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Magnesium Hydroxide

Magnesium Hydroxide


Magnesium hydroxide (mag-NEE-zee-um hye-DROK-side) is a white powder with no odor, found in nature as the mineral brucite. Perhaps the best known form of the compound is a milky liquid known as milk of magnesia, a product used to treat upset stomach and constipation. Milk of magnesia was invented in 1817 by the Irish pharmacist Sir James Murray (1788–1871). Murray built a plant to produce a mixture of magnesium hydroxide in water that he sold for the treatment of a variety of ailments, including heartburn, stomach acidity, bladder and bowel problems, and "female problems." He said that the liquid mixture was much more effective than powdery magnesium hydroxide which had previously been used for the same purposes.



Magnesium hydrate; milk of magnesia; magnesia magma




Magnesium, hydrogen, oxygen


Inorganic base




58.33 g/mol


Decomposes at 350°C (660°F)


Not applicable


Virtually insoluble in water and alcohol, soluble in dilute acids and solutions of ammonium salts

In 1880, New York chemist Charles Henry Phillips (1820–1882) invented the name "milk of magnesia" and opened his own factory for producing the product. The name Phillips Milk of Magnesia is one of the oldest and best known over-the-counter medicines ever made in the United States.


Magnesium hydroxide is prepared by reacting a magnesium salt, such as magnesium chloride (MgCl2), with sodium hydroxide (NaOH). In a similar procedure, seawater (which contains small amounts of magnesium chloride) is treated with lime (calcium oxide; CaO). The water, lime, and magnesium chloride react to produce magnesium hydroxide, which settles out of solution as a precipitate.

Interesting Facts

  • The element magnesium is named after a region in Greece called Magnesia in ancient times. Today the same region is called Manisa. Compounds of magnesium, including magnesium hydroxide, were abundant in the region and were given the name of magnesia lithos, or "stones of Magnesia."


The best known use for magnesium hydroxide is as an antacid in the form of milk of magnesia. Since magnesium hydroxide is a base, it reacts with excess stomach acid, which reduces heartburn and the discomfort of upset stomach. Milk of magnesia also acts as a laxative because it increases the flow of water into the intestines, stimulating a bowel movement. Milk of magnesia consists of an 8 percent suspension of solid magnesium hydroxide in water. Other chemicals, such as calcium carbonate and aluminum hydroxide, are sometimes added to the mixture to increase its effectiveness.

Magnesium hydroxide also has a number of important industrial uses, such as:

  • A clarifier (a substance that removes impurities) in the refining of sugar;
  • An additive in the treatment of wastewater to neutralize acids present in the wastes;
  • A flame retardant coating on fabrics and other materials used by consumers and industries;
  • An additive to fuel oils;
  • An additive in toothpastes; and
  • A drying agent in some food products.

Words to Know

A medicine used for the treatment of upset stomach, acid indigestion, and related symptoms.
A substance that removes impurities from another substance.
A solid material that settles out of a solution, often as the result of a chemical reaction.
A mixture of two substances that do not dissolve in each other.


Dean, Carolyn. The Miracle of Magnesium. New York: Ballantine Books, 2003.

"Magnesium Hydroxide." Chemical Land 21. (accessed on September 14, 2005).

See AlsoMagnesium Oxide

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