Magnesium chloride

Mg 1
Cl 2

3 atoms in one formula unit of an ionic lattice.

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

IUPAC name: magnesium dichloride
Molar mass: 95.205(22) g/mol
Atoms: 3
SMILES: [Mg+2].[Cl-].[Cl-]
InChIKey: TWRXJAOTZQYOKJ-UHFFFAOYSA-L
PubChem CID: 5360315

Made of

Sea salt is 17 per cent magnesium chloride by weight, on Haz-Map’s figure, and salt lakes and natural brines hold it too. Its IUPAC name, magnesium dichloride, spells out how magnesium and chlorine combine: one magnesium ion and two chloride ions, as MeSH, the Medical Subject Headings index, puts it. The dry salt forms colourless, odourless flakes or crystals that are very deliquescent, drawing water out of the air until they dissolve in it.

At 20 °C, 100 g of water takes up 54.6 g of it, by the figure of HSDB, the Hazardous Substances Data Bank. Roads put it to work twice. On gravel the salt pulls in moisture that holds dust down, and in winter its brine stays liquid far below the freezing point of water. De-icing is its primary market, in the account of the US Geological Survey (USGS); it also goes into cement, food and medicine.

Structure and bonding

Three separate pieces make up the drawing: one Mg²⁺ ion and two Cl⁻ ions, with no line between them. Their charges cancel, and the trio is one formula unit, the ratio in which the ions build the solid. In the crystal no trio stands alone: the ions form a lattice held by the attraction of opposite charges, as chemical bonding sets out.

The drawing leaves out water as well. The hexahydrate, MgCl₂·6H₂O, carries six water molecules per formula unit and occurs in nature as the mineral bischofite; the anhydrous salt is the mineral chloromagnesite. The salt’s crystals are lustrous and hexagonal, in HSDB’s description. Heated, the hydrate loses two water molecules at 100 °C, by HSDB’s account, and starts to give off some hydrogen chloride at 110 °C.

In water each Mg²⁺ ion holds its nearest water molecules by ion–dipole attraction, a charged ion’s pull on the oppositely charged end of a polar water molecule. How many water molecules fill an ion’s first shell depends on its radius and charge, Chem1 explains. The positive field of Mg²⁺ also makes its nearest water molecules more acidic, so a magnesium chloride solution is faintly acidic.

Where it turns up

Industrial supplies include the mineral carnallite and the end brines of the potash industry, in HSDB’s list, and titanium manufacture yields the salt as a by-product. The seawater route to it is told on the magnesium page. For ice control in the United States, the principal source is Great Salt Lake brine, according to the Federal Highway Administration (FHWA). A Utah smelter once drew magnesium metal from the same lake’s brines by electrolysis, but the USGS estimates that it stopped in 2022. Israel supplied 56 per cent of US imports of the salt from 2020 to 2023.

Magnesium oxychloride cement pairs caustic-calcined magnesia, a magnesium oxide powder, with a concentrated magnesium chloride solution. Limited water resistance has kept it to niche indoor uses, the USGS reports. In medicine it serves as a source of magnesium ions, MeSH notes.

Why it matters

Before a storm, highway crews often spray liquid magnesium chloride on pavement to lower the freezing point of water, the US Environmental Protection Agency (EPA) notes. FHWA puts its ice-melting capacity about 40 per cent above calcium chloride’s. The EPA finds 17 to 56 per cent more chloride ion in it, weight for weight, than in other salt-type de-icers. Environmentally it counts as safer than sodium chloride, the EPA notes, but twice as much is needed, raising the cost.

Concrete can be damaged. In hardened cement paste, magnesium chloride from de-icers reacts with calcium hydroxide and calcium silicate hydrate to form calcium chloride, FHWA’s tech brief explains. The calcium chloride page traces what happens next. In tests the brief cites, mortar soaked in magnesium chloride cracked at the surface, unlike mortar soaked in sodium chloride or lime water.

Metal is at risk as well. In humid air the salt makes steel rust very quickly, HSDB warns. After inspections found corrosion in airtankers, the US Forest Service suspended aerial fire retardants based on magnesium chloride for 2024, the USGS reports.

Properties

Properties of magnesium chloride Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 5360315.
Property	Value	Unit	Source
Formula (Hill order)	Cl₂Mg
Molar mass	95.205(22) g/mol	g/mol	IUPAC/CIAAW 2024
Magnesium atoms	1
Chlorine atoms	2
PubChem molecular weight	95.21 g/mol	g/mol	PubChem

Safety

In US food law magnesium chloride is affirmed as generally recognised as safe, yet its hazard classification carries the signal word Danger. Of 1,600 reports to the European Chemicals Agency, 60.6% find it meets no hazard criteria; of the 631 giving hazards, 37.6% list skin irritation and 27.6% serious eye damage. The International Chemical Safety Cards call it mildly irritating to the eyes and airways, and in a fire it gives off irritating or toxic fumes. The PubChem Laboratory Chemical Safety Summary for magnesium chloride sets out the full classification and exposure guidance.

Questions

How cold can magnesium chloride melt ice?

Though it is sold as flakes, it goes on roads as a liquid, the Federal Highway Administration's anti-icing manual notes. The manual puts the eutectic of magnesium chloride and water, the lowest temperature at which their mixture stays liquid, at about −33 °C. The US Environmental Protection Agency counts it effective down to −13 °F, about −25 °C.

Why is magnesium chloride put on dirt roads?

It holds down dust. Spread on gravel, it absorbs small amounts of water from the air; the moisture gathers fine particles into clumps, and suction holds the surface together, the Federal Highway Administration's dust handbook explains. The handbook found it the treatment of choice in many areas, for its record, cost-effectiveness, availability and ease of application, amid growing concern about possible long-term environmental effects of yearly use.

What does magnesium chloride do in food?

It sets tofu, flavours food and supplies magnesium. Nigari, the traditional Japanese agent that curdles soy milk into tofu, is what seawater leaves after its salt crystallises out, magnesium chloride and magnesium sulfate among it. A technical report for the US Department of Agriculture explains that magnesium acts on the soy protein. US federal regulations, at 21 CFR 184.1426, affirm magnesium chloride as generally recognised as safe, and JECFA, the international food-additive committee, classes it as a firming agent.