Magnesium, Mg²⁺

Most common oxidation state: +2

M.P. 650^o

B.P. 1120^o

Density 1.74 g/cm³

Characteristics: This silvery metal is quite active, reacting slowly with boiling (but not cold) water to give hydrogen and the rather insoluble magnesium hydroxide, Mg(OH)₂. It combines easily with oxygen and at high temperatures reacts with such nonmetals as the halogens, sulfur, and even nitrogen.

Characteristic reactions of Mg²⁺:

Magnesium ion rarely forms complex ions. All salts are white; most are soluble in water.

Aqueous Ammonia:

Aqueous ammonia precipitates white gelatinous Mg(OH)₂:

Ammonium salts dissolve Mg(OH)₂ or prevent its precipitation, when added to aqueous ammonia. This is a buffer effect and shifts the pH to a lower value, causing a shift of the precipitation equilibrium to the left.

Sodium Hydroxide:

Sodium hydroxide gives the same precipitate as aqueous ammonia:

Sodium Monohydrogen Phosphate:

Na₂HPO₄ gives a characteristic crystalline precipitate in an ammonia-ammonium chloride buffer.

Magnesium Reagent:

Solid magnesium hydroxide forms a blue "lake" with a dilute solution of 4-(p-nitrophenylazo)resorcinol (magnesium reagent).

No Reaction:

Cl^-, SO₄^2-

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