Magnesium hydroxide

Magnesium hydroxide is an inorganic compound with the chemical formula Mg(OH)₂. As a suspension in water, it may be referred to as Milk of Magnesia. The solid mineral form of magnesium hydroxide is known as brucite.

Magnesium hydroxide is common component of antacids and laxatives. Magnesium hydroxide interferes with the absorption of folic acid and iron.

Magnesium hydroxide is not very soluble in water, with a K_sp of 1.5x10^-11. While the solubility of magnesium hydroxide is low, all of the magnesium hydroxide that does dissolve in the water does dissociate. Since the dissociation of this small amount of dissolved magnesium hydroxide is complete, magnesium hydroxide is considered a strong base.

Preparation

Magnesium hydroxide is commercially available and there is no need to prepare it in the laboratory. It can be precipitated by the metathesis reaction between magnesium salts and sodium, potassium, or ammonium hydroxide:

Mg²⁺ (aq) + 2OH⁻ (aq) → Mg(OH)₂ (s)

Uses

Magnesium hydroxide, in the form of Milk of Magnesia, is used as an antacid to neutralize stomach acid, and a laxative. The diarrhea caused by magnesium hydroxide carries away much of the body's supply of potassium, and failure to take extra potassium may lead to muscle cramps. In addition it can be used as a non antiperspirant armpit deodorant. It is also used in bleaching solutions to whiten clothes.

Magnesium hydroxide is used industrially as a non-hazardous alkali to neutralise acidic wastewaters.

Magnesium hydroxide is also used for growing new reefs called Seacrete or Biorock

Magnesium hydroxide also has smoke suppressing and fire retarding properties. This is due to the endothermic decomposition it undergoes at 332°C (630°F) :

Mg(OH)₂ → MgO + H₂O

The heat absorbed by the reaction acts as a retardant by delaying ignition of the associated substance. The water released dilutes any combustible gases and inhibits oxygen from aiding the combustion. Common uses of Magnesium hydroxide as a fire retardant include plastics, roofing, and coatings.

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