What does pure calcium look like? Calcium and its characteristics

Calcium compounds- limestone, marble, gypsum (as well as lime - a product of limestone) have been used in construction since ancient times. Until the end of the 18th century, chemists considered lime to be a simple substance. In 1789, A. Lavoisier suggested that lime, magnesia, barite, alumina and silica are complex substances. In 1808, Davy, subjecting a mixture of wet slaked lime with mercury oxide to electrolysis with a mercury cathode, prepared a calcium amalgam, and after driving mercury out of it, he obtained a metal called "calcium" (from lat. Calx, genus. case calcis - lime).

Arrangement of electrons in orbits.

+20Ca… |3s 3p 3d | 4s

Calcium is called an alkaline earth metal, it is classified as an S element. At the external electronic level, calcium has two electrons, so it gives compounds: CaO, Ca (OH) 2, CaCl2, CaSO4, CaCO3, etc. Calcium belongs to typical metals - it has a high affinity for oxygen, reduces almost all metals from their oxides, and forms a fairly strong base Ca (OH) 2.

The crystal lattices of metals can be of various types, however, calcium is characterized by a face-centered cubic lattice.

The sizes, shape and mutual arrangement of crystals in metals are emitted by metallographic methods. The most complete assessment of the metal structure in this respect is given by microscopic analysis of its thin section. A sample is cut out of the metal under test, and its plane is ground, polished and etched with a special solution (etchant). As a result of etching, the structure of the sample is highlighted, which is examined or photographed using a metallographic microscope.

Calcium is a light metal (d = 1.55), silver-white in color. It is harder and melts at a higher temperature (851°C) than sodium, which is next to it in the periodic table. This is because there are two electrons per calcium ion in the metal. Therefore, the chemical bond between ions and electron gas is stronger than that of sodium. In chemical reactions, calcium valence electrons are transferred to atoms of other elements. In this case, doubly charged ions are formed.

Calcium is highly reactive with metals, especially with oxygen. In air, it oxidizes more slowly than alkali metals, since the oxide film on it is less permeable to oxygen. When heated, calcium burns with the release of huge amounts of heat:

Calcium reacts with water, displacing hydrogen from it and forming a base:

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

Owing to its great reactivity with oxygen, calcium finds some use in obtaining rare metals from their oxides. Metal oxides are heated together with calcium chips; as a result of the reactions, calcium oxide and a metal are obtained. The use of calcium and some of its alloys for the so-called deoxidation of metals is based on the same property. Calcium is added to molten metal and it removes traces of dissolved oxygen; the resulting calcium oxide floats to the surface of the metal. Calcium is part of some alloys.

Calcium is obtained by electrolysis of molten calcium chloride or by the aluminothermic method. Calcium oxide, or slaked lime, is a white powder that melts at 2570°C. It is obtained by calcining limestone:

CaCO3 \u003d CaO + CO2 ^

Calcium oxide is a basic oxide, so it reacts with acids and acid anhydrides. With water, it gives a base - calcium hydroxide:

CaO + H2O = Ca(OH)2

The addition of water to calcium oxide, called lime slaking, proceeds with the release of a large amount of heat. Part of the water is converted into steam. Calcium hydroxide, or slaked lime, is a white substance, slightly soluble in water. An aqueous solution of calcium hydroxide is called lime water. Such a solution has rather strong alkaline properties, since calcium hydroxide dissociates well:

Ca (OH) 2 \u003d Ca + 2OH

Compared to hydrates of alkali metal oxides, calcium hydroxide is a weaker base. This is explained by the fact that the calcium ion is doubly charged and more strongly attracts hydroxyl groups.

Hydrated lime and its solution, called lime water, react with acids and acid anhydrides, including carbon dioxide. Lime water is used in laboratories to discover carbon dioxide, since the resulting insoluble calcium carbonate causes the water to become cloudy:

Ca + 2OH + CO2 = CaCO3v + H2O

However, when carbon dioxide is passed for a long time, the solution becomes transparent again. This is due to the fact that calcium carbonate is converted into a soluble salt - calcium bicarbonate:

CaCO3 + CO2 + H2O = Ca(HCO3)2

In industry, calcium is obtained in two ways:

By heating a briquetted mixture of CaO and Al powder at 1200 ° C in a vacuum of 0.01 - 0.02 mm. rt. Art.; released by the reaction:

6CaO + 2Al = 3CaO Al2O3 + 3Ca

Calcium vapor condenses on a cold surface.

By electrolysis of a melt of CaCl2 and KCl with a liquid copper-calcium cathode, an alloy of Cu - Ca (65% Ca) is prepared, from which calcium is distilled off at a temperature of 950 - 1000 ° C in a vacuum of 0.1 - 0.001 mm Hg.

A method has also been developed for obtaining calcium by thermal dissociation of calcium carbide CaC2.

Calcium is one of the most abundant elements in nature. It contains approximately 3% (mass) in the earth's crust. Calcium salts form in nature large accumulations in the form of carbonates (chalk, marble), sulfates (gypsum), phosphates (phosphorites). Under the action of water and carbon dioxide, carbonates pass into solution in the form of hydrocarbons and are transported by underground and river waters over long distances. When calcium salts are washed out, caves can form. Due to the evaporation of water or an increase in temperature, deposits of calcium carbonate can form in a new place. So, for example, stalactites and stalagmites are formed in caves.

Soluble calcium and magnesium salts determine the overall hardness of water. If they are present in water in small quantities, then the water is called soft. With a high content of these salts (100 - 200 mg of calcium salts - in 1 liter in terms of ions), water is considered hard. In such water, soap foams poorly, since calcium and magnesium salts form insoluble compounds with it. In hard water, food products are poorly boiled, and when boiled, it gives scale on the walls of steam boilers. Scale does not conduct heat well, causes an increase in fuel consumption and accelerates the wear of the boiler walls. Scale formation is a complex process. When heated, the acid salts of calcium and magnesium carbonic acid decompose and turn into insoluble carbonates:

Ca + 2HCO3 = H2O + CO2 + CaCO3v

The solubility of calcium sulfate CaSO4 also decreases when heated, so it is part of the scale.

The hardness caused by the presence of calcium and magnesium bicarbonates in water is called carbonate or temporary, since it is eliminated by boiling. In addition to carbonate hardness, non-carbonate hardness is also distinguished, which depends on the content of sulfates and chlorides of calcium and magnesium in the water. These salts are not removed by boiling, and therefore non-carbonate hardness is also called constant hardness. Carbonate and non-carbonate hardness add up to total hardness.

To completely eliminate hardness, water is sometimes distilled. Boil water to remove carbonate hardness. General hardness is eliminated either by adding chemicals or by using so-called cation exchangers. When using the chemical method, soluble calcium and magnesium salts are converted into insoluble carbonates, for example, milk of lime and soda are added:

Ca + 2HCO3 + Ca + 2OH = 2H2O + 2CaCO3v

Ca + SO4 + 2Na + CO3 = 2Na + SO4 + CaCO3v

Removing stiffness with cation exchangers is a more advanced process. Cation exchangers are complex substances (natural compounds of silicon and aluminum, high molecular weight organic compounds), the composition of which can be expressed by the formula Na2R, where R is a complex acid residue. When water is filtered through a layer of cation exchanger, Na ions (cations) are exchanged for Ca and Mg ions:

Ca + Na2R = 2Na + CaR

Consequently, Ca ions from the solution pass into the cation exchanger, and Na ions pass from the cation exchanger into the solution. To restore the used cation exchanger, it is washed with a solution of common salt. In this case, the reverse process occurs: Ca ions in the cation exchanger are replaced by Na ions:

2Na + 2Cl + CaR = Na2R + Ca + 2Cl

The regenerated cation exchanger can be used again for water treatment.

In the form of a pure metal, Ca is used as a reducing agent for U, Th, Cr, V, Zr, Cs, Rb and some rare earth metals and their compounds. It is also used for the deoxidation of steels, bronzes and other alloys, for the removal of sulfur from petroleum products, for the dehydration of organic liquids, for the purification of argon from nitrogen impurities, and as a gas absorber in electric vacuum devices. Antifiction materials of the Pb - Na - Ca system, as well as Pb - Ca alloys, which are used to make the sheath of electric cables, have received great use in technology. Alloy Ca - Si - Ca (silicocalcium) is used as a deoxidizer and degasser in the production of quality steels.

Calcium is one of the biogenic elements necessary for the normal course of life processes. It is present in all tissues and fluids of animals and plants. Only rare organisms can develop in an environment devoid of Ca. In some organisms, the content of Ca reaches 38%: in humans - 1.4 - 2%. Cells of plant and animal organisms need strictly defined ratios of Ca, Na and K ions in extracellular media. Plants get Ca from the soil. According to their relation to Ca, plants are divided into calcephiles and calcephobes. Animals get Ca from food and water. Ca is necessary for the formation of a number of cellular structures, maintaining the normal permeability of outer cell membranes, for fertilizing the eggs of fish and other animals, and activating a number of enzymes. Ca ions transmit excitation to the muscle fiber, causing its contraction, increase the strength of heart contractions, increase the phagocytic function of leukocytes, activate the system of protective blood proteins, and participate in its coagulation. In cells, almost all Ca is in the form of compounds with proteins, nucleic acids, phospholipids, in complexes with inorganic phosphates and organic acids. In the blood plasma of humans and higher animals, only 20-40% Ca can be associated with proteins. In animals with a skeleton, up to 97 - 99% of all Ca is used as a building material: in invertebrates, mainly in the form of CaCO3 (mollusc shells, corals), in vertebrates, in the form of phosphates. Many invertebrates store Ca before molting to build a new skeleton or to provide vital functions in adverse conditions. The content of Ca in the blood of humans and higher animals is regulated by the hormones of the parathyroid and thyroid glands. Vitamin D plays the most important role in these processes. Ca absorption occurs in the anterior part of the small intestine. Assimilation of Ca worsens with a decrease in acidity in the intestine and depends on the ratio of Ca, phosphorus and fat in food. The optimal Ca/P ratios in cow's milk are about 1.3 (in potatoes 0.15, in beans 0.13, in meat 0.016). With an excess of P and oxalic acid in food, the absorption of Ca worsens. Bile acids accelerate its absorption. The optimal ratio of Ca/fat in human food is 0.04 - 0.08 g of Ca per 1 g. fat. Excretion of Ca occurs mainly through the intestines. Mammals during lactation lose a lot of Ca with milk. With violations of phosphorus-calcium metabolism in young animals and children, rickets develop, in adult animals - a change in the composition and structure of the skeleton (osteomalacia).

