Calcium (Ca)

Chemical element

Written by:Timothy P. Hanusa

http://www.britannica.com/EBchecked/topic/88956/calcium-Ca/1018/Compounds#ref89814

Alternate title:Ca

Compounds

The most important calciumcompoundiscalcium carbonate, CaCO3, the major constituent oflimestone,marble,chalk,oystershells, andcorals. Calcium carbonate obtained from its natural sources is used as a filler in a variety of products, such asceramics,glass,plastics, andpaint, and as a starting material for the production of calcium oxide. Synthetic calcium carbonate, called precipitated calcium carbonate, is employed when high purity is required, as inmedicine(antacids and dietary calcium supplements), in food (baking powder), and for laboratory purposes.

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Calcium oxide, CaO, also known as lime or more specifically quicklime, is a white or grayish white solid produced in large quantities by roasting calcium carbonate so as to drive offcarbon dioxide. At roomtemperature, CaO will spontaneously absorb carbon dioxide from theatmosphere, reversing the reaction. It will also absorbwater, converting itself into calcium hydroxide and releasing heat in the process. The bubbling that accompanies the reaction is the source of its name as quick, or living, lime. The reaction of quicklime with water is sometimes used in portable heat sources. One of the oldest known products of achemical reaction, quicklime is used extensively as a building material. It is sometimes used directly as a fertilizer, although calcium carbonate is usually preferred for that purpose. Large quantities of quicklime are used in various industrial neutralization reactions.Limelights, used in the 19th century in stage lighting, emit a very brilliant white light upon heating a block of calcium oxide to incandescence in an oxyhydrogen flame, hence the expression to be in the limelight.

A large amount of calcium oxide also is used as starting material in the production ofcalcium carbide, CaC2, also known simply as carbide, or calcium acetylide. Colourless when pure (though technical grades are typically grayish brown), this solid decomposes in water, forming flammableacetylenegasand calcium hydroxide, Ca(OH)2. The decomposition reaction is used for the production of acetylene, which serves as an important fuel for welding torches. The drip of water on calcium carbide produces a steady stream of acetylene that is ignited in carbide lamps. Such lamps were commonly used inlighthousebeacons and by miners in the early 20th century and still find some use in spelunking. Calcium carbide also is used to make calcium cyanamide, CaCN2, afertilizercomponent and starting material for certain plastic resins.

Calcium hydroxide, also called slaked lime, Ca(OH)2, is obtained by the action of water on calcium oxide. When mixed with water, a small proportion of it dissolves, forming a solution known as limewater, the rest remaining as a suspension called milk of lime. Calcium hydroxide is used as an industrialalkaliand as a constituent ofmortars,plasters, andcement. It is used in the kraft paper process and as a flocculant in sewage treatment.

Another important compound iscalcium chloride, CaCl2, a colourless or white solid produced in large quantities either as a by-product of the manufacture of sodium carbonate by the Solvay process or by the action of hydrochloric acid on calcium carbonate. The anhydrous solid is used as a drying agent and for dust and ice control on roads. Calcium hypochlorite, Ca(ClO2), widely used as bleaching powder, is produced by the action ofchlorineon calcium hydroxide. The hydride CaH2, formed by the direct action of the elements, liberateshydrogenwhen treated with water. Traces of water can be removed from many organic solvents by refluxing them in the presence of CaH2.

Calcium sulfate, CaSO4, is a naturally occurring calcium salt. It is commonly known in its dihydrate form, CaSO42H2O, a white or colourless powder calledgypsum. As uncalcined gypsum, the sulfate is employed as a soil conditioner. Calcined gypsum is used in making tile, wallboard, lath, and various plasters. When gypsum is heated to about 120 C (250 F), it loses three-quarters of its water, becoming the hemihydrate CaSO41/2H2O,plaster of paris. If mixed with water, plaster of paris can be molded into shapes before it hardens by recrystallizing to dihydrate form. Calcium sulfate may occur in groundwater, causing hardness that cannot be removed by boiling.

Calcium phosphatesoccur abundantly in nature in several forms and are the principal minerals for the production of phosphate fertilizers and for a range ofphosphoruscompounds. For example, the tribasic variety (precipitated calcium phosphate), Ca3(PO4)2, is the principal inorganic constituent of bone ash. The acid salt Ca(H2PO4)2, produced by treating mineral phosphates withsulfuric acid, is employed as aplantfood and stabilizer for plastics.

Thehydrogen sulfite, Ca(HSO3)2, is made by the action ofsulfur dioxideon a slurry of Ca(OH)2. Its aqueous solution under pressure dissolves thelignininwoodto leavecellulosefibres and thus finds considerable application in thepaperindustry.

Thefluoride, CaF2, is important to the production of hydrofluoric acid, which is made from CaF2by the action of sulfuric acid. CaF2is used in laboratory instruments as a window material for bothinfraredandultraviolet radiation.

