Magnesium Nitrate, Mg(NO₃)₂

Metallic magnesium, magnesia, and magnesium carbonate dissolve easily in nitric acid to magnesium nitrate. From neutral solutions at ordinary temperatures the hexahydrate, Mg(NO₃)₂.6H₂O, crystallises in monoclinic prisms and needles which do not effloresce over sulphuric acid. Mg(NO₃)₂.9H₂O forms when the solution is cooled to -20° C.; at -17° C. it reverts to the hexahydrate.

The heat of solution of the hexahydrate is -4.200 Cal.; it melts in its water of crystallisation, and with rising temperature loses water and, apparently, nitric acid. The basic salts 3MgO.2N₂O₅.5H₂O and 4MgO.N₂O₅ have been said to result from this double loss.

According to Lescoeur, the hexahydrate gradually dissociates into the trihydrate at 65° C., and the latter also results when a saturated, neutral solution of magnesium nitrate is mixed with an equal volume of fuming nitric acid. Ditte, by heating the hexahydrate, obtained a syrupy mass which suddenly solidified into the trihydrate. He also obtained transparent prisms of the dihydrate by heating the nitrate till nitric oxide began to come off, dissolving in nitric acid, and crystallising. The crystals were very deliquescent.

Magnesia is the final product of heat upon magnesium nitrate: according to Graham, the monohydrate is formed at the temperature of melting lead.

A nearly saturated solution of magnesium nitrate evolves heat on dilution. A saturated aqueous solution of magnesium nitrate at 0° C. contains 39.96 per cent, of the anhydrous salt.

Mg(OH)₂+2HNO₃Aq. = Mg(NO₃)₂Aq.+27.600 Cal.

Didier obtained 3MgO.N₂O₅.5H₂O in slender, white, microscopic needles, which were optically active, by treating a solution of magnesium nitrate at 100° C. with small quantities of magnesium at a time, heating to 150° C., filtering through cloth, and allowing to stand in a closed vessel. It was decomposed by water and alcohol.