In medicine, Ca drugs eliminate disorders associated with a lack of Ca ions in the body (with tetany, spasmophilia, rickets). Ca preparations reduce hypersensitivity to allergens and are used to treat allergic diseases (serum sickness, sleeping fever, etc.). Ca preparations reduce increased vascular permeability and have an anti-inflammatory effect. They are used for hemorrhagic vasculitis, radiation sickness, inflammatory processes (pneumonia, pleurisy, etc.) and some skin diseases. It is prescribed as a hemostatic agent, to improve the activity of the heart muscle and enhance the effect of digitalis preparations, as an antidote for poisoning with magnesium salts. Together with other drugs, Ca preparations are used to stimulate labor. Ca chloride is administered by mouth and intravenously. Ossocalcinol (15% sterile suspension of specially prepared bone powder in peach oil) has been proposed for tissue therapy.

Ca preparations also include gypsum (CaSO4), used in surgery for plaster casts, and chalk (CaCO3), administered orally with increased acidity of gastric juice and for the preparation of tooth powder.

CALCIUM (Latin Calcium), Ca, a chemical element of group II of the short form (2nd group of the long form) of the periodic system; refers to alkaline earth metals; atomic number 20; atomic mass 40.078. In nature, there are 6 stable isotopes: 40 Ca (96.941%), 42 Ca (0.647%), 43 Ca (0.135%), 44 Ca (2.086%), 46 Ca (0.004%), 48 Ca (0.187%); artificially obtained radioisotopes with mass numbers 34-54.

History reference. Many natural calcium compounds were known in ancient times and were widely used in construction (for example, gypsum, lime, marble). Metallic calcium was first isolated by G. Davy in 1808 during the electrolysis of a mixture of CaO and HgO oxides and subsequent decomposition of the formed calcium amalgam. The name comes from the Latin calx (genitive calcis) - lime, soft stone.

Distribution in nature. The calcium content in the earth's crust is 3.38% by mass. Due to its high chemical activity, it does not occur in the free state. The most common minerals are anorthite Ca, anhydrite CaSO 4, apatite Ca 5 (PO 4) 3 (F, Cl, OH), gypsum CaSO 4 2H 2 O, calcite and aragonite CaCO 3, perovskite CaTiO 3, fluorite CaF 2, scheelite CaWO 4 . Calcium minerals are part of sedimentary (for example, limestone), igneous and metamorphic rocks. Calcium compounds are found in living organisms: they are the main components of bone tissues of vertebrates (hydroxyapatite, fluorapatite), coral skeletons, mollusk shells (calcium carbonate and phosphates), etc. The presence of Ca 2+ ions determines the hardness of water.

Properties. The configuration of the outer electron shell of the calcium atom is 4s 2 ; in compounds it exhibits an oxidation state of +2, rarely +1; Pauling electronegativity 1.00, atomic radius 180 pm, Ca 2+ ion radius 114 pm (coordination number 6). calcium is a silvery-white soft metal; up to 443 °С, the modification with a cubic face-centered crystal lattice is stable, above 443 °С - with a cubic body-centered lattice; t pl 842°С, t kip 1484 °С, density 1550 kg/m3; thermal conductivity 125.6 W/(m K).

Calcium is a metal of high chemical activity (stored in hermetically sealed vessels or under a layer of mineral oil). Under normal conditions, it easily interacts with oxygen (calcium oxide CaO is formed), when heated - with hydrogen (CaH 2 hydride), halogens (calcium halides), boron (CaB 6 boride), carbon (calcium carbide CaC 2), silicon (Ca silicides 2 Si, CaSi, CaSi 2, Ca 3 Si 4), nitrogen (Ca 3 N 2 nitride), phosphorus (Ca 3 P 2, CaP, CaP 5 phosphides), chalcogens (CaX chalcogenides, where X is S, Se, Those). Calcium interacts with other metals (Li, Cu, Ag, Au, Mg, Zn, Al, Pb, Sn, etc.) to form intermetallic compounds. Metallic calcium reacts with water to form calcium hydroxide Ca(OH) 2 and H 2 . Vigorously interacts with most acids, forming the corresponding salts (for example, calcium nitrate, calcium sulfate, calcium phosphates). It dissolves in liquid ammonia to form a dark blue solution with metallic conductivity. When ammonia evaporates, ammonia is released from such a solution. Gradually, calcium reacts with ammonia to form the amide Ca(NH 2) 2 . It forms various complex compounds, complexes with oxygen-containing polydentate ligands, for example, Ca complexonates, are of the greatest importance.

Biological role. Calcium refers to biogenic elements. The daily human need for calcium is about 1 g. In living organisms, calcium ions are involved in the processes of muscle contraction and the transmission of nerve impulses.

Receipt. Calcium metal is obtained by electrolytic and metallothermic methods. The electrolytic method is based on the electrolysis of molten calcium chloride with a touch cathode or a liquid copper-calcium cathode. Calcium is distilled off from the resulting copper-calcium alloy at a temperature of 1000-1080 °C and a pressure of 13-20 kPa. The metallothermic method is based on the reduction of calcium from its oxide with aluminum or silicon at 1100-1200 °C. This produces aluminate or calcium silicate, as well as gaseous calcium, which is then condensed. World production of calcium compounds and materials containing calcium, about 1 billion tons/year (1998).

Application. Calcium is used as a reducing agent in the production of many metals (Rb, Cs, Zr, Hf, V, etc.). Calcium silicides, as well as calcium alloys with sodium, zinc and other metals, are used as deoxidizers and desulfurizers for some alloys and oil, for purifying argon from oxygen and nitrogen, and as a gas absorber in vacuum devices. CaCl 2 chloride is used as a drying agent in chemical synthesis, gypsum is used in medicine. Calcium silicates are the main components of cement.

Lit .: Rodyakin VV Calcium, its compounds and alloys. M., 1967; Spitsyn V.I., Martynenko L.I. Inorganic chemistry. M., 1994. Part 2; Inorganic Chemistry / Edited by Yu. D. Tretyakov. M., 2004. T. 2.

L. N. Komissarova, M. A. Ryumin.

Calcium (Latin Calcium, denoted by the symbol Ca) is an element with atomic number 20 and atomic mass 40.078. It is an element of the main subgroup of the second group, the fourth period of the periodic table of chemical elements of Dmitry Ivanovich Mendeleev. Under normal conditions, a simple substance calcium is a light (1.54 g / cm3) malleable, soft, reactive alkaline earth metal of a silvery white color.

In nature, calcium is presented as a mixture of six isotopes: 40Ca (96.97%), 42Ca (0.64%), 43Ca (0.145%), 44Ca (2.06%), 46Ca (0.0033%) and 48Ca ( 0.185%. The main isotope of the twentieth element - the most common - is 40Ca, its isotopic abundance is about 97%. Of the six natural calcium isotopes, five are stable, the sixth isotope 48Ca, the heaviest of the six and quite rare (its isotopic abundance is only 0.185%), has recently been found to undergo double β-decay with a half-life of 5.3∙1019 years. Artificially produced isotopes with mass numbers 39, 41, 45, 47 and 49 are radioactive. Most often, they are used as an isotope tracer in the study of mineral metabolism processes in a living organism. 45Ca, obtained by irradiating metallic calcium or its compounds with neutrons in a uranium reactor, plays an important role in studying the metabolic processes occurring in soils and in studying the processes of calcium assimilation by plants. Thanks to the same isotope, it was possible to detect sources of contamination of various grades of steel and ultrapure iron with calcium compounds during the smelting process.

Calcium compounds - marble, gypsum, limestone and lime (limestone roasting product) have been known since ancient times and were widely used in construction and medicine. The ancient Egyptians used calcium compounds in the construction of their pyramids, and the inhabitants of the great Rome invented concrete - using a mixture of crushed stone, lime and sand. Until the very end of the 18th century, chemists were convinced that lime was a simple body. Only in 1789 did Lavoisier suggest that lime, alumina and some other compounds are complex substances. In 1808, metallic calcium was obtained by G. Davy by electrolysis.

The use of metallic calcium is associated with its high chemical activity. It is used to recover from compounds of certain metals, for example, thorium, uranium, chromium, zirconium, cesium, rubidium; for removal from steel and from some other alloys of oxygen, sulfur; for dehydration of organic liquids; for absorption of the remains of gases in vacuum devices. In addition, metallic calcium serves as an alloying component of some alloys. Calcium compounds are much more widely used - they are used in construction, pyrotechnics, glass production, medicine and many other areas.

Calcium is one of the most important biogenic elements; it is necessary for most living organisms for the normal course of life processes. The body of an adult contains up to one and a half kilograms of calcium. It is present in all tissues and fluids of living organisms. The twentieth element is necessary for the formation of bone tissue, maintaining a heart rhythm, blood clotting, maintaining normal permeability of outer cell membranes, and the formation of a number of enzymes. The list of functions that calcium performs in plant and animal organisms is very large. Suffice it to say that only rare organisms are able to develop in an environment devoid of calcium, while other organisms are 38% composed of this element (the human body contains only about 2% calcium).

