Magnesium hydroxide

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Magnesium hydroxide
Magnesium hydroxide.jpg
IUPAC name
Magnesium hydroxide
Systematic IUPAC name
Magnesium hydroxide
Other names
Milk of magnesia
Jmol-3D images Image
Molar mass 58.3197 g/mol
Appearance White solid
Odor Odorless
Density 2.3446 g/cm3
Melting point 350 °C (662 °F; 623 K) decomposes
Boiling point Decomposes
0.00064 g/100 ml (25 °C)
0.004 g/100 ml (100 °C)
Solubility Reacts with acids
Vapor pressure ~ 0 mmHg
64 J·mol−1·K−1
−924.7 kJ/mol
Safety data sheet Sigma-Aldrich
Flash point Non-flammable
Related compounds
Related compounds
Calcium hydroxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Magnesium hydroxide is inorganic compound with formula Mg(OH)2. It is often known as milk of magnesia, because of its milk-like appearance as a suspension. The solid mineral form of magnesium hydroxide is known as brucite. Suspensions of magnesium hydroxide in water are used as an antacid to neutralize stomach acid, and as 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. Magnesium hydroxide is also used as an antiperspirant underarm deodorant.



Magnesium hydroxide reacts with acids, forming the magnesium salt of said acid and water.

Mg(OH)2 + 2 HCl → MgCl2 + 2 H2O

Magnesium hydroxide decomposes when heated at high temperatures.

Mg(OH)2 → MgO + H2O


Magnesium hydroxide is an inorganic compound with the chemical formula of hydrated Mg(OH)2. While magnesium hydroxide has a low solubility in water, with a Ksp of 1.5×10−11, it is large enough that it will partially dissolve to produce ions in the solution, forming the suspension. A relatively high concentration of magnesium or hydroxide ions would be required to revert the suspension to the solid precipitate by reversing the equilibrium.

In this suspended form, magnesium hydroxide is a common component of antacids and laxatives; it interferes with the absorption of folic acid and iron. The antacid properties come from the hydroxide ions which are responsible for neutralizing the acid.


Magnesium hydroxide is composed of magnesium ions and hydroxide ions and it will precipitate whenever the two are present together - ie, combine in a metathesis reaction. The reaction is as follows:

Mg2+ (aq) + 2 OH (aq) → Mg(OH)2 (s)

It can be easily prepared by mixing sodium hydroxide (or potassium hydroxide) with magnesium sulfate. Note that it is extremely hard to filter milk of magnesia: magnesium hydroxide forms a slime-like gel that clogs most filters, and even strong vacuum does not help much.


Magnesium hydroxide can be bought from companies like

It's also available in some drugstores as an antacid.


  • Make magnesium salts
  • Make magnesium oxide



Magnesium hydroxide has low to no toxicity, though if too much is ingested it may cause digestion issues and hypermagnesemia. It's non-flammable and non-explosive.


Magnesium hydroxide should be stored in closed bottles, away from open air to prevent it from absorbing carbon dioxide or sulfur dioxide from air.


Magnesium hydroxide can be safely thrown out with your normal garbage or dumped in plant-less soil.


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