Magnesium hydroxide

Magnesium hydroxide

Names
IUPAC name Magnesium hydroxide
Systematic IUPAC name Magnesium hydroxide
Other names Brucite Magensium dihydroxide Milk of magnesia
Identifiers
Jmol-3D images	Image
SMILES [OH-].[OH-].[Mg+2]
Properties
Chemical formula	Mg(OH)2
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
Solubility in water	0.00064 g/100 ml (25 °C) 0.004 g/100 ml (100 °C)
Solubility	Reacts with acids
Vapor pressure	~ 0 mmHg
Thermochemistry
Std molar entropy (So298)	64 J·mol−1·K−1
Std enthalpy of formation (ΔfHo298)	−924.7 kJ/mol
Hazards
Safety data sheet	Sigma-Aldrich
Flash point	Non-flammable
Related compounds
Related compounds	Calcium hydroxide Strontium hydroxide Barium 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.

Properties

Chemical

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

Physical

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.

Preparation

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.

2 NaOH + MgSO₄ → Mg(OH)₂ + Na₂SO₄

2 KOH + MgSO₄ → Mg(OH)₂ + K₂SO₄

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.

Very pure magnesium hydroxide can be obtained by reacting magnesium metal with water, which works much better with hot water than cold water.

Mg + 2 H₂O → Mg(OH)₂ + H₂

Magnesium metal does not react with alkali hydroxides, thus the hydroxide cannot be produced this way.

Availability

Magnesium hydroxide is sold by chemical suppliers and can be bought cheaply online.

Some companies, like Martin Marietta Magnesia Specialties, deal in high purity products.

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

Projects

• Make magnesium salts
• Make magnesium oxide
• Antiacid

Handling

Safety

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.

Storage

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

Disposal

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

References

Relevant Sciencemadness threads

• Separation of Mg(OH)2 from Ca(OH)2