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Chemical Properties of Magnesium

Magnesium ribbon burns with a bright light in air to the oxide and nitride: Winkler says no nitride is formed unless inactive substances which absorb heat are present. The great brightness of this light makes magnesium useful in flashlights, signalling, and pyrotechny. Burning magnesium gives a continuous spectrum.

Heated magnesium decomposes carbon dioxide (the formation of magnesium carbide may interfere with the reaction), carbon monoxide (this has been denied), sulphur dioxide, hydrogen sulphide, nitrous oxide, nitric oxide, all oxides except those of the alkalies and alkaline earths, some sulphides, and combines directly with the halogens, sulphur, nitrogen, phosphorus, and other elements.

Magnesium does not tarnish in dry air, but in moist air becomes covered lightly with oxide. Distilled water acts on it very slowly, and though it has been suggested that perfectly pure water attacks magnesium until the action is stopped by a coating of oxide, there seems to be no action if dissolved gases are entirely absent. The heated metal reacts readily with steam -

Mg+H2O = MgO+H2.

Magnesium amalgam decomposes water violently, and many magnesium alloys act similarly.

In general, action occurs in systems containing magnesium, water, and an electrolyte. Powdered magnesium reacts with the water of crystallisation of many substances, and the metal decomposes water rapidly in the presence of a drop of platinic chloride or of a little palladious chloride. Magnesium reduces solutions of nitrates, nitrites, and chlorates. It liberates its equivalent of hydrogen from dilute hydrochloric or sulphuric acid; it also evolves hydrogen, more slowly and at very different rates,13 from solutions of many salts - including its own.

Getman concluded that the reaction

Mg+2H2O = Mg(OH)2+H2

is catalytically accelerated by dissolved potassium chloride and other dissolved salts: the reaction is equally rapid with impure and perfectly pure metal. In solutions of hydrolysed salts, the evolution of hydrogen is accelerated if the base is weak, and retarded if it is strong.

Magnesium, according to Getman, acts in four distinguishable ways on aqueous solutions of salts -

  1. Neutral salts function solely as catalysts, and the magnesium forms hydrogen and magnesium hydroxide with the water. A basic salt may result.
  2. When the base is weak, the magnesium reacts first with the acid from the hydrolysis and then with the solvent. In the latter reaction the saline constituents act as catalytic stimulants.
  3. When the acid is weak, the hydrogen-ion concentration is depressed, and the magnesium reacts very slightly, if at all, with the solvent.
  4. When the cation is less electro-positive than the magnesium, the metal of the salt is replaced by the latter. Since such salts usually have an acid reaction, the magnesium generally directly displaces the hydrogen of the acid as well.

Magnesium precipitates its equivalent of pure copper, gold, or platinum from solutions containing no other metals than the alkalies or alkaline earths. Many other metals alloy with the magnesium. Tellurium can be estimated by precipitating it with magnesium, and zinc is also quantitatively precipitated from its acetate in the presence of alkaline earth metals.

The formation of oxides or hydroxides of metals has been noted during their displacement from their salts by magnesium, and it has been suggested that the action of magnesium on salts primarily converts their metal into its oxide or hydroxide.

Nitric oxide, nitrous oxide, and nitrogen are evolved, according to Ac worth and Armstrong, when nitric acid acts upon magnesium. When the acid is mixed with an equal volume of water these gases correspond to 64.23 per cent, of the metal dissolved. They correspond to 4451 per cent, when the acid is mixed with 12 parts of water. Nitric oxide predominates. If some hydrochloric acid is present hydroxylamine is formed, but it decomposes rapidly. Ammonium nitrate is also produced during the action. Much hydrogen is also said to be evolved, and the proportion of ammonia increases with the concentration of the nitric acid till the latter reaches 40 per cent., and then decreases.

Alkaline hydroxides do not act on magnesium; sodium carbonate solution acts on it slightly, and borax solution with considerable vigour.

Concentrated sulphuric acid begins to attack magnesium at 215° C. with the evolution of sulphur dioxide and a trace of hydrogen sulphide. Sulphurous acid converts the metal into its sulphite.

Potassium dichromate inhibits the solution of magnesium in acetic acid: the addition of an alkali or of a magnesium salt of a strong acid destroys the " passivity."

The metal acts on methyl alcohol to form magnesium methoxide, and enters into many important reactions in organic chemistry.

A green unstable colloidal solution of magnesium has been prepared by the electrical method.

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