Biological properties

Calcium is one of the biogenic elements, its compounds are found in almost all living organisms (few organisms are able to develop in an environment devoid of calcium), ensuring the normal course of life processes. The twentieth element is present in all tissues and fluids of animals and plants, most of it (in vertebrate organisms - including humans) is found in the skeleton and teeth in the form of phosphates (for example, hydroxyapatite Ca5 (PO4) 3OH or 3Ca3 (PO4) 2 Ca (OH)2). The use of the twentieth element as a building material for bones and teeth is due to the fact that calcium ions are not used in the cell. The concentration of calcium is controlled by special hormones, their combined action preserves and maintains the structure of the bones. The skeletons of most groups of invertebrates (mollusks, corals, sponges, and others) are built from various forms of calcium carbonate CaCO3 (lime). Many invertebrates store calcium before molting to build a new skeleton or to provide vital functions in adverse conditions. Animals receive calcium from food and water, and plants from the soil and in relation to this element are divided into calcephiles and calcephobes.

The ions of this important trace element are involved in the processes of blood coagulation, as well as in ensuring a constant osmotic pressure of the blood. In addition, calcium is necessary for the formation of a number of cellular structures, maintaining the normal permeability of outer cell membranes, for fertilizing the eggs of fish and other animals, and activating a number of enzymes (perhaps this circumstance is due to the fact that calcium replaces magnesium ions). Calcium ions transmit excitation to the muscle fiber, causing it to contract, increase the strength of heart contractions, increase the phagocytic function of leukocytes, activate the system of protective blood proteins, regulate exocytosis, including the secretion of hormones and neurotransmitters. Calcium affects the patency of blood vessels - without this element, fats, lipids and cholesterol would settle on the walls of blood vessels. Calcium promotes the excretion of salts of heavy metals and radionuclides from the body, performs antioxidant functions. Calcium affects the reproductive system, has an anti-stress effect and has an anti-allergic effect.

The content of calcium in the body of an adult (weighing 70 kg) is 1.7 kg (mainly in the composition of the intercellular substance of bone tissue). The need for this element depends on age: for adults, the required daily allowance is from 800 to 1,000 milligrams, for children from 600 to 900 milligrams. For children, it is especially important to consume the required dose for intensive growth and development of bones. The main source of calcium in the body is milk and dairy products, the rest of calcium comes from meat, fish, and some plant products (especially legumes). The absorption of calcium cations occurs in the large and small intestines, the absorption is facilitated by an acidic environment, vitamins C and D, lactose (lactic acid), and unsaturated fatty acids. In turn, aspirin, oxalic acid, estrogen derivatives significantly reduce the absorption of the twentieth element. So, combining with oxalic acid, calcium gives water-insoluble compounds that are components of kidney stones. The role of magnesium in calcium metabolism is great - with its deficiency, calcium is “washed out” of the bones and deposited in the kidneys (kidney stones) and muscles. In general, there is a complex system of storage and release of the twentieth element in the body, for this reason the calcium content in the blood is precisely regulated, and with proper nutrition, there is no deficiency or excess. Long-term calcium diet can cause cramps, joint pain, constipation, fatigue, drowsiness, growth retardation. Prolonged lack of calcium in the diet leads to the development of osteoporosis. Nicotine, caffeine and alcohol are some of the reasons for the lack of calcium in the body, as they contribute to its intensive excretion in the urine. However, an excess of the twentieth element (or vitamin D) leads to negative consequences - hypercalcemia develops, the consequence of which is intense calcification of bones and tissues (mainly affects the urinary system). Long-term calcium surplus disrupts the functioning of muscle and nerve tissues, increases blood clotting and reduces the absorption of zinc by bone cells. Perhaps the appearance of osteoarthritis, cataracts, problems with blood pressure. From the foregoing, we can conclude that the cells of plant and animal organisms need strictly defined ratios of calcium ions.

In pharmacology and medicine, calcium compounds are used for the manufacture of vitamins, tablets, pills, injections, antibiotics, as well as for the manufacture of ampoules and medical utensils.

It turns out that a fairly common cause of male infertility is a lack of calcium in the body! The fact is that the head of the spermatozoon has an arrow-shaped formation, which consists entirely of calcium, with a sufficient amount of this element, the spermatozoon is able to overcome the membrane and fertilize the egg, with insufficient infertility occurs.

American scientists have found that the lack of calcium ions in the blood leads to a weakening of memory and a decrease in intelligence. For example, from the well-known US journal Science News, it became known about experiments that confirmed that cats develop a conditioned reflex only if their brain cells contain more calcium than blood.

The calcium cyanamide compound, highly valued in agriculture, is used not only as a nitrogen fertilizer and a source of obtaining urea - the most valuable fertilizer and raw material for the production of synthetic resins, but also as a substance with which it was possible to mechanize the harvesting of cotton fields. The fact is that after processing with this compound, the cotton immediately sheds foliage, which allows people to leave cotton picking to machines.

When talking about foods rich in calcium, dairy products are always mentioned, but milk itself contains from 120 mg (cow) to 170 mg (sheep) of calcium per 100 g; cottage cheese is even poorer - only 80 mg per 100 grams. Of dairy products, only cheese contains from 730 mg (gouda) to 970 mg (emmental) of calcium per 100 g of product. However, the record holder for the content of the twentieth element is poppy - 100 grams of poppy seeds contain almost 1,500 mg of calcium!

Calcium chloride CaCl2, which is used, for example, in refrigeration plants, is a waste product of many chemical-technological processes, in particular, large-scale production of soda. However, despite the widespread use of calcium chloride in various fields, its consumption is significantly inferior to its production. For this reason, for example, near the factories producing soda, whole lakes are formed from calcium chloride brine. Such storage ponds are not uncommon.

In order to understand how much calcium compounds are consumed, it is worth giving just a couple of examples. In the production of steel, lime is used to remove phosphorus, silicon, manganese and sulfur; in the oxygen-converter process, 75 kilograms of lime are consumed per ton of steel! Another example is from a completely different area - the food industry. In the production of sugar, to precipitate calcium saccharate, raw sugar syrup is reacted with lime. So, cane sugar usually requires about 3-5 kg ​​of lime per ton, and beet sugar - a hundred times more, that is, about half a ton of lime per ton of sugar!

"Hardness" of water is a number of properties that are given to water by calcium and magnesium salts dissolved in it. Rigidity is divided into temporary and permanent. Temporary or carbonate hardness is caused by the presence of soluble bicarbonates Ca (HCO3) 2 and Mg (HCO3) 2 in water. It is very easy to get rid of carbonate hardness - when boiling water, bicarbonates turn into water-insoluble calcium and magnesium carbonates, precipitating. Permanent hardness is created by sulfates and chlorides of the same metals, but getting rid of it is much more difficult. Hard water is terrible not so much because it prevents the formation of soap foam and therefore washes clothes worse, it is much worse that it forms a layer of scale in steam boilers and boiler plants, thereby reducing their efficiency and leading to emergencies. Interestingly, they knew how to determine the hardness of water in ancient Rome. Red wine was used as a reagent - its dyes form a precipitate with calcium and magnesium ions.

The process of preparing calcium for storage is very interesting. Metallic calcium is stored for a long time in the form of pieces weighing from 0.5 to 60 kg. These "pigs" are packed in paper bags, then placed in galvanized iron containers with soldered and dyed seams. Tightly closed containers are placed in wooden boxes. Pieces weighing less than half a kilogram cannot be stored for a long time - when oxidized, they quickly turn into oxide, hydroxide and calcium carbonate.

Story

Metallic calcium was obtained relatively recently - in 1808, however, mankind has been familiar with the compounds of this metal for a very long time. Since ancient times, people have used limestone, chalk, marble, alabaster, gypsum and other calcium-containing compounds in construction and medicine. Limestone CaCO3 was most likely the first building material used by man. It was used in the construction of the Egyptian pyramids and the Great Wall of China. Many temples and churches in Russia, as well as most of the buildings of ancient Moscow, were built using limestone - white stone. Even in ancient times, a person, burning limestone, received quicklime (CaO), as evidenced by the works of Pliny the Elder (I century AD) and Dioscorides, a doctor in the Roman army, whom he introduced for calcium oxide in his essay “On Medicines” the name "quicklime", which has survived to this day. And all this despite the fact that pure calcium oxide was first described by the German chemist I. Then, only in 1746, and in 1755, the chemist J. Black, studying the firing process, revealed that the mass loss of limestone during firing occurs due to the release of carbon dioxide gas:

CaCO3 ↔ CO2 + CaO

The Egyptian mortars used in the pyramids of Giza were based on partially dehydrated gypsum CaSO4 2H2O, or in other words, alabaster 2CaSO4∙H2O. It is also the basis of all plaster in the tomb of Tutankhamun. Burnt gypsum (alabaster) was used by the Egyptians as a binder in the construction of irrigation facilities. By firing natural gypsum at high temperatures, Egyptian builders achieved its partial dehydration, and not only water, but also sulfuric anhydride was split off from the molecule. Later, when diluted with water, a very strong mass was obtained, which was not afraid of water and temperature fluctuations.

The Romans can rightly be called the inventors of concrete, because in their buildings they used one of the varieties of this building material - a mixture of crushed stone, sand and lime. There is a description by Pliny the Elder of the construction of cisterns from such concrete: “For the construction of cisterns, five parts of pure gravel sand, two parts of the best slaked lime and fragments of silex (hard lava) weighing no more than a pound each are taken, after mixing, the lower and side surfaces are compacted with blows of an iron rammer ". In the humid climate of Italy, concrete was the most stable material.

