Magnesium hydride, MgH₂

The hydride is prepared by the action of hydrogen under pressure on heated magnesium powder in presence of magnesium iodide as catalyst:

Mg + H₂ MgH₂

It is a grayish-white solid reacts with water giving hydrogen:

MgH₂ + 2H₂O Mg(OH)₂ + 2H₂

It is not readily decomposed by heat.

Magnesium nitride, Mg₃N₂

The nitride is formed when magnesium is heated to redness in nitrogen or ammonia gas:

3Mg + 2NH₃ Mg₃N₃ + 3H₂

It is a very pale yellow solid which is decomposed by water to give ammonia:

Mg₃N₂ + 5H₂O 3Mg(OH)₂ + 2NH₃

Magnesium oxide, MgO

The oxide is prepared by heating magnesite, MgCO₃, or burning magnesium in oxygen. It is a basic oxide, reacting with acids to give salts. With water, magnesium hydroxide is formed slowly.

Magnesium hydroxide, Mg(OH)₂

The hydroxide is only slightly soluble in water and is precipitated when hydroxide ions are added to a magnesium salt:

Mg²⁺ + 2OH^- Mg(OH)₂

Magnesium carbonate, MgCO₃

The carbonate occurs naturally as magnesite. When solutions containing magnesium and carbonate ions are mixed, a white precipitate is obtained; this is a basic carbonate and can have a precipitate is obtained; this is a basic carbonate is obtained if the reaction is carried out in the presence of carbon dioxide, or by the addition of sodium bicarbonate to a solution of a magnesium salt. No precipitate is obtained if ammonium carbonate is added to a solution of a magnesium salt in presence of ammonium chloride and ammonium hydroxide classical analysis table.

Magnesium carbonate is fairly insoluble in water, but if carbon dioxide is bubbled through a suspension in water, the soluble bicarbonate is formed:

MgCO₃ + H₂O + CO₂ Mg²⁺ + 2HCO₃^-

Magnesium sulphate, MgSO₄

The sulphate is prepared by the usual methods for sulphates and crystallizes as MgSO₄.7H₂O. On heating, the hydrate water is first lost and then sulphur trioxide, leaving finally the oxide. This reaction shows the weaker basic character of magnesium compared with the alkali metals.

Magnesium fluoride, MgF₂

The fluoride is sparingly soluble in water (the fluorides of Group IIA metals are insoluble).

Magnesium chloride, MgCl₂

The chloride forms several hydrates. When a solution is crystallized at room temperature the hexahydrate, MgCl₂.6H₂O, is obtained. This deliquesces in air and is soluble in water. Hydrated magnesium chloride decomposes on heating to give basic salts, the simplest of which is Mg(OH)Cl:

MgCl₂.6H₂O Mg(OH)Cl + HCl + 5H₂O

Magnesium bromide, MgBr, and magnesium iodide, MgI₂

These are similar to the chloride, but dissolve in some organic solvents.

Magnesium ammonium phosphate, MgNH₄PO₄.6H₂O

Magnesium ammonium phosphate is formed as a white crystalline precipitate when a solution of disodium hydrogen phosphate, Na₂HPO₄, is added to a solution of a magnesium salt containing ammonium chloride and ammonia:

6H₂O + Mg²⁺ + NH₄⁺ + PO₄^3- MgNH₄PO₄.6H₂O

The formation of this precipitate is used as a test for magnesium in qualitative analysis (Group VI) after all the other metals (except the alkali metals) have been removed, for most metals give insoluble phosphates. The same reaction may be used to estimate magnesium gravimetrically. The precipitate is filtered off and strongly heated in a crucible, when it decomposes:

2MgNH₄PO₄.6H₂O Mg₂P₂O₇ + 13H₂O + 2NH₃

The pyrophosphate is cooled and weighed.

Magnesium perchlorate, MgClO₄

The perchlorate is an excellent drying agent (‘anhydrone’) for use in a desiccators, because it can take up almost half its own weight of water; the anhydrous salt can be obtained by heating.

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