74 | The History and Use of Our Earth’s Chemical Elements Characteristics
74 | The History and Use of Our Earth’s Chemical Elements Characteristics
Finely powdered calcium metal is flammable in air because it liberates hydrogen from the moisture. It can be extremely reactive in water but can be dissolved in acids. Calcium is harder than sodium metal, but softer than aluminum. In its elemental form it can be machined (cut on a lathe), extruded (pushed through a die), and drawn (stretched into rods or wires).
Calcium is present in a number of products used in our everyday lives. It is found in classroom chalk, teeth, and bones. About 2% of the human body consists of various forms of calcium compounds. Calcium is an essential inorganic element (usually in compound form) for plant and animal life.
AbundanceandSource Calcium is the fifth most abundant element found in the Earth’s crust. It is not found as
a free element, but as calcium compounds (mostly salts and oxides), which are found on all landmasses of the world as limestone, marble, and chalk. Calcium, particularly as the com- pound calcium chloride (CaCl 2 ), is found in the oceans to the extent of 0.15%.
Calcium is produced by two methods. One method is the electrolysis of calcium chloride
(Ca ++ + 2Cl - → CaCl 2 ) as the electrolyte at a temperature of ˜800°C, during which process metallic calcium cations (Ca ++ ) are deposited at the cathode as elemental calcium metal. Calcium can also be produced through a thermal process under very low pressure (vacuum) in which lime is reduced by using aluminum.
In addition to limestone, calcium is also found in other rocks, coral, shells, eggshells, bones, teeth, and stalactites and stalagmites.
History Several calcium compounds were used by ancient civilizations for thousands of years. The
Egyptians used gypsum to form plasters. Gypsum (CaSO 4 •2H 2 O) is the common name for calcium sulfate dihydrate and is a mineral produced from seawater. Gypsum plasters are found in Egyptian tombs. The ancient Greeks and Romans also made mixtures of gypsum plaster somewhat similar to the modern drywall boards used extensively in today’s construction industry. A desiccated form of gypsum, known as plaster of Paris, when soaked into cloth ban- dages, was used to support broken bones as far back as the tenth century. The same procedure is still employed to form casts to support broken bones.
By the early 1800s several chemists had separated potassium and sodium as elements from compounds. It was believed that metallic calcium could be obtained by similar methods. In 1808 Sir Humphry Davy finally produced the metallic element calcium from a mixture of lime and mercuric oxide by his experimental electrolysis apparatus. This was the same process
he had previously used to discover several other alkali earth metals. CommonUses Calcium oxide was used in ancient times to make mortar for building with stone. Both
the metal and calcium compounds have many industrial as well as biological uses. Metallic calcium is used as an alloy agent for copper and aluminum. It is also used to purify lead and is a reducing agent for beryllium.
It is used to remove carbon and sulfur impurities during the processing of iron, producing
Guide to the Elements | 75 Calcium is an important ingredient in the diets of all plants and animals. It is found in the
soft tissues and fluids of animals (e.g., blood) as well as in bones and teeth. Calcium makes up about 2% of human body weight.
Calcium is the main ingredient of Portland cement and is used to reduce the acid content of soils.
ExamplesofCompounds Many calcium compounds are found on both land and sea.
Calcium acetate [Ca(CH 3 COO) 2 •H 2 O] is used as a food additive and a mordant to fix dyes in the textile industry. It is used as an alkali (base) in the manufacture of soaps, to improve some lubricants, and as an antimold to preserve baked goods for a longer shelf life.
Calcium bromide (CaBr 2 ) is used as a developer for photographic film and paper and as a dehydrating agent (drying agent), food preservative, and fire retardant. Calcium carbide (CaC 2 ) has a garlic-like odor and reacts with water to form acetylene gas plus calcium hydroxide and heat. In the past, it was used in miners’ lamps to continuously produce a small acetylene flame to provide some illumination in coal mines.
Calcium chloride (CaCl 2 ) has many uses. It is used as a drying agent and to melt ice and snow on highways, to control dust, to thaw building materials (sand, gravel, concrete, and so on). It is also used in various food and pharmaceutical industries and as a fungicide.
Calcium carbonate (CaCO 3 ) can be in the form of an odorless crystal or powder and is one of calcium’s most stable compounds, better known in its natural state as limestone, marble, chalk, calcite, oyster shells, and the minerals marl and travertine. Calcium carbonate is the source of lime and is used as a “filler” for many products, including paints, plastics, and foods (bread), and as an antacid.
Calcium oxide (CaO) is formed when calcium metal is exposed to atmospheric oxygen (2Ca +O 2 → 2CaO). It is known as quick lime, unslaked lime, calx, or just lime. It is used as a flux in steel industries and as a food additive and is also used to make paper pulp, insecticides, and fungicides, to remove hair from animal hides, and to produce calcium carbide (CaC 2 ). Calcium oxide becomes incandescent when heated to a high temperature. This attribute made it useful in bright spotlights for stage lighting. It became known as “limelight” and thus its use as a term for public attention.
Calcium hydroxide [Ca (OH) 2 ] is known as slaked or hydrated lime and is formed by expos- ing calcium oxide to water. Slaked lime is less caustic than quick lime. Therefore, it is used to line football fields. (Unslaked lime, CaO, is very caustic when wet, and if it is used on playing fields, players may receive caustic burns.) Calcium hydroxide has many uses, including as an ingredient for stonemasons’ mortar, cements, whitewash, and soil conditioner (high pH), as a food additive, and as a human depilatory.
Calcium nitrate [Ca(NO 3 ) 2 ] is known as Norwegian saltpeter. It is a strong oxidizer (because
of the NO 3 ) that is flammable in the presence of organic materials (such as hands). It explodes when given a hard shock. It is used in fireworks, matches, and fertilizers.
Hazards The metallic form of calcium, particularly the powdered form, combines with water or
oxidizing agents to release hydrogen that may explode, as do the other alkali metals. There are many useful calcium compounds; some are excellent reducing agents, some are explosive, and