It turns out that calcium compounds, which they widely used, have long been known to mankind. However, until the end of the 18th century, chemists considered lime to be a simple body, only on the eve of the new century did the study of the nature of lime and other calcium compounds begin. So Stahl suggested that lime is a complex body consisting of earthy and watery principles, and Black established a difference between caustic lime and carbonic lime, which contained "fixed air". Antoine Laurent Lavoisier attributed calcareous earth (CaO) to the number of elements, that is, to simple substances, although in 1789 he suggested that lime, magnesia, barite, alumina and silica are complex substances, but it will be possible to prove this only by decomposing "stubborn earth" (calcium oxide). And the first to succeed was Humphrey Davy. After the successful decomposition of potassium and sodium oxides by electrolysis, the chemist decided to obtain alkaline earth metals in the same way. However, the first attempts were unsuccessful - the Englishman tried to decompose lime by electrolysis in air and under a layer of oil, then he calcined the lime with potassium metal in a tube and made many other experiments, but to no avail. Finally, in a device with a mercury cathode, he obtained an amalgam by electrolysis of lime, and from it metallic calcium. Pretty soon, this method of obtaining metal was improved by I. Berzelius and M. Pontin.

The new element got its name from the Latin word "calx" (in the genitive case calcis) - lime, soft stone. Calx (calx) was called chalk, limestone, in general, a pebble stone, but most often a mortar based on lime. This concept was also used by ancient authors (Vitruvius, Pliny the Elder, Dioscorides), describing the burning of limestone, slaking lime and preparing mortars. Later, in the circle of alchemists, "calx" denoted the product of roasting in general - in particular, metals. So, for example, metal oxides were called metallic limes, and the firing process itself was called calcination (calcinatio). In the ancient Russian prescription literature, the word feces (mud, clay) is found, so in the collection of the Trinity-Sergius Lavra (XV century) it says: “take feces, from it they make gold for the furnace.” Only later did the word cal, which is undoubtedly related to the word "calx", become synonymous with the word dung. In Russian literature of the early 19th century, calcium was sometimes called the base of calcareous earth, calcareous (Shcheglov, 1830), calcareous (Iovskiy), calcium, calcium (Hess).

Being in nature

Calcium is one of the most common elements on our planet - the fifth in terms of quantitative content in nature (of non-metals, only oxygen is more common - 49.5% and silicon - 25.3%) and the third among metals (only aluminum is more common - 7.5% and iron - 5.08%). Clarke (average content in the earth's crust) of calcium, according to various estimates, ranges from 2.96% by weight to 3.38%, we can definitely say that this figure is about 3%. In the outer shell of the calcium atom, there are two valence electrons, the bond of which with the nucleus is rather fragile. For this reason, calcium has a high chemical activity and does not occur in nature in a free form. However, it actively migrates and accumulates in various geochemical systems, forming approximately 400 minerals: silicates, aluminosilicates, carbonates, phosphates, sulfates, borosilicates, molybdates, chlorides, and others, ranking fourth in this indicator. During the melting of basalt magmas, calcium accumulates in the melt and enters into the composition of the main rock-forming minerals, during the fractionation of which its content decreases during the differentiation of magma from basic to acidic rocks. For the most part, calcium lies in the lower part of the earth's crust, accumulating in the main rocks (6.72%); there is little calcium in the earth's mantle (0.7%) and, probably, even less in the earth's core (in iron meteorites of the twentieth element similar to the core, only 0.02%).

True, the calcium clarke in stony meteorites is 1.4% (rare calcium sulfide is found), in medium rocks - 4.65%, acidic rocks contain 1.58% calcium by weight. The main part of calcium is contained in the composition of silicates and aluminosilicates of various rocks (granites, gneisses, etc.), especially in feldspar - anorthite Ca, as well as diopside CaMg, wollastonite Ca3. In the form of sedimentary rocks, calcium compounds are represented by chalk and limestone, consisting mainly of the mineral calcite (CaCO3).

Calcium carbonate CaCO3 is one of the most common compounds on Earth - minerals based on calcium carbonate cover approximately 40 million square kilometers of the earth's surface. In many parts of the Earth's surface there are significant sedimentary deposits of calcium carbonate, which were formed from the remains of ancient marine organisms - chalk, marble, limestone, shell rocks - all this is CaCO3 with minor impurities, and calcite is pure CaCO3. The most important of these minerals is limestone, more precisely, limestones - after all, each deposit differs in density, composition and amount of impurities. For example, shell rock is limestone of organic origin, and calcium carbonate, which has fewer impurities, forms transparent crystals of lime or Icelandic spar. Chalk is another common variety of calcium carbonate, but marble, the crystalline form of calcite, is much less common in nature. It is generally accepted that marble was formed from limestone in ancient geological epochs. During the movement of the earth's crust, individual deposits of limestone were buried under layers of other rocks. Under the action of high pressure and temperature, the process of recrystallization took place, and the limestone turned into a denser crystalline rock - marble. Bizarre stalactites and stalagmites - the mineral aragonite, which is another variety of calcium carbonate. Orthorhombic aragonite is formed in warm seas - the Bahamas, the Florida Keys and the Red Sea basin are formed by huge layers of calcium carbonate in the form of aragonite. Also quite widespread are such calcium minerals as fluorite CaF2, dolomite MgCO3 CaCO3, anhydrite CaSO4, phosphorite Ca5 (PO4) 3 (OH, CO3) (with various impurities) and apatites Ca5 (PO4) 3 (F, Cl, OH) - forms of calcium phosphate, alabaster CaSO4 0.5H2O and gypsum CaSO4 2H2O (forms of calcium sulfate) and others. In calcium-containing minerals, there are isomorphically replacing elements-impurities (for example, sodium, strontium, rare earth, radioactive and other elements).

A large amount of the twentieth element is found in natural waters due to the existence of a global “carbonate balance” between poorly soluble CaCO3, highly soluble Ca(HCO3)2 and CO2 found in water and air:

CaCO3 + H2O + CO2 = Ca(HCO3)2 = Ca2+ + 2HCO3-

This reaction is reversible and is the basis for the redistribution of the twentieth element - with a high content of carbon dioxide in the waters, calcium is in solution, and with a low content of CO2, the mineral calcite CaCO3 precipitates, forming powerful deposits of limestone, chalk, marble.

A considerable amount of calcium is included in the composition of living organisms, for example, hydroxyapatite Ca5 (PO4) 3OH, or, in another way, 3Ca3 (PO4) 2 Ca (OH) 2 - the basis of the bone tissue of vertebrates, including humans. Calcium carbonate CaCO3 is the main component of the shells and shells of many invertebrates, eggshells, corals and even pearls.

Application

Metallic calcium is used quite rarely. Basically, this metal (as well as its hydride) is used in the metallothermic production of hard-to-recover metals - uranium, titanium, thorium, zirconium, cesium, rubidium and a number of rare earth metals from their compounds (oxides or halides). Calcium is used as a reducing agent in the production of nickel, copper and stainless steel. Also, the twentieth element is used for the deoxidation of steels, bronzes and other alloys, for the removal of sulfur from petroleum products, for the dehydration of organic solvents, for the purification of argon from nitrogen impurities and as a gas absorber in electric vacuum devices. Metallic calcium is used in the production of antifriction alloys of the Pb-Na-Ca system (used in bearings), as well as the Pb-Ca alloy used to make the sheath of electric cables. Silicocalcium alloy (Ca-Si-Ca) is used as a deoxidizer and degasser in the production of high-quality steels. Calcium is used both as an alloying element for aluminum alloys and as a modifying additive for magnesium alloys. For example, the introduction of calcium increases the strength of aluminum bearings. Pure calcium is also used for doping lead, which is used for the manufacture of battery plates, maintenance-free starter lead-acid batteries with low self-discharge. Also, metallic calcium is used for the production of high-quality calcium babbits BKA. With the help of calcium, the carbon content in cast iron is regulated and bismuth is removed from lead, oxygen, sulfur and phosphorus are purified from steel. Calcium, as well as its alloys with aluminum and magnesium, are used in reserve thermal electric batteries as an anode (for example, calcium-chromate element).

However, compounds of the twentieth element are much more widely used. And first of all we are talking about natural calcium compounds. One of the most common calcium compounds on Earth is CaCO3 carbonate. Pure calcium carbonate is the mineral calcite, and limestone, chalk, marble, shell rock - CaCO3 with minor impurities. A mixture of calcium and magnesium carbonate is called dolomite. Limestone and dolomite are mainly used as building materials, road surfaces or soil deacidifiers. Calcium carbonate CaCO3 is necessary to obtain calcium oxide (quicklime) CaO and calcium hydroxide (slaked lime) Ca(OH)2. In turn, CaO and Ca (OH) 2 are the main substances in many areas of the chemical, metallurgical and engineering industries - calcium oxide, both in free form and as part of ceramic mixtures, is used in the production of refractory materials; colossal volumes of calcium hydroxide are needed by the pulp and paper industry. In addition, Ca (OH) 2 is used in the production of bleach (a good bleaching and disinfectant), Berthollet salt, soda, and some pesticides to control plant pests. A huge amount of lime is consumed in the production of steel - to remove sulfur, phosphorus, silicon and manganese. Another role of lime in metallurgy is the production of magnesium. Lime is also used as a lubricant in steel wire drawing and in the neutralization of waste pickling liquids containing sulfuric acid. In addition, it is lime that is the most common chemical reagent in the treatment of drinking and industrial water (together with alum or iron salts, it coagulates suspensions and removes sediment, and also softens water by removing temporary - hydrocarbonate - hardness). In everyday life and medicine, precipitated calcium carbonate is used as an acid neutralizing agent, a mild abrasive in toothpastes, a source of additional calcium in diets, an ingredient in chewing gum, and a filler in cosmetics. CaCO3 is also used as a filler in rubbers, latexes, paints and enamels, and plastics (about 10% by weight) to improve their heat resistance, stiffness, hardness and machinability.

