Calcium Oxide
Overview
Calcium oxide (KAL-see-um OK-side) is an odorless crystalline or powdery solid that, in a pure form, is white to off-gray. It often appears with a yellowish or brownish tint to the presence of impurities, especially iron. Calcium oxide reacts with water to form calcium hydroxide (Ca(OH)2) with the evolution of significant amounts of heat. The compound is strongly caustic.
Key Facts
Other Names:
Lime; quicklime; burnt lime; calx; unslaked lime; fluxing lime
Formula:
CaO
Elements:
Calcium, oxygen
Compound Type:
Metallic oxide
State:
Solid
Molecular Weight:
56.08 g/mol
Melting Point:
2,898°C (5,248°F)
Boiling Point:
Not available
Solubility:
Reacts with water to form calcium hydroxide; soluble in acids; insoluble in alcohol and most organic solvents
Calcium oxide has been known since ancient times. The Roman writer Cato the Elder (234–149 BCE) described one method of producing the compound in 184 BCE. Another early Roman scholar, Pliny the Elder (23–79 CE) discussed the compound at length in his book Historia Naturalis (Natural History), published in 70 CE. By the early fifteenth century, most of Europe was using calcium oxide (widely referred to as lime) in the construction of buildings. The Scottish chemist Joseph Black (1728–1799) carried out some of the earliest scientific studies of calcium oxide. He found that when the compound is exposed to air, it combines with carbon dioxide to form calcium carbonate.
How It Is Made
The process for making calcium oxide is believed to be one of the first chemical reactions known to humans, dating back to prehistoric times. When limestone (calcium carbonate; CaCO3) is heated, carbon dioxide (CO2) is driven off, leaving calcium oxide behind. The reaction was probably discovered very early in human history because limestone is a common, readily available material in the form of chalk and sea shells, and the amount of heat needed to produce the reaction can easily be produced in a simple wood fire. A more efficient method for carrying out the reaction is to heat the limestone in a kiln (oven) at temperatures of 500°C to 900°C (900°F to 1,600°F), resulting in a more complete conversion of calcium carbonate to calcium oxide. This method is still used today for the commercial preparation of calcium oxide.
Common Uses and Potential Hazards
The most important single use of calcium oxide is in metallurgy, particularly in the production of steel. When calcium oxide is added to the furnace in which steel is made, it reacts with sulfur, phosphorus, silica, and other impurities present in the mixture from which steel is produced. The complex mixture that results can be poured off the top of the molten steel in the form of a slag, a nonmetallic waste formed during the production of metals. Calcium oxide plays a comparable role in the manufacture of other metals, such as aluminum and magnesium. About 40 percent of all the calcium oxide produced in the United States goes to metallurgical applications.
The next most important use of calcium oxide is in pollution control devices. Smoke that leaves a factory's smokestack, for example, contains oxides of sulfur and nitrogen that, in the atmosphere, combine with water to form sulfuric acid and nitric acid. To prevent the formation of these pollutants, "scrubbers" can be installed in smokestacks. The scrubbers contain some chemical that reacts with and neutralizes the oxides of sulfur and nitrogen. One of the most common compounds used for this purpose is calcium oxide. About 15 percent of all calcium oxide used in the United States goes to this application.
Interesting Facts
- Calcium oxide is often used to "lime" lake waters that have been acidified by acid rain. It reacts with and neutralizes acids in the lake formed when nitric and sulfuric acid in acid rain are carried to earth by rain, snow, sleet, and other forms of precipitation.
- When calcium oxide is heated near its melting point, it gives off a brilliant white light. In the years before electricity was available for lighting, particularly during the second half of the nineteenth century, heated lime was used to produce the bright lights needed to illuminate stage productions. From this practice came the expression "being in the limelight" to refer to anyone who was in public view of large groups of people.
- Because it was thought to accelerate the decomposition of soft tissue, quicklime has historically been used in the burial of diseased animals and humans. For example, bodies of plague victims in London in 1666 were directed to be buried in quicklime.
Some other uses of calcium oxide include:
- In water treatment plants, to control acidity of the water being treated and to remove impurities present in the water;
- In the construction industry, where it is used to make plaster, mortar, stucco, bricks, and other building materials;
- As a filler to strengthen paper products;
- As a refractory, a heat-resistant material used to line the insides of furnaces;
- In the production of other chemical materials;
- As an additive for poultry feed;
- In insecticides and fungicides;
- In the removal of hair from hides before tanning; and
- As a food additive to maintain proper acidity and give bulk to a food product.
Exposure to calcium oxide can cause damage to the skin, eyes, nose, and respiratory system. People who use the product in their line of work or at home (for garden purposes, for example) must exercise extreme caution to avoid swallowing, breathing in, or otherwise coming into contact with the chemical. If such contact occurs, it should be washed off completely and medical assistance requested.
Words to Know
Strongly basic or alkaline material that irritates or corrodes living tissue. The science of working with metals.For Further Information
"Calcium Oxide or Quicklime." Peters Chemical Company. http://www.peterschemical.com/calcium_oxide_or_quicklime.htm (accessed on September 27, 2005).
"Chemical of the Week—Lime." http://scifun.chem.wisc.edu/chemweek/Lime/lime.html (accessed on September 27, 2005).
"Cheminfo." Canadian Centre for Occupational Health and Safety. http://intox.org/databank/documents/chemical/calcoxid/cie11.htm (accessed on September 27, 2005).
"Metallurgy Information Area." National Lime Association. http://www.lime.org/ENV02/Metal802.htm#IS (accessed on September 27, 2005).
See Also
Calcium Carbonate; Calcium Hydroxide
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