Magnesium hydroxide

Mg 1
O 2
H 2

Formula unit Lattice

Choose Lattice to see eight unit cells. Then drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

5 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; magnesium hydroxide has no molecular shape.

IUPAC name: magnesium dihydroxide
Molar mass: 58.3190(44) g/mol
Atoms: 5
SMILES: [OH-].[OH-].[Mg+2]
InChIKey: VTHJTEIRLNZDEV-UHFFFAOYSA-L
PubChem CID: 73981

Made of

Mg O H

Four-fifths of the magnesium hydroxide used in the United States in 2020 went into water treatment, on US Geological Survey figures. It is also the compound in milk of magnesia. The compound pairs magnesium ions, Mg²⁺, with hydroxide ions, OH⁻, each made of one oxygen and one hydrogen atom, two hydroxides to every magnesium. Its IUPAC name is magnesium dihydroxide and its molar mass 58.32 g/mol.

HSDB, the Hazardous Substances Data Bank quoted on PubChem, describes an odourless solid that comes as an amorphous powder or as granules. Water and ethanol leave it practically undissolved, but it dissolves in dilute acids. In nature it occurs as the mineral brucite. Medical Subject Headings (MeSH), the US National Library of Medicine’s vocabulary, classes it as an antacid that also acts as a laxative. It stands on a US list of antacid active ingredients first published in June 1974.

Structure and bonding

The Formula unit drawing holds one magnesium ion, charge +2, and two hydroxide ions, charge −1 each. The only lines are two O–H single bonds, one inside each hydroxide ion. Nothing joins magnesium to either oxygen, and the drawing falls into three separate pieces. It is one formula unit of an ionic lattice, not a molecule; how ions hold such a lattice together is set out in chemical bonding.

Its unit cell, the smallest block that repeats through the crystal, holds 1 magnesium ion and 2 hydroxide ions; the Lattice view draws eight such cells. The magnesium ion is drawn at its ionic radius, and each hydroxide as a small unit of two bonded atoms. Measured from the crystal structure, every magnesium ion has six neighbours. The calcium hydroxide cell carries the same count, one metal ion to two hydroxide ions, with six neighbours around the metal.

HSDB describes the real crystals as white and hexagonal, with a hardness of 2.5 on the Mohs scale. Very little of the lattice dissolves in water. In Chem1’s account, even so sparingly soluble a hydroxide yields a solution containing hydroxide ions, since what dissolves separates into Mg²⁺ and OH⁻.

Where it turns up

Brucite, the natural mineral, is colourless and crystalline. The US Geological Survey puts identified world resources of it at several million tons, against 13 billion for magnesite. The compound is made by precipitation, in HSDB’s description: sodium hydroxide brings it out of a magnesium salt solution as a solid. US plants drew on well brines and seawater in 2020; the lime-and-seawater route is told under magnesium.

Toothpastes contain it, and so do frozen desserts and other foods, where it dries ingredients and helps them keep their colour. US food rules list it as a nutrient supplement, a pH control agent and a processing aid. The US Food and Drug Administration (FDA) affirmed it as generally recognised as safe in April 1985. Pulp and paper mills use it as well.

Why it matters

In the bowel, magnesium hydroxide acts as a saline laxative in the FDA’s terms: an agent that adds water to the intestine’s contents and thereby brings on a bowel movement. On HSDB’s account, laxatives carrying magnesium ions are thought to hold water in the bowel by osmosis, water’s passage through a membrane that holds back dissolved particles. That water then stimulates peristalsis, the movement of the intestine. Other mechanisms may also contribute. In the US National Cancer Institute’s thesaurus (NCI Thesaurus), the hydroxide ions are the agents: they pass from the stomach into the intestines and hold water there.

Beyond water treatment, chemical intermediates took 11 per cent of US use in 2020, when domestic producers shipped 196,000 metric tons. A thin slurry removes sulfur dioxide from waste gases, and the compound stands in for lime in pH control. Plastics ignite at low temperatures, and the US Geological Survey names magnesium hydroxide among the compounds that can replace bromine-based fire retardants in some uses.

Properties

Properties of magnesium hydroxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 73981.
Property	Value	Unit	Source
Formula (Hill order)	H₂MgO₂
Molar mass	58.3190(44) g/mol	g/mol	IUPAC/CIAAW 2024
Magnesium atoms	1
Oxygen atoms	2
Hydrogen atoms	2
PubChem molecular weight	58.32 g/mol	g/mol	PubChem

Safety

Magnesium hydroxide is essentially harmless in its everyday forms, and the US Environmental Protection Agency’s Safer Choice programme marks it as verified to be of low concern. Of 736 company reports to the European Chemicals Agency, 630 found that it meets no hazard criteria of the Globally Harmonized System (GHS). It irritates the eyes and may injure them, according to Haz-Map, an occupational-health database. The PubChem Laboratory Chemical Safety Summary for magnesium hydroxide (CID 73981) collects these entries.

Questions

What is milk of magnesia made of?

It is magnesium hydroxide suspended in water, an opaque, fairly thick liquid that usually lets water separate out when left standing. HSDB notes that, unlike other magnesium hydroxide products, its solid is made from basic magnesium carbonate, and the fine particles formed keep it a stable slurry. The US government's institute for digestive and kidney diseases (NIDDK) lists it among osmotic agents for constipation.

How does magnesium hydroxide neutralise stomach acid?

The powder stays practically undissolved, according to HSDB, until its hydroxide reacts with hydrochloric acid to give magnesium chloride. A base this poorly soluble can need more than one step, Chem1 explains. In water the reaction runs between H⁺ from the acid and the small amount of OH⁻ released as the base dissolves. Its neutralising capacity is high and its onset rapid, and the stomach empties it slowly, prolonging its action.

Does magnesium hydroxide dissolve in water?

Barely. HSDB gives 0.0009 g dissolving in 100 cm³ of water at 18 °C and 0.004 g at 100 °C, and it is practically insoluble in ethanol too, though dilute acids dissolve it. The little that enters water gives a slightly alkaline reaction, and a slurry of the powder measures pH 9.5 to 10.5. Kept wet, the solid also absorbs carbon dioxide.