Of particular importance is calcium fluoride CaF2, because in the form of a mineral (fluorite) it is the only industrially important source of fluorine! Calcium fluoride (fluorite) is used in the form of single crystals in optics (astronomical objectives, lenses, prisms) and as a laser material. The fact is that only calcium fluoride glasses are permeable to the entire spectrum region. Calcium tungstate (scheelite) in the form of single crystals is used in laser technology, and also as a scintillator. No less important is calcium chloride CaCl2 - a component of brines for refrigeration units and for filling tires of tractors and other vehicles. With the help of calcium chloride, roads and sidewalks are cleaned of snow and ice, this compound is used to protect coal and ore from freezing during transportation and storage, wood is impregnated with its solution to make it fire resistant. CaCl2 is used in concrete mixtures to accelerate the onset of setting, increase the initial and final strength of concrete.

Artificially obtained calcium carbide CaC2 (during calcination in electric furnaces of calcium oxide with coke) is used to obtain acetylene and to reduce metals, as well as in the production of calcium cyanamide, which, in turn, releases ammonia under the action of water vapor. In addition, calcium cyanamide is used for the production of urea, a valuable fertilizer and raw material for the production of synthetic resins. By heating calcium in a hydrogen atmosphere, CaH2 (calcium hydride) is obtained, which is used in metallurgy (metallothermy) and in the production of hydrogen in the field (more than a cubic meter of hydrogen can be obtained from 1 kilogram of calcium hydride), which is used to fill balloons, for example. In laboratory practice, calcium hydride is used as an energetic reducing agent. The insecticide calcium arsenate, which is obtained by neutralizing arsenic acid with lime, is widely used to control cotton weevil, codling moth, tobacco worm, Colorado potato beetle. Important fungicides are lime-sulfate sprays and Bordeaux mixtures, which are obtained from copper sulfate and calcium hydroxide.

Production

The first to obtain metallic calcium was the English chemist Humphry Davy. In 1808, he produced an electrolysis of a mixture of wet slaked lime Ca (OH) 2 with mercury oxide HgO on a platinum plate that served as an anode (a platinum wire immersed in mercury acted as a cathode), as a result of which Davy obtained a calcium amalgam by driving mercury out of it. , the chemist discovered a new metal, which he called calcium.

In modern industry, free metallic calcium is obtained by electrolysis of a calcium chloride CaCl2 melt, the proportion of which is 75-85%, and potassium chloride KCl (it is possible to use a mixture of CaCl2 and CaF2) or by aluminothermic reduction of calcium oxide CaO at a temperature of 1 170-1 200 ° C. The pure anhydrous calcium chloride required for electrolysis is obtained by chlorination of calcium oxide by heating in the presence of coal or by dehydration of CaCl2 ∙ 6H2O obtained by the action of hydrochloric acid on limestone. The electrolytic process takes place in an electrolysis bath, in which a dry, purified calcium chloride salt and potassium chloride are placed, which is necessary to lower the melting point of the mixture. Graphite blocks are placed above the bath - an anode, a cast-iron or steel bath filled with a copper-calcium alloy, acts as a cathode. In the process of electrolysis, calcium passes into the copper-calcium alloy, significantly enriching it; goes to the chlorination of milk of lime. The enriched copper-calcium alloy can be used directly as an alloy or sent for purification (distillation), where it is distilled in vacuum (at a temperature of 1000-1080 ° C and a residual pressure of 13-20 kPa) from which metallic calcium of nuclear purity is obtained. To obtain high-purity calcium, it is distilled twice. The electrolysis process is carried out at a temperature of 680-720 °C. The fact is that this is the most optimal temperature for the electrolytic process - at a lower temperature, the calcium-enriched alloy floats to the surface of the electrolyte, and at a higher temperature, calcium dissolves in the electrolyte with the formation of CaCl. During electrolysis with liquid cathodes, alloys of calcium and lead or calcium and zinc are directly used in engineering to obtain alloys of calcium with lead (for bearings) and with zinc (for producing foam concrete - when the alloy interacts with moisture, hydrogen is released and a porous structure is created). Sometimes the process is carried out with an iron cooled cathode, which is only in contact with the surface of the molten electrolyte. As calcium is released, the cathode is gradually raised, a rod (50-60 cm) of calcium is pulled out of the melt, protected from atmospheric oxygen by a layer of solidified electrolyte. The “touch method” is used to obtain calcium heavily contaminated with calcium chloride, iron, aluminum, sodium, purification is carried out by remelting in an argon atmosphere.

Another method for obtaining calcium - metallothermic - was theoretically substantiated as early as 1865 by the famous Russian chemist N. N. Beketov. The aluminothermic method is based on the reaction:

6CaO + 2Al → 3CaO Al2O3 + 3Ca

Briquettes are pressed from a mixture of calcium oxide with powdered aluminum, they are placed in a chromium-nickel steel retort and the resulting calcium is distilled off at 1170-1200 ° C and a residual pressure of 0.7-2.6 Pa. Calcium is obtained in the form of vapor, which is then condensed on a cold surface. The aluminothermic method of obtaining calcium is used in China, France and a number of other countries. On an industrial scale, the metallothermic method of obtaining calcium was the first to be used by the United States during the Second World War. In the same way, calcium can be obtained by reduction of CaO with ferrosilicon or silicoaluminum. Calcium is produced in the form of ingots or sheets with a purity of 98-99%.

Pros and cons exist in both methods. The electrolytic method is multi-operational, energy-intensive (40-50 kWh of energy is consumed per 1 kg of calcium), besides, it is not environmentally safe, it requires a large amount of reagents and materials. However, the yield of calcium with this method is 70-80%, while with the aluminothermic method the yield is only 50-60%. In addition, with the metallothermic method of obtaining calcium, the minus is that it is necessary to carry out repeated distillation, and the plus is in low power consumption, and in the absence of gas and liquid harmful emissions.

Not so long ago, a new method for obtaining metallic calcium was developed - it is based on the thermal dissociation of calcium carbide: the carbide heated in a vacuum to 1,750 ° C decomposes with the formation of calcium vapor and solid graphite.

Until the middle of the 20th century, metallic calcium was produced in very small quantities, since it was almost never used. For example, in the United States of America during the Second World War no more than 25 tons of calcium were consumed, and in Germany only 5-10 tons. Only in the second half of the 20th century, when it became clear that calcium is an active reducing agent of many rare and refractory metals, a rapid growth in consumption (about 100 tons per year) and, as a result, the production of this metal began. With the development of the nuclear industry, where calcium is used as a component of the metallothermic reduction of uranium from uranium tetrafluoride (with the exception of the USA, where magnesium is used instead of calcium), the demand (about 2,000 tons per year) for element number twenty, as well as its production, has increased many times. At the moment, China, Russia, Canada and France can be considered the main producers of metallic calcium. From these countries calcium is sent to the USA, Mexico, Australia, Switzerland, Japan, Germany, Great Britain. The price of calcium metal rose steadily until China began to produce the metal in such volumes that a surplus of the twentieth element appeared on the world market, which led to a sharp decrease in the price.

Physical properties

What is metallic calcium? What are the properties of this element, obtained in 1808 by the English chemist Humphrey Davy, a metal whose mass in the body of an adult can be up to 2 kilograms?

The simple substance calcium is a silvery-white light metal. The density of calcium is only 1.54 g/cm3 (at a temperature of 20 °C), which is significantly less than the density of iron (7.87 g/cm3), lead (11.34 g/cm3), gold (19.3 g/cm3) or platinum (21.5 g/cm3). Calcium is even lighter than such "weightless" metals as aluminum (2.70 g/cm3) or magnesium (1.74 g/cm3). Few metals can "boast" of a density less than that of the twentieth element - sodium (0.97 g / cm3), potassium (0.86 g / cm3), lithium (0.53 g / cm3). In terms of density, calcium is very similar to rubidium (1.53 g/cm3). The melting point of calcium is 851 °C, the boiling point is 1480 °C. Similar melting (albeit slightly lower) and boiling points for other alkaline earth metals are strontium (770 °C and 1380 °C) and barium (710 °C and 1640 °C).

Metallic calcium exists in two allotropic modifications: at normal temperatures up to 443 ° C, α-calcium is stable with a cubic face-centered lattice of the copper type, with parameters: a = 0.558 nm, z = 4, space group Fm3m, atomic radius 1.97 A, ionic radius Ca2+ 1.04 A; in the temperature range of 443-842 °C, β-calcium is stable with a cubic body-centered lattice of the α-iron type, with parameters a = 0.448 nm, z = 2, space group Im3m. The standard enthalpy of transition from the α-modification to the β-modification is 0.93 kJ/mol. The temperature coefficient of linear expansion for calcium in the temperature range 0-300 °C is 22 10-6. The thermal conductivity of the twentieth element at 20 °C is 125.6 W/(m K) or 0.3 cal/(cm sec °C). The specific heat capacity of calcium in the range from 0 to 100°C is 623.9 J/(kg K) or 0.149 cal/(g°C). The electrical resistivity of calcium at 20°C is 4.6 10-8 ohm m or 4.6 10-6 ohm cm; temperature coefficient of electrical resistance of element number twenty 4.57 10-3 (at 20 °C). Modulus of elasticity of calcium 26 Gn/m2 or 2600 kgf/mm2; ultimate tensile strength 60 Mn/m2 (6 kgf/mm2); the elastic limit for calcium is 4 MN / m2 or 0.4 kgf / mm2, the yield strength is 38 MN / m2 (3.8 kgf / mm2); relative elongation of the twentieth element 50%; Brinell calcium hardness 200-300 MN/m2 or 20-30 kgf/mm2. With a gradual increase in pressure, calcium begins to exhibit the properties of a semiconductor, but does not become one in the full sense of the word (at the same time, it is no longer a metal either). With a further increase in pressure, calcium returns to the metallic state and begins to exhibit superconducting properties (superconductivity temperature is six times higher than that of mercury, and far exceeds the conductivity of all other elements). The unique behavior of calcium is similar in many ways to strontium (that is, the parallels in the periodic table are preserved).

