Carbonate calcium

The carbonate of calcium (Ca C O ₃) is composed of a Ion carbonate (CO₃^2-) and of an ion calcium (Ca²⁺), its molar mass is of 100 g/mole.

It is the component main thing of the Calcaire and the Craie, but also of the Marbre. It is also the principal one constituting shells of marine animals and snail S.

$Ca^ {2+} + CO_3^ {2} \ rightarrow CaCO_3$

The calcium carbonate crystallizes naturally with two principal crystalline forms: the Aragonite and the Calcite. The two other crystalline forms are rarer: the Vatérite & a Hexahydrate, not very stable intermediate form which evolves to the Calcite. It can be presented, in the processes of crystallization, in the form of Colloïde container of fine crystals of the more stable forms.

The ion Bicarbonate is at the base of the formation of the Carbonate of calcium ( $CaCO_3$ ) which is insoluble in the Eau and is the constituent main thing of the Calcaire:

$Ca^ {2+} + 2 HCO_3^- \ rightarrow CaCO_3 + H_2CO_3$

(

H_2CO_3

is the carbon Dioxide )

On the other hand, into Acid medium, the Carbonate of calcium transforms into Bicarbonate of calcium ( $Ca (HCO_3) _2$ or Ca²⁺, 2 HCO₃⁻ ), very soluble in the Water.

It is thus formally a salt, of a weak base (Ca (OH) ₂ - pKa = 12,6) and of a weak acid (H₂CO₃ - pKa = 6,35), able to react with the acids to give salts of calcium, Eau and Carbon dioxide, which is with the atmospheric pressure and room temperature, gas and volatile:

CaCO₃+2H₃O⁺ → CO₂+3H₂O+Ca²⁺

CaCO₃+2RCOOH → CO₂+H₂O+Ca (RCOO) ₂

From 800 °C, it is formed a strongly endothermic reaction of decomposition:

CaCO₃ → CAD +CO₂

CAD is the Quicklime, which is the subject of an important industry. It is besides for this reaction that calcium carbonates it bears, in certain industries, the name of carbonate of lime , which was the old name before calcium is not isolated.

Chemistry of the natural state

The calcium carbonate is very slightly soluble in pure water (about 15 to 20 mg/l with 25°C), but water soluble charged with Carbonic gas. However the solubility of gases in water increases with the pressure and, contrary to that of the majority of the solids, decreases when the temperature increases. This explains certain natural phenomena.

For example, the calcium carbonate settles in the boilers and on the bottom of the kettles. It also settles on the outlet side of the taps, where the pressure decreases abruptly.

Certain sources, called petrifying , with water very rich in carbonic gas, contain a very great quantity of calcium carbonate. While arriving at the free air, they lose their carbonic gas and deposit carbonate partly dissolves. Exposed objects a few days with the action of this water are encrusted in a layer with calcite.

a similar phenomenon explains the concretions caves. Water, charged with carbonic gas, dissolves the limestone of the rocks which it crosses and, while arriving in contact with the air, hotter, of the cavities, it deposits transported calcite. This one accumulates in stalactites at the places where water is detached from the ceiling or in stalagmites at the places where the water drops fall on the ground.

Biochemistry of the secretion of their shell by molluscs, and of the formation of the skeleton in the animals, is not yet completely included/understood. But it is probable that the content of gases of dissolution plays a central role.

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