Magnesium oxide plus water
Magnesium is present in seawater in amounts of about 1300 ppm. After sodium, it is the most commonly found cation in oceans. Rivers contains approximately 4 ppm of magnesium, marine algae 6000-20, 000 ppm, and oysters 1200 ppm.
Dutch drinking water contains between 1 and 5 mg of magnesium per liter.
Magnesium and other alkali earth metals are responsible for water hardness. Water containing large amounts of alkali earth ions is called hard water, and water containing low amounts of these ions is called soft water.
In what way and in what form does magnesium react with water?
Magnesium metals are not affected by water at room temperature. Magnesium generally is a slow-reacting element, but reactivity increases with oxygen levels. Furthermore, magnesium reacts with water vapor to magnesium hydroxide and hydrogen gas:
Mg (s) + 2H2O(g) -> Mg(OH)2(aq) + H2(g)
Magnesium fires cannot be extinguished by water. Magnesium continues to burn after oxygen is depleted. It than reacts with nitrogen from air to form magnesium nitride (Mg3N2). When attempts are made to extinguish magnesium fires with water, magnesium aggressively reacts with hydrogen gas. To prevent any damage, a magnesium fire must be covered in sand.
An example of a magnesium compound is magnesium phosphide (Mg3P2), an odorous, grey solid. When this compound comes in contact with water or moist air, it is decomposed and phosphine (PH3) is formed. This is a toxic compound, and it is also very flammable in air.
Solubility of magnesium and magnesium compounds
Magnesium is mainly present as Mg2+ (aq) in watery solutions, but also as MgOH+ (aq) and Mg(OH)2 (aq). In seawater it can also be found as MgSO4.
Water solubility of magnesium hydroxide is 12 mg/L. Other magnesium compounds are clearly more water soluble, for example magnesium carbonate (600 mg/L). Magnesium sulphate adds a bitter flavour to water, and has a water solubility of 309 g/L at 10oC.
Why is magnesium present in water?
A large number of minerals contains magnesium, for example dolomite (calcium magnesium carbonate; CaMg(CO3)2) and magnesite (magnesium carbonate; MgCO3). Magnesium is washed from rocks and subsequently ends up in water.
Magnesium has many different purposes and consequently may end up in water in many different ways. Chemical industries add magnesium to plastics and other materials as a fire protection measure or as a filler. It also ends up in the environment from fertilizer application and from cattle feed. Magnesium sulphate is applied in beer breweries, and magnesium hydroxide is applied as a flocculant in wastewater treatment plants. Magnesium is also a mild laxative. Magnesium alloys are applied in car and plane bodies.
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