Magnesium Chloride

Preparation

(i) From carnallite: The main source of magnesium chloride is carnallite (KCl.MgCl₂.6H₂O). The mineral is powdered and then boiled with water. Upon cooling, KCl crystallizes but white MgCl2 remains in the mother liquor. It is concentrated and upon cooling gives crystals of MgCl₂.6H₂O.

(ii) From sea water: Sea water also contains magnesium chloride. Sea water is concentrated and then treated with lime when magnesium hydroxide gets precipitated.

MgCl₂ + Ca(OH)₂ →  Mg(OH)₂↓ + CaCl₂

(sea water)(ppt.)

The precipitate is filtered and dissolved in HCl. Upon concentration and cooling, crystals of MgCl₂.6H₂O separate out.

(iii) Laboratory preparation: In the laboratory, magnesium chloride is prepared by the action of hydrochloric acid on magnesium oxide or carbonate.

MgO + 2HCl  →  MgCl₂ + H₂O

MgCO₃ + 2HCl →  MgCl₂ + CO₂ + H₂O

The reaction mixture is cooled and concentrated when crystals of MgCl₂.6H₂O separate out.

Properties

(i) It is a deliquescent white crystalline solid. It is very soluble in water.

(ii) It decomposes on heating to give magnesium oxide.

MgCl₂.6H₂O MgO + 5H₂O + 2HCl

(iii) When a current of dry hydrogen chloride is passed, it loses its water of crystallization to give anhydrous magnesium chloride.

MgCl₂.6H₂O MgCl₂ + 6H₂O

(iv) When a saturated solution of magnesium chloride is added to magnesium oxide, it sets into a hard mass called magnesia cement or sorel cement.

MgCl₂.6H₂O + 5MgO + (x − 6) H2O  → MgCl₂.5MgO.xH₂O

Magnesia cement or Sorel cement

Uses

(i) Magnesium chloride is chiefly used for the manufacture of magnesia cement which resembles marble and is used for making synthetic tiles.

(ii) Anhydrous magnesium chloride is used in the electrolytic extraction of magnesium.