Why is Magnesium Chloride a Lewis Acid

Magnesium chloride

Crystal structure
__Mg2+ __Cl
Surname Magnesium chloride
other names

E 511

Ratio formula MgCl2
CAS number
  • 7786-30-3 (anhydrous)
  • 7791-18-6 (hexahydrate)
Brief description

colorless, bitter-tasting, deliquescent, hexagonal crystals[1]

Molar mass
  • 95.21 g mol−1 (anhydrous)
  • 203.3 g mol−1 (Hexahydrate)
Physical state



2.32 g cm−3 (20 ° C, anhydrous) [2]

Melting point

708 ° C[2]

boiling point

1412 ° C [2]


well in water (542 g l−1 at 20 ° C) [2]

safety instructions
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions.
Template: Infobox chemical / empirical formula search not possible

Magnesium chloride, MgCl2, is the magnesium salt of hydrochloric acid. It forms several hydrates.


Magnesium chloride occurs naturally in the mineral bischofite (MgCl2 • 6H2O) and as double salt carnallite (KMgCl3 · 6 H.2O) before. Another natural source is sea water. In some salt lakes the concentration of magnesium ions is even higher than that of sodium ions.

Extraction and presentation

The industrial production of magnesium chloride takes place by evaporating the final liquors from the production of potassium chloride. The magnesium chloride hexahydrate (MgCl2 · 6H2O) received. Further evaporation yields a product that is less watery. Anhydrous magnesium chloride is obtained by reacting magnesium oxide with coke and chlorine:

$ \ mathrm {MgO + \ Cl_2 + \ C \ longrightarrow MgCl_2 + \ CO} $

In the laboratory, magnesium chloride can be made from magnesium hydroxide and hydrochloric acid

$ \ mathrm {Mg (OH) _2 + 2 \ HCl \ longrightarrow MgCl_2 + 2 \ H_2O} $

or by reacting magnesium with hydrochloric acid

$ \ mathrm {Mg + 2 \ HCl \ longrightarrow MgCl_2 + H_2} $

be won.


Magnesium chloride is very hygroscopic. Its tendency to hydrolysis is less pronounced than that of aluminum chloride (AlCl3). Anhydrous magnesium chloride crystallizes in CdCl2-Grid type.

Magnesium chloride hexahydrate has a molar mass of 203.3 g · mol−1, a density of 1.57 g · cm−3 and a melting point of approx. 117 ° C (decomposition).[2] The solubility of the hexahydrate is 1700 g / L (at 20 ° C).

The standard enthalpy of formation of magnesium chloride is -642 kJ mol−1.[5]


Magnesium chloride is used to extract elemental magnesium by means of fused-salt electrolysis:[1]

$ \ mathrm {MgCl_2 \ longrightarrow Mg + Cl_2} $

It is used together with magnesium oxide in screed cements.

In food technology, magnesium chloride is used as an acid regulator, firming agent, flavor enhancer, carrier or separating agent. It is number one in the EU as a food additive E 511 without a maximum quantity restriction (quantum satis) for all foodstuffs approved for additives as well as for organic foodstuffs. It is the main component of the coagulant Nigari and is used in the production of tofu. It is still used as a road salt supplement.

Magnesium chloride is also used to increase the magnesium concentration in reef aquariums.

It is also used to bind dust (explosion protection) in hard coal mining.

To bind road dust, it was used experimentally in a lye called anti-dustite.

As Magnesium Oil, a saturated magnesium chloride-water solution with an oily consistency, it is used in concentrated form for transdermal magnesium substitution (absorption through the skin).

Individual evidence

  1. 1,01,1Helmut Sitzmann, in: Roempp Online - Version 3.5, 2009, Georg Thieme Verlag, Stuttgart.
  2. 2,02,12,22,32,42,5Entry to Magnesium chloride in the GESTIS substance database of the IFA, accessed on March 30, 2008 (JavaScript required)
  3. ↑ data sheet Magnesium chloride from Sigma-Aldrich, accessed April 9, 2011.
  4. ↑ Since December 1, 2012, only GHS hazardous substance labeling has been permitted for substances. The R-phrases of this substance may still be used to classify preparations until June 1, 2015, after which the EU hazardous substance labeling is of purely historical interest.
  5. ↑ Entry at www.thieme.de