The mechanical properties of elemental calcium do not differ from those of other members of the family of metals, which are excellent structural materials: high-purity metallic calcium is ductile, well pressed and rolled, drawn into a wire, forged and amenable to cutting - it can be turned on a lathe. However, despite all these excellent qualities of a structural material, calcium is not such - the reason for everything is its high chemical activity. True, one should not forget that calcium is an indispensable structural material of bone tissue, and its minerals have been a building material for many millennia.

Chemical properties

The configuration of the outer electron shell of the calcium atom is 4s2, which determines the valence of 2 of the twentieth element in compounds. The two electrons of the outer layer are comparatively easily split off from the atoms, which are then converted into positive doubly charged ions. For this reason, in terms of chemical activity, calcium is only slightly inferior to alkali metals (potassium, sodium, lithium). Like the latter, even at ordinary room temperature, calcium easily interacts with oxygen, carbon dioxide and humid air, while being covered with a dull gray film from a mixture of CaO oxide and Ca (OH) 2 hydroxide. Therefore, calcium is stored in a hermetically sealed vessel under a layer of mineral oil, liquid paraffin or kerosene. When heated in oxygen and air, calcium ignites, burning with a bright red flame, and the basic oxide CaO is formed, which is a white, highly flammable substance, the melting point of which is approximately 2,600 ° C. Calcium oxide is also known in the art as quicklime or burnt lime. Calcium peroxides - CaO2 and CaO4 - have also been obtained. Calcium reacts with water with the release of hydrogen (in the series of standard potentials, calcium is located to the left of hydrogen and is able to displace it from water) and the formation of calcium hydroxide Ca (OH) 2, and in cold water the reaction rate gradually decreases (due to the formation of a slightly soluble layer on the metal surface calcium hydroxide):

Ca + 2H2O → Ca(OH)2 + H2 + Q

Calcium interacts more vigorously with hot water, rapidly displacing hydrogen and forming Ca(OH)2. Calcium hydroxide Ca (OH) 2 is a strong base, slightly soluble in water. A saturated solution of calcium hydroxide is called lime water and is alkaline. In air, lime water quickly becomes cloudy due to the absorption of carbon dioxide and the formation of insoluble calcium carbonate. Despite such violent processes occurring during the interaction of the twentieth element with water, nevertheless, unlike alkali metals, the reaction of interaction of calcium with water proceeds less vigorously - without explosions and ignitions. In general, the reactivity of calcium is lower than that of other alkaline earth metals.

Calcium actively combines with halogens, thus forming compounds of the CaX2 type - it reacts with fluorine in the cold, and with chlorine and bromine at temperatures above 400 ° C, giving CaF2, CaCl2 and CaBr2, respectively. These halides in the molten state form with calcium monohalides of the CaX type - CaF, CaCl, in which calcium is formally monovalent. These compounds are stable only above the melting points of the dihalides (they disproportionate on cooling to form Ca and CaX2). In addition, calcium actively interacts, especially when heated, with various non-metals: when heated, calcium sulfide CaS is obtained with sulfur, the latter attaches sulfur, forming polysulfides (CaS2, CaS4 and others); interacting with dry hydrogen at a temperature of 300-400 ° C, calcium forms a hydride CaH2 - an ionic compound in which hydrogen is an anion. Calcium hydride CaH2 is a white salt-like substance that reacts violently with water to release hydrogen:

CaH2 + 2H2O → Ca(OH)2 + 2H2

When heated (about 500 ° C) in a nitrogen atmosphere, calcium ignites and forms Ca3N2 nitride, known in two crystalline forms - high-temperature α and low-temperature β. Nitride Ca3N4 was also obtained by heating calcium amide Ca(NH2)2 in vacuum. When heated without access to air with graphite (carbon), silicon or phosphorus, calcium gives, respectively, calcium carbide CaC2, silicides Ca2Si, Ca3Si4, CaSi, CaSi2 and phosphides Ca3P2, CaP and CaP3. Most of the calcium compounds with non-metals are easily decomposed by water:

CaH2 + 2H2O → Ca(OH)2 + 2H2

Ca3N2 + 6H2O → 3Ca(OH)2 + 2NH3

With boron, calcium forms calcium boride CaB6, with chalcogens - chalcogenides CaS, CaSe, CaTe. Polychalcogenides CaS4, CaS5, Ca2Te3 are also known. Calcium forms intermetallic compounds with various metals - aluminum, gold, silver, copper, lead and others. Being an energetic reducing agent, calcium displaces almost all metals from their oxides, sulfides and halides when heated. Calcium dissolves well in liquid ammonia NH3 with the formation of a blue solution, the evaporation of which releases ammonia [Ca (NH3) 6] - a golden-colored solid compound with metallic conductivity. Calcium salts are usually obtained by the interaction of acid oxides with calcium oxide, the action of acids on Ca(OH)2 or CaCO3, and exchange reactions in aqueous electrolyte solutions. Many calcium salts are highly soluble in water (CaCl2 chloride, CaBr2 bromide, CaI2 iodide and Ca(NO3)2 nitrate), they almost always form crystalline hydrates. CaF2 fluoride, CaCO3 carbonate, CaSO4 sulfate, Ca3(PO4)2 orthophosphate, CaC2O4 oxalate and some others are insoluble in water.

Calcium is an element of the main subgroup of the second group, the fourth period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 20. It is designated by the symbol Ca (lat. Calcium). The simple substance calcium is a soft, reactive, silver-white alkaline earth metal.

Calcium in the environment

There is a lot of it in nature: mountain ranges and clay rocks are formed from calcium salts, it is found in sea and river water, and is part of plant and animal organisms. Calcium accounts for 3.38% of the mass of the earth's crust (5th place in abundance after oxygen, silicon, aluminum and iron).

Isotopes of calcium

Calcium occurs in nature as a mixture of six isotopes: 40 Ca, 42 Ca, 43 Ca, 44 Ca, 46 Ca and 48 Ca, among which the most common - 40 Ca - is 96.97%.

Of the six naturally occurring calcium isotopes, five are stable. The sixth 48Ca isotope, the heaviest of the six and very rare (its isotopic abundance is only 0.187%), was recently discovered to undergo double beta decay with a half-life of 5.3×10 19 years.

The content of calcium in rocks and minerals

Most of the calcium is contained in the composition of silicates and aluminosilicates of various rocks (granites, gneisses, etc.), especially in feldspar - anorthite Ca.

In the form of sedimentary rocks, calcium compounds are represented by chalk and limestone, consisting mainly of the mineral calcite (CaCO 3). The crystalline form of calcite - marble - is found in nature much less frequently.

Calcium minerals such as calcite CaCO 3 , anhydrite CaSO 4 , alabaster CaSO 4 0.5H 2 O and gypsum CaSO 4 2H 2 O, fluorite CaF 2 , apatites Ca 5 (PO 4) 3 (F, Cl, OH), dolomite MgCO 3 CaCO 3 . The presence of calcium and magnesium salts in natural water determines its hardness.

Calcium, which migrates vigorously in the earth's crust and accumulates in various geochemical systems, forms 385 minerals (fourth in terms of the number of minerals).

Migration of calcium in the earth's crust

In the natural migration of calcium, a significant role is played by the “carbonate equilibrium”, associated with the reversible reaction of the interaction of calcium carbonate with water and carbon dioxide with the formation of soluble bicarbonate:

CaCO 3 + H 2 O + CO 2 ↔ Ca (HCO 3) 2 ↔ Ca 2+ + 2HCO 3 -

(the equilibrium shifts to the left or right depending on the concentration of carbon dioxide).

Biogenic migration plays an important role.

The content of calcium in the biosphere

Calcium compounds are found in almost all animal and plant tissues (see also below). A significant amount of calcium is part of living organisms. So, hydroxyapatite Ca 5 (PO 4) 3 OH, or, in another entry, 3Ca 3 (PO 4) 2 Ca (OH) 2 - the basis of the bone tissue of vertebrates, including humans; shells and shells of many invertebrates, egg shells, etc. are made of calcium carbonate CaCO 3. In living tissues of humans and animals, 1.4-2% Ca (by mass fraction); in a human body weighing 70 kg, the calcium content is about 1.7 kg (mainly in the composition of the intercellular substance of bone tissue).

Getting calcium

Calcium was first obtained by Davy in 1808 by electrolysis. But, like other alkali and alkaline earth metals, element No. 20 cannot be obtained by electrolysis from aqueous solutions. Calcium is obtained by electrolysis of its molten salts.

This is a complex and energy intensive process. Calcium chloride is melted in the electrolyzer with the addition of other salts (they are needed in order to lower the melting point of CaCl 2).

The steel cathode only touches the electrolyte surface; the released calcium sticks and freezes on it. As calcium is released, the cathode is gradually raised and, ultimately, a calcium "bar" 50 ... 60 cm long is obtained. Then it is removed, beaten off from the steel cathode and the process starts all over again. The “touch method” is used to obtain calcium heavily contaminated with calcium chloride, iron, aluminum, and sodium. It is purified by remelting in an argon atmosphere.

