Calcium carbonate is a chemical compound with the formula CaCO3. It is found in rocks such as the minerals calcite and aragonite (most notably limestone, a type of sedimentary rock composed primarily of calcite) and is the primary component of eggshells, gastropod shells, shellfish skeletons, and pearls. The active ingredient in agricultural lime is calcium carbonate, which is formed when calcium ions in hard water react with carbonate ions to form limescale. It is used in medicine as a calcium supplement or as an antacid, but excessive consumption can be harmful, causing hypercalcemia and digestive problems.
Preparation
Calcium carbonate occurs naturally in a variety of mineral forms, including pure minerals such as calcite, aragonite, and vaterite, as well as impure minerals such as limestone, chalk, marble, and travertine. It is also the primary chemical constituent of eggshells, seashells, oyster shells, snail shells, corals, and other similar materials.
The vast majority of calcium carbonate used in industry is mined or quarried. Pure calcium carbonate (for food or pharmaceutical applications) can be produced from a pure, quarried source (usually marble).
Calcium carbonate can also be made from calcium oxide. Water is added to produce calcium hydroxide, and then carbon dioxide is passed through the solution to precipitate the desired calcium carbonate, also known as precipitated calcium carbonate (PCC) in the industry. This is known as carbonatation:
CaO + H2O → Ca(OH)2
Ca(OH)2 + CO2 → CaCO3 + H2O
Calcite can also be obtained by adding sodium carbonate to calcium chloride. When too much carbon dioxide is released, calcium hydrogen-carbonate is formed.
CaCl2 + Na2CO3 → CaCO3 + 2NaCl
Properties
It is a white, insoluble powder-like substance found in minerals, chalk, marble, limestone, calcite, shells, and pearls. The molecular weight of the compound is 100.09 amu. The common name of calcium carbonate is limestone. It decomposes to give carbon dioxide when heated up to 1200 K. In a flame test, the calcium compound turns brick red. Hence, calcium carbonate (chalk or limestone) produces a red-orange flame.
When it reacts with dilute acid, it liberates carbon dioxide as a by-product.
CaCO3 + H2SO4 → CaSO4 + H2O +CO2
At 1200K, calcium carbonate decomposes to give carbon dioxide and calcium oxide.
CaCO3 → CaO + CO2
On reacting with dilute acids, calcium carbonate gives carbon dioxide.
CaCO3 + 2HCl → CaCl2 + H2O + CO2
Uses
Calcination of limestone involves the decomposition of carbonate ores and the removal of carbon dioxide. Limestone undergoes calcination with charcoal fires to produce quicklime and has been practised by cultures all over the world since antiquity. The temperature at which limestone produces calcium oxide is commonly stated to be above 840°C.
Calcium carbonate is primarily used in the construction industry, either as a building material or as limestone aggregate for road construction, or as an ingredient in cement.
In a blast furnace, calcium carbonate is also used to purify iron from iron ore.
It is also used in the form of chalk, which has traditionally been a significant component of blackboard chalk.
Calcium carbonate is widely used medicinally as a low-cost dietary calcium supplement for gastric antacids.
Agricultural lime, powdered chalk or limestone is used as a low-cost method of neutralising acidic soil, making it suitable for planting. It is also used in the aquaculture industry to regulate the pH of pond soil before starting culture.
