Silicon dioxide (SILL-uh-kon dye-OK-side) is one of the most abundant chemical compounds on Earth. It makes up about 60 percent of the weight of the Earth's crust either as an independent compound (SiO2) or in combination with metallic oxides that form silicates. Silicates are inorganic compounds whose negative part is the SiO3− ion (grouping of atoms). An example is magnesium silicate, MgSiO3.
Silica, quartz, sand, amorphous silica, silica gel, and others
Nonmetallic oxide (inorganic)
Varies depending on crystalline state; typically above 1700°C (3100°F)
Solubility depends on crystalline state; generally insoluble in water; soluble in many acids and alkalis
Silicon dioxide occurs as colorless, odorless, tasteless white or colorless crystals or powder. Its many different forms can be classified as crystalline, amorphous, or vitreous. In crystalline forms of silicon dioxide, all of the atoms that make up the substances are arranged in orderly patterns that have the shape of cubes, rhombohedrons, or other geometric figures. In amorphous silicon dioxide, silicon and oxygen atoms are arranged randomly, without any clear-cut pattern. Vitreous silicon dioxide is a glassy form of the compound that may be transparent, translucent, or opaque. The various forms of silicon dioxide can be converted from one form to another by heating and changes in pressure.
An especially interesting form of silicon dioxide is silica gel, a powdery form of amorphous silicon dioxide that is highly adsorbent. An adsorbent material (in contrast to an absorbent material) is one that is capable of removing a material, such as water, ammonia, alcohol, or other gases, out of the air. The second material bonds weakly to the outer surface of silica gel particles. Silica gel is able to adsorb anywhere from 30 to 50 percent of its own weight in water from the surrounding atmosphere before it becomes saturated. The silica gel is not chemically altered by the process of adsorption and still feels dry even when saturated. The adsorbed water can be driven off simply by heating the silica gel, allowing the material to regain its adsorbent properties.
HOW IT IS MADE
Although methods are available for synthesizing silicon dioxide, there is no practical reason for doing so. The abundant quantities of silicon dioxide found in the earth's crust are sufficient to satisfy all industrial needs. Among the minerals and earths that contain silicon dioxide in an uncombined form are quartz, flint, diatomite, stishovite, agate, amethyst, chalcedony, cristobalite, and tridymite.
- Stardust, a U.S. National Aeronautics and Space Administration (NASA) spacecraft, used silica gel to collect particles of debris from the tail of comet Wild-2.
- Although silica gel has been known since the mid-seventeenth century, practical applications for the material were not discovered until 1919 when American Walter A. Patrick (1888–1969) patented a number of processes for the manufacture of the compound. It still did not become widely popular until World War II (1939–1945), when the American military found a number of important uses for the compound.
Naturally occurring silicon dioxide can be treated by a variety of physical processes to change its form. For example, heating crystalline silicon dioxide above its melting point and then cooling it again converts the compound into its vitreous form, sometimes called natural glass. Silica gel is made by treating sodium silicate (Na2SiO3) with sulfuric acid (H2SO4): Na2SiO3 + H2SO4 → SiO2 + H2O + Na2SO4.
COMMON USES AND POTENTIAL HAZARDS
The primary use of silicon dioxide is in the building industry. It is used to make ceramics, enamels, concrete, and specialized silica bricks used as refractory materials. It is also one of the raw materials from which all kinds of glass are made. Vitreous silicon dioxide is an important constituent of specialized types of glass, such as that used in making laboratory equipment, mirrors, windows, prisms, cells, and other kinds of optical devices. Silicon dioxide is also used as an anti-caking or thickening agent in a variety of foods and pharmaceutical products. Some other applications of silicon dioxide include:
- In the manufacture of polishing and grinding materials;
- As molds for casting;
- In the production of elemental silicon;
- As a filler in many different kinds of products, including paper, insecticides, rubber products, pharmaceuticals, and cosmetics;
- As an additive in paints to produce a low-gloss finish;
- In the reinforcement of certain types of plastics.
The primary application of silica gel is as a drying agent. Packets of silica gel are found in many consumer products, such as electronic equipment, hardware tools, clothing, CD and DVD discs, and foodstuffs. Because of its ability to adsorb moisture from the surrounding air, silica gel prevents rust and other forms of oxidation. Silica gel also has similar applications in industry. For example, it is used to dry compressed air, air conditioning systems, and natural gas. The compound is also used to bleach petroleum oils and as an anti-caking agent for cosmetics and pharmaceuticals.
Normal exposure to silicon dioxide is not considered hazardous to the health of humans or other animals. Inhaling silicon dioxide dust or fumes, however, may create problems in the respiratory system. In low doses, inhaled silica can cause coughing, wheezing, and difficulty breathing. In higher doses, particles of silicon dioxide may lodge in the lungs and block the openings through which oxygen is absorbed. Over long periods of exposure, a condition known as silicosis may develop. Silicosis is a condition similar to tuberculosis, lung cancer, or emphysema in which a person's ability to breathe normally is reduced, causing severe and life-threatening long-term health problems. Individuals at greatest risk for silicosis and other silicon dioxide-related problems are those who cut, chip, drill, or grind objects that contain silica. During these processes, silica is reduced to a fine powder that is easily inhaled. Wearing a mask during these operations is generally sufficient to protect a worker from inhaling these fumes and particles.
Words to Know
- A strong base.
- A material with a high melting point, resistant to melting, often used to line the interior of industrial furnaces.
FOR FURTHER INFORMATION
Brown, David. "Walter Patrick: Preservationist of the First Order." Mount Washington Newsletter. Spring 2003. Also available online at http://www.mwia.org/Newsletters/MWIANewsletter_Spring%202003.pdf (accessed on December 10, 2005).
Brownlee, Don. "An Exciting Encounter with a Cold Dark Mysterious Body from the Edge of the Solar System." Jet Propulsion Laboratory. California Institute of Technology. http://stardust.jpl.nasa.gov/news/news101.html (accessed on December 10, 2005).
Scheer, James F. "Silica: Health and Beauty from Nature." Better Nutrition (December 1997): 38-43. Also available online at http://www.findarticles.com/p/articles/mi_m0FKA/is_n12_v59/ai_20086185 (accessed on December 10, 2005).
"Silica Gel." J. T. Baker. http://www.jtbaker.com/msds/englishhtml/s1610.htm (accessed on December 10, 2005).
"Silica—Silicon Dioxide." Azom.com. http://www.azom.com/details.asp?ArticleID=1114 (accessed on December 10, 2005).
"What Is Silica Gel and Why Do I Find Packets of It in Everything I Buy?" How Stuff Works. http://science.howstuffworks.com/question206.htm (accessed on December 10, 2005).