If the steel cathode is replaced by a metal cathode capable of alloying with calcium, then the corresponding alloy will be obtained during electrolysis. Depending on the purpose, it can be used as an alloy, or pure calcium can be obtained by distillation in a vacuum. This is how calcium alloys with zinc, lead and copper are obtained.

Another method for obtaining calcium - metal-thermal - was theoretically substantiated as early as 1865 by the famous Russian chemist N.N. Beketov. Calcium is reduced with aluminum at a pressure of only 0.01 mmHg. Process temperature 1100...1200°C. Calcium is thus obtained in the form of vapor, which is then condensed.

In recent years, another method for obtaining the element has been developed. It is based on the thermal dissociation of calcium carbide: heated in a vacuum to 1750°C, the carbide decomposes with the formation of calcium vapor and solid graphite.

Physical properties of calcium

Calcium metal exists in two allotropic modifications. Up to 443 °C, α-Ca with a cubic face-centered lattice is stable (parameter a = 0.558 nm), above β-Ca is stable with a cubic body-centered lattice of the α-Fe type (parameter a = 0.448 nm). Standard enthalpy Δ H 0 of the α → β transition is 0.93 kJ/mol.

With a gradual increase in pressure, it begins to show the properties of a semiconductor, does not become a semiconductor in the full sense of the word (it is no longer a metal either). With a further increase in pressure, it returns to the metallic state and begins to exhibit superconducting properties (the superconductivity temperature is six times higher than that of mercury, and far exceeds all other elements in conductivity). The unique behavior of calcium is similar in many ways to strontium.

Despite the ubiquity of the element, even chemists have not all seen elemental calcium. But this metal, both externally and in behavior, is completely different from alkali metals, contact with which is fraught with the danger of fires and burns. It can be safely stored in air, it does not ignite from water. The mechanical properties of elemental calcium do not make it a "black sheep" in the family of metals: calcium surpasses many of them in strength and hardness; it can be turned on a lathe, drawn into a wire, forged, pressed.

And yet, elemental calcium is almost never used as a structural material. He's too active for that. Calcium easily reacts with oxygen, sulfur, halogens. Even with nitrogen and hydrogen, under certain conditions, it reacts. The environment of carbon oxides, inert for most metals, is aggressive for calcium. It burns in an atmosphere of CO and CO 2 .

Naturally, having such chemical properties, calcium cannot be found in nature in a free state. But calcium compounds - both natural and artificial - have become of paramount importance.

Chemical properties of calcium

Calcium is a typical alkaline earth metal. The chemical activity of calcium is high, but lower than that of all other alkaline earth metals. It easily reacts with oxygen, carbon dioxide and moisture in the air, which is why the surface of calcium metal is usually dull gray, so calcium is usually stored in the laboratory, like other alkaline earth metals, in a tightly closed jar under a layer of kerosene or liquid paraffin.

In the series of standard potentials, calcium is located to the left of hydrogen. The standard electrode potential of the Ca 2+ / Ca 0 pair is −2.84 V, so that calcium actively reacts with water, but without ignition:

Ca + 2H 2 O \u003d Ca (OH) 2 + H 2 + Q.

With active non-metals (oxygen, chlorine, bromine), calcium reacts under normal conditions:

2Ca + O 2 \u003d 2CaO, Ca + Br 2 \u003d CaBr 2.

When heated in air or oxygen, calcium ignites. With less active non-metals (hydrogen, boron, carbon, silicon, nitrogen, phosphorus and others), calcium interacts when heated, for example:

Ca + H 2 \u003d CaH 2, Ca + 6B \u003d CaB 6,

3Ca + N 2 \u003d Ca 3 N 2, Ca + 2C \u003d CaC 2,

3Ca + 2P = Ca 3 P 2 (calcium phosphide), calcium phosphides of CaP and CaP 5 compositions are also known;

2Ca + Si \u003d Ca 2 Si (calcium silicide), calcium silicides of the compositions CaSi, Ca 3 Si 4 and CaSi 2 are also known.

The course of the above reactions, as a rule, is accompanied by the release of a large amount of heat (that is, these reactions are exothermic). In all compounds with non-metals, the oxidation state of calcium is +2. Most of the calcium compounds with non-metals are easily decomposed by water, for example:

CaH 2 + 2H 2 O \u003d Ca (OH) 2 + 2H 2,

Ca 3 N 2 + 3H 2 O \u003d 3Ca (OH) 2 + 2NH 3.

The Ca 2+ ion is colorless. When soluble calcium salts are added to the flame, the flame turns brick red.

Calcium salts such as CaCl 2 chloride, CaBr 2 bromide, CaI 2 iodide and Ca(NO 3) 2 nitrate are highly soluble in water. CaF 2 fluoride, CaCO 3 carbonate, CaSO 4 sulfate, Ca 3 (PO 4) 2 orthophosphate, CaC 2 O 4 oxalate and some others are insoluble in water.

Important is the fact that, unlike calcium carbonate CaCO 3, acidic calcium carbonate (hydrocarbonate) Ca (HCO 3) 2 is soluble in water. In nature, this leads to the following processes. When cold rain or river water, saturated with carbon dioxide, penetrates underground and falls on limestones, their dissolution is observed:

CaCO 3 + CO 2 + H 2 O \u003d Ca (HCO 3) 2.

In the same places where water saturated with calcium bicarbonate comes to the surface of the earth and is heated by the sun's rays, the reverse reaction occurs:

Ca (HCO 3) 2 \u003d CaCO 3 + CO 2 + H 2 O.

So in nature there is a transfer of large masses of substances. As a result, huge gaps can form underground, and beautiful stone "icicles" - stalactites and stalagmites - form in the caves.

The presence of dissolved calcium bicarbonate in water largely determines the temporary hardness of water. It is called temporary because when boiling water, the bicarbonate decomposes, and CaCO 3 precipitates. This phenomenon leads, for example, to the fact that scale forms in the kettle over time.

Application calcium

Until recently, metallic calcium has almost never been used. The United States, for example, before the Second World War consumed only 10...25 tons of calcium per year, Germany - 5...10 tons. But for the development of new areas of technology, many rare and refractory metals are needed. It turned out that calcium is a very convenient and active reducing agent for many of them, and the element began to be used in the production of thorium, vanadium, zirconium, beryllium, niobium, uranium, tantalum and other refractory metals. Pure metallic calcium is widely used in metallothermy to obtain rare metals.

Pure calcium is used to alloy lead, which is used for the manufacture of battery plates, maintenance-free starter lead-acid batteries with low self-discharge. Also, metallic calcium is used for the production of high-quality calcium babbits BKA.

Applications of metallic calcium

The main use of calcium metal is as a reducing agent in the production of metals, especially nickel, copper and stainless steel. Calcium and its hydride are also used to obtain hard-to-recover metals such as chromium, thorium and uranium. Alloys of calcium with lead are used in batteries and bearing alloys. Calcium granules are also used to remove traces of air from electrovacuum devices.

Natural chalk in the form of a powder is included in the compositions for polishing metals. But it is impossible to brush your teeth with natural chalk powder, as it contains the remains of shells and shells of the smallest animals, which have increased hardness and destroy tooth enamel.

Usagecalciumin nuclear fusion

The 48 Ca isotope is the most effective and widely used material for the production of superheavy elements and the discovery of new elements in the periodic table. For example, in the case of using 48 Ca ions to produce superheavy elements in accelerators, the nuclei of these elements are formed hundreds and thousands of times more efficiently than when using other "projectiles" (ions). Radioactive calcium is widely used in biology and medicine as an isotope tracer in the study of mineral metabolism processes in a living organism. With its help, it was found that in the body there is a continuous exchange of calcium ions between plasma, soft tissues and even bone tissue. 45 Ca also played an important role in the study of the metabolic processes occurring in soils and in the study of the processes of calcium assimilation by plants. Using the same isotope, it was possible to detect sources of contamination of steel and ultrapure iron with calcium compounds during the smelting process.

The ability of calcium to bind oxygen and nitrogen made it possible to use it for the purification of inert gases and as a getter (A getter is a substance that serves to absorb gases and create a deep vacuum in electronic devices.) in vacuum radio equipment.

The use of calcium compounds

Some artificial calcium compounds have become even more famous and familiar than limestone or gypsum. Thus, slaked Ca(OH) 2 and quicklime CaO lime was used by the builders of antiquity.

Cement is also a calcium compound obtained artificially. First, a mixture of clay or sand with limestone is fired and clinker is obtained, which is then ground into a fine gray powder. You can talk a lot about cement (or rather, about cements), this is the topic of an independent article.

The same applies to glass, which also usually contains an element.

calcium hydride

By heating calcium in a hydrogen atmosphere, CaH 2 (calcium hydride) is obtained, which is used in metallurgy (metallothermy) and in the production of hydrogen in the field.

Optical and laser materials

Calcium fluoride (fluorite) is used in the form of single crystals in optics (astronomical objectives, lenses, prisms) and as a laser material. Calcium tungstate (scheelite) in the form of single crystals is used in laser technology, and also as a scintillator.

calcium carbide

Calcium carbide is a substance discovered by chance when testing a new furnace design. More recently, calcium carbide CaCl 2 was used mainly for oxy-fuel welding and cutting of metals. When carbide interacts with water, acetylene is formed, and the combustion of acetylene in an oxygen jet makes it possible to obtain a temperature of almost 3000°C. Recently, acetylene, and with it carbide, is used less and less for welding and more and more - in the chemical industry.

calcium aschemical current source

Calcium, as well as its alloys with aluminum and magnesium, are used in reserve thermal electric batteries as an anode (for example, a calcium-chromate element). Calcium chromate is used in such batteries as the cathode. The peculiarity of such batteries is an extremely long shelf life (decades) in a usable condition, the ability to operate in any conditions (space, high pressures), high specific energy by weight and volume. The disadvantage is the short duration. Such batteries are used where it is necessary to create colossal electric power for a short time (ballistic missiles, some spacecraft, etc.).

