is nearly insoluble in water (forming a slightly alkaline aqueous slurry with a pH of 9.5–10.5) and insoluble in alcohol, but it dissolves in dilute acids and ammonium salt solutions. Magnesium hydroxide is thermally unstable; upon heating to 350°C, it decomposes into magnesium oxide and water. It reacts with dilute acids to form the corresponding magnesium salts and water, and it can also react with salts or acidic oxides-for instance, dissolving in excess ammonium chloride to form (NH₄)₂MgCl₄ or reacting with carbon dioxide to produce a magnesium carbonate precipitate.
Magnesium hydroxide is produced by reacting refined brine with milk of lime to form a precipitate, followed by filtration and drying; alternatively, it can be prepared by dissolving magnesium chloride in water, reacting it with sodium hydroxide or aqueous ammonia, and then filtering, washing, and drying the resulting precipitate. It is used to manufacture magnesium oxide (a catalyst carrier) as well as magnesium salts, pharmaceuticals, fine ceramics, thermal insulation materials, and refined sugar. It also serves as a flue-gas desulfurization agent, an anti-corrosion additive for petroleum products, a neutralizing agent for acidic wastewater, and a flame retardant for plastic and rubber products.
Magnesium hydroxide is non-toxic, although its dust can cause mild irritation to mucous membranes and the conjunctiva; the oral LD₅₀ in rats is 8,500 mg/kg.