Carbon dioxide

Carbon dioxide is a heavy, odorless, colorless, faintly acid-tasting, and non-flammable gas (at room temperature) that is released during respiration, combustion, and by the decomposition of organic substances, which is then mostly absorbed from the air by plants in the process called photosynthesis. Sometimes called carbonic acid gas, its molecular makeup consists of one atom of carbon (C) attached to two atoms of oxygen (O): CO₂. Carbon dioxide was the first gas to be distinguished from ordinary air, perhaps because it is so intimately connected with the cycles of plant and animal life. When humans breathe air or when wood and other fuels are burned, carbon dioxide is released; when plants store energy in the form of food, they use up carbon dioxide. Early scientists were able to observe the effects of carbon dioxide long before they knew exactly what it was.

Around 1630, Flemish scientist Jan van Helmont (1580–1644) discovered that certain vapors differed from air, which at the time was thought to be a single substance or element. Van Helmont coined the term gas to describe these vapors and collected the gas given off by burning wood, calling it gas sylvestre. Today, this gas is known as carbon dioxide. Van Helmont is credited with its discovery. He also recognized that carbon dioxide was produced by the fermentation of wine and from other natural processes. Before long, other scientists began to notice similarities between the processes of respiration and combustion, both of which used up and gave off carbon dioxide. For example, they discovered that a candle flame will eventually be extinguished when enclosed in a jar with a limited supply of air, as will the life of a bird or small animal.

Then in 1756, Scottish chemist Joseph Black (1728–1799) proved that carbon dioxide, which he called fixed air, is present in the atmosphere and that it combines with other chemicals to form new compounds. Black also identified carbon dioxide in exhaled breath, determined that the gas is heavier than air, and characterized its chemical behavior as that of a weak acid. The pioneering work of van Helmont and Black soon led to the discovery of other gases by English chemist and physicist Henry Cavendish (1731–1810), French chemist Antoine-Laurent Lavoisier (1743–1794), Swedish chemist Carl Wilhelm Scheele (1742–1786), and other chemists. As a result, scientists began to realize that gases must be weighed and accounted for in the analysis of chemical compounds, just like solids and liquids.

The first practical use for carbon dioxide was invented by English chemist Joseph Priestley (1733–1804) in the mid-1700s. Priestley had duplicated Black’s experiments using a gas produced by fermenting grain and showed that it had the same properties as Black’s fixed air, or carbon dioxide. When Priestley dissolved the gas in water, he found that it created a refreshing drink with a slightly tart flavor. This was the first artificially carbonated water, known as soda water or seltzer. Carbon dioxide is still used today to make colas and other soft drinks. In addition to supplying bubbles and zest, the gas acts as a preservative.

The early study of carbon dioxide also gave rise to the expression to be a guinea pig, meaning to subject oneself to an experiment. In 1783, French mathematician, Pierre Simon de Laplace (1749–1827) used a guinea pig to demonstrate quantitatively that oxygen from the air is used to burn carbon stored in the body and produce carbon dioxide in exhaled breath. Around the same time, chemists began drawing the connection between carbon dioxide and plant life. Like animals, plants breathe, using up oxygen and releasing carbon dioxide. However, plants also have the unique ability to store energy in the form of carbohydrates, the human body’s primary source of food. This energy-storing process, called photosynthesis, is essentially the reverse of respiration. It uses up carbon dioxide and releases oxygen in a complex series of reactions that also require sunlight and chlorophyll (the green substance that gives plants their color). In the 1770s, Dutch physiologist Jan Ingen-Housz (1730–1799) established the principles of photosynthesis that helped explain the age-old superstition that plants purify air during the day and poison it at night.

Since these early discoveries, chemists have learned much more about carbon dioxide. English chemist John Dalton (1766–1844) guessed in 1803 that the molecule contains one carbon atom and two oxygen atoms (CO₂); this was later proved to be true. The decay of all organic materials produces carbon dioxide very slowly, and Earth’s atmosphere contains a small amount of the gas (about 0.033%). Spectroscopic analysis has shown that in the solar system, the planets of Venus and Mars have atmospheres very rich in carbon dioxide, both with atmospheres of over 95%. The gas also exists in ocean water, where it plays a vital role in marine plant photosynthesis.

In modern life, carbon dioxide has many practical applications. For example, fire extinguishers use CO ₂ to control electrical and oil fires that cannot be put out with water. Because carbon dioxide is heavier than air, it spreads into a blanket and smothers the flames. Carbon dioxide is also a very effective refrigerant. In its solid form, known as dry ice, it is used to chill perishable food during transport. Many industrial processes are also cooled by carbon dioxide, which allows faster production rates. For these commercial purposes, carbon dioxide can be obtained from either natural gas wells, fermentation of organic material, or combustion of fossil fuels.

Recently, carbon dioxide has received negative attention as a greenhouse gas. When it accumulates in the upper atmosphere, it traps the Earth’s heat, eventually causing global warming. Since the beginning of the Industrial Revolution in the mid-1800s, factories and power plants have significantly increased the amount of carbon dioxide in the atmosphere by burning coal and other fossil fuels. This effect was first predicted by Swedish physicist Svante August Arrhenius (1859–1927) in the 1880s. Then in 1938, British physicist G. S. Callendar (1898–1964) suggested that higher CO₂ levels had caused the warmer temperatures observed in America and Europe since Arrhenius’ day. Modern scientists have confirmed these views and identified other causes of increasing carbon dioxide levels, such as the clearing of the world’s forests. Because trees extract CO₂ from the air, their depletion has contributed to upsetting the delicate balance of gases in the atmosphere.

In very rare circumstances, carbon dioxide can endanger life. In 1986, a huge cloud of the gas exploded from Lake Nyos, a volcanic lake in northwestern Cameroon, and quickly suffocated more than 1, 700 people and 8, 000 animals. Scientists have attempted to control this phenomenon by slowly pumping the gas up from the bottom of the lake.

See also Air pollution; Carbon cycle; Planetary atmospheres.