Refractory materials fromcalcium

Calcium oxide, both in free form and as part of ceramic mixtures, is used in the production of refractory materials.

Medicines

Calcium compounds are widely used as an antihistamine.

• Calcium chloride
• Calcium gluconate
• calcium glycerophosphate

In addition, calcium compounds are introduced into preparations for the prevention of osteoporosis, into vitamin complexes for pregnant women and the elderly.

calcium in the human body

Calcium is a common macronutrient in plants, animals and humans. In humans and other vertebrates, most of it is found in the skeleton and teeth in the form of phosphates. The skeletons of most groups of invertebrates (sponges, coral polyps, mollusks, etc.) are composed of various forms of calcium carbonate (lime). The need for calcium depends on age. For adults, the required daily allowance is from 800 to 1000 milligrams (mg), and for children from 600 to 900 mg, which is very important for children due to the intensive growth of the skeleton. Most of the calcium that enters the human body with food is found in dairy products, the remaining calcium is found in meat, fish, and some plant foods (legumes are especially rich).

Assimilation of calcium is prevented by aspirin, oxalic acid, estrogen derivatives. Combining with oxalic acid, calcium gives water-insoluble compounds that are components of kidney stones.

Excessive doses of calcium and vitamin D can cause hypercalcemia, followed by intense calcification of bones and tissues (mainly affecting the urinary system). The maximum daily safe dose for an adult is 1500 to 1800 milligrams.

calcium in hard water

The complex of properties defined by one word "hardness" is given to water by calcium and magnesium salts dissolved in it. Hard water is unsuitable in many cases of life. It forms a layer of scale in steam boilers and boiler plants, makes it difficult to dye and wash fabrics, but is suitable for making soap and emulsifying in the perfume industry. Therefore, in the past, when water softening methods were imperfect, textile and perfume enterprises were usually located near sources of “soft” water.

Distinguish between temporary and permanent hardness. Temporary (or carbonate) hardness is given to water by soluble bicarbonates Ca (HCO 3) 2 and Mg (HCO 3) 2. It can be eliminated by simple boiling, in which bicarbonates are converted into water-insoluble calcium and magnesium carbonates.

Permanent hardness is created by sulfates and chlorides of the same metals. And it can be eliminated, but it is much more difficult to do it.

The sum of both hardnesses is the total hardness of the water. It is valued differently in different countries. It is customary to express the hardness of water as the number of milligram equivalents of calcium and magnesium in one liter of water. If there is less than 4 mEq in a liter of water, then the water is considered soft; as their concentration increases, more and more rigid and, if the content exceeds 12 units, very rigid.

Water hardness is usually determined using a soap solution. Such a solution (of a certain concentration) is added dropwise to a measured amount of water. As long as there are Ca 2+ or Mg 2+ ions in the water, they will interfere with the formation of foam. According to the costs of the soap solution before the appearance of foam, the content of Ca 2+ and Mg 2+ ions is calculated.

Interestingly, the hardness of water was determined in a similar way back in ancient Rome. Only red wine served as a reagent - its coloring substances also form a precipitate with calcium and magnesium ions.

Calcium storage

Metallic calcium can be stored for a long time in pieces weighing from 0.5 to 60 kg. Such pieces are stored in paper bags enclosed in galvanized iron drums with soldered and painted seams. Tightly closed drums are placed in wooden boxes. Pieces weighing less than 0.5 kg cannot be stored for a long time - they quickly turn into oxide, hydroxide and calcium carbonate.

Calcium is a chemical element of group II with atomic number 20 in the periodic system, denoted by the symbol Ca (lat. Calcium). Calcium is a soft, silvery-gray alkaline earth metal.

20 element of the periodic table The name of the element comes from lat. calx (in the genitive case calcis) - "lime", "soft stone". It was proposed by the English chemist Humphry Davy, who isolated metallic calcium in 1808.
Calcium compounds - limestone, marble, gypsum (as well as lime - a product of burning limestone) have been used in construction for several millennia ago.
Calcium is one of the most abundant elements on earth. Calcium compounds are found in almost all animal and plant tissues. It accounts for 3.38% of the mass of the earth's crust (5th place in abundance after oxygen, silicon, aluminum and iron).

Finding calcium in nature

Due to the high chemical activity of calcium in the free form in nature is not found.
Calcium accounts for 3.38% of the mass of the earth's crust (5th place in abundance after oxygen, silicon, aluminum and iron). The content of the element in sea water is 400 mg/l.

isotopes

Calcium occurs in nature in the form of a mixture of six isotopes: 40Ca, 42Ca, 43Ca, 44Ca, 46Ca and 48Ca, among which the most common - 40Ca - is 96.97%. Calcium nuclei contain the magic number of protons: Z = 20. Isotopes
40
20
Ca20 and
48
20
Ca28 are two of the five doubly magic number nuclei found in nature.
Of the six naturally occurring calcium isotopes, five are stable. The sixth 48Ca isotope, the heaviest of the six and very rare (its isotopic abundance is only 0.187%), undergoes double beta decay with a half-life of 1.6 1017 years.

In rocks and minerals

Most calcium is contained in the composition of silicates and aluminosilicates of various rocks (granites, gneisses, etc.), especially in feldspar - anorthite Ca.
In the form of sedimentary rocks, calcium compounds are represented by chalk and limestone, consisting mainly of the mineral calcite (CaCO3). The crystalline form of calcite, marble, is much less common in nature.
Calcium minerals such as calcite CaCO3, anhydrite CaSO4, alabaster CaSO4 0.5H2O and gypsum CaSO4 2H2O, fluorite CaF2, apatites Ca5(PO4)3(F,Cl,OH), dolomite MgCO3 CaCO3 are quite widespread. The presence of calcium and magnesium salts in natural water determines its hardness.
Calcium, which migrates vigorously in the earth's crust and accumulates in various geochemical systems, forms 385 minerals (fourth in terms of the number of minerals).

The biological role of calcium

Calcium is a common macronutrient in plants, animals and humans. In humans and other vertebrates, most of it is in the skeleton and teeth. Calcium is found in bones in the form of hydroxyapatite. The "skeletons" of most groups of invertebrates (sponges, coral polyps, mollusks, etc.) consist of various forms of calcium carbonate (lime). Calcium ions are involved in blood coagulation processes, and also serve as one of the universal second messengers inside cells and regulate a variety of intracellular processes - muscle contraction, exocytosis, including the secretion of hormones and neurotransmitters. The concentration of calcium in the cytoplasm of human cells is about 10−4 mmol/l, in intercellular fluids about 2.5 mmol/l.

The need for calcium depends on age. For adults aged 19-50 years and children aged 4-8 inclusive, the daily requirement (RDA) is 1000 mg (contained in approximately 790 ml of milk with a fat content of 1%), and for children aged 9 to 18 years inclusive - 1300 mg per day (contained in approximately 1030 ml of milk with a fat content of 1%). In adolescence, adequate calcium intake is very important due to the intensive growth of the skeleton. However, according to research in the US, only 11% of girls and 31% of boys aged 12-19 achieve their needs. In a balanced diet, most of the calcium (about 80%) enters the child's body with dairy products. The remaining calcium comes from cereals (including whole grain bread and buckwheat), legumes, oranges, greens, nuts. Dairy products based on milk fat (butter, cream, sour cream, cream-based ice cream) contain practically no calcium. The more milk fat in a dairy product, the less calcium it contains. Calcium absorption in the intestine occurs in two ways: transcellular (transcellular) and intercellular (paracellular). The first mechanism is mediated by the action of the active form of vitamin D (calcitriol) and its intestinal receptors. It plays a big role in low to moderate calcium intake. With a higher calcium content in the diet, intercellular absorption begins to play the main role, which is associated with a large calcium concentration gradient. Due to the transcellular mechanism, calcium is absorbed to a greater extent in the duodenum (due to the highest concentration of receptors in calcitriol there). Due to intercellular passive transfer, calcium absorption is most active in all three sections of the small intestine. Calcium absorption is paracellularly promoted by lactose (milk sugar).

Calcium absorption is hindered by some animal fats (including cow's milk fat and beef fat, but not lard) and palm oil. The palmitic and stearic fatty acids contained in such fats are cleaved off during digestion in the intestines and, in the free form, firmly bind calcium, forming calcium palmitate and calcium stearate (insoluble soaps). In the form of this soap with a chair, both calcium and fat are lost. This mechanism is responsible for decreased calcium absorption, reduced bone mineralization, and reduced indirect measures of bone strength in infants with palm oil (palm olein) based infant formula. In these children, the formation of calcium soaps in the intestines is associated with hardening of the stool, a decrease in its frequency, as well as more frequent regurgitation and colic.

The concentration of calcium in the blood, due to its importance for a large number of vital processes, is precisely regulated, and with proper nutrition and sufficient intake of low-fat dairy products and vitamin D, deficiency does not occur. Prolonged deficiency of calcium and/or vitamin D in the diet leads to an increased risk of osteoporosis and causes rickets in infancy.

Excessive doses of calcium and vitamin D can cause hypercalcemia. The maximum safe dose for adults aged 19 to 50 inclusive is 2500 mg per day (about 340 g of Edam cheese).

Thermal conductivity