Timothy P. Hanusa

Element Properties

atomic number

20

atomic weight

40.078

melting point

842 C (1,548 F)

boiling point

1,484 C (2,703 F)

specific gravity

1.55 (20 C, or 68 F)

oxidation state

+2

electron configuration

1s22s22p63s23p64s2

Calcium (Ca)

Chemical element

Written by:Timothy P. Hanusa

http://www.britannica.com/EBchecked/topic/88956/calcium-Ca

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Alternate title:Ca

TABLE OF CONTENTS

Introduction

Occurrence, properties, and uses

Compounds

Calcium(Ca),chemical element, one of thealkaline-earth metalsof Group 2 (IIa) of the periodic table. It is the most abundant metallic element in the human body and the fifth most abundant element inEarths crust.

Occurrence, properties, and uses

Calcium does not occur naturally in the free state, butcompoundsof the element are widely distributed. One calcium compound, lime (calcium oxide, CaO) was extensively used by the ancients. The silvery, rather soft, lightweightmetalitself was first isolated (1808) bySir Humphry Davyafter distillingmercuryfrom anamalgamformed byelectrolyzinga mixture of lime and mercuric oxide. The name for the element was taken from the Latin word for lime,calx.

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Calcium constitutes 3.64 percent ofEarths crust and 8 percent of theMoons crust, and its cosmic abundance is estimated at 4.9 104atoms(on a scale where the abundance ofsiliconis 106atoms). Ascalcite(calcium carbonate), it occurs on Earth inlimestone,chalk,marble,dolomite, eggshells,pearls,coral, stalactites, stalagmites, and the shells of many marine animals. Calcium carbonate deposits dissolve inwaterthat containscarbon dioxideto form calcium bicarbonate, Ca(HCO3)2. This process frequently results in the formation ofcavesand may reverse to deposit limestone asstalactites and stalagmites. As calcium hydroxyl phosphate, it is the principal inorganic constituent ofteethandbonesand occurs as themineralapatite. As calcium fluoride, it occurs asfluorite, or fluorspar. And as calcium sulfate, it occurs asanhydrite. Calcium is found in many other minerals, such asaragonite(a type of calcium carbonate) andgypsum(another form of calcium sulfate), and in manyfeldsparsandzeolites. It is also found in a large number ofsilicatesand aluminosilicates, insaltdeposits, and in natural waters, including the sea.

Formerly produced by electrolysis of anhydrous calcium chloride, pure calciummetalis now made commercially by heating lime with aluminum. The metal reacts slowly with oxygen, water vapour, andnitrogenof the air to form a yellow coating of the oxide, hydroxide, and nitride. It burns in air or pure oxygen to form the oxide and reacts rapidly with warm water (and more slowly with cold water) to producehydrogengas and calcium hydroxide. On heating, calcium reacts with hydrogen,halogens,boron,sulfur,carbon, andphosphorus. Although it compares favourably withsodiumas a reducing agent, calcium is more expensive and less reactive than the latter. In many deoxidizing, reducing, and degasifying applications, however, calcium is preferred because of its lower volatility and is used to preparechromium,thorium,uranium,zirconium, and other metals from their oxides.

The metal itself is used as analloyingagent foraluminum,copper,lead, magnesium, and other base metals; as a deoxidizer for certain high-temperaturealloys; and as a getter inelectron tubes. Small percentages of calcium are used in many alloys for special purposes. Alloyed with lead (0.04 percent calcium), for example, it is employed as sheaths fortelephonecables and as grids for storagebatteriesof the stationary type. When added to magnesium-based alloys in amounts from 0.4 to 1 percent, it improves the resistance of degradable orthopedic implants to biological fluids, permittingtissuesto heal fully before the implants lose their structural integrity.

Naturally occurring calcium consists of a mixture of sixisotopes: calcium-40 (96.94 percent), calcium-44 (2.09 percent), calcium-42 (0.65 percent), and, in smaller proportions, calcium-48, calcium-43, and calcium-46. Calcium-48 undergoes doublebeta decaywith ahalf-lifeof roughly 4 1019years, so it is stable for all practical purposes. It is particularlyneutron-rich and is used in the synthesis of new heavy nuclei inparticle accelerators. Theradioactive isotopecalcium-41 occurs in trace quantities onEarththrough the natural bombardment of calcium-40 by neutrons incosmic rays.

Calcium is essential to bothplantandanimallifeand is broadly employed as a signal transducer,enzymecofactor, and structural element (e.g.,cell membranes, bones, and teeth). A large number of living organisms concentrate calcium in their shells or skeletons, and in higher animals calcium is the most abundant inorganic element. Many important carbonate and phosphate deposits owe their origin to living organisms.

The human body is 2 percent calcium. Major sources of calcium in the human diet aremilk, milk products,fish, and green leafy vegetables. The bone diseasericketsoccurs when a lack ofvitamin Dimpairs the absorption of calcium from the gastrointestinal tract into the extracellular fluids. The disease especially affects infants and children.