Chemical and physical properties
Carbon dioxide is a colourless, odorless gas. When inhaled at concentrations
much higher than usual atmospheric levels, it can produce a sour taste in the
mouth and a stinging sensation in the nose and throat. These effects result from
the gas dissolving in the
mucous membranes and saliva, forming a weak solution of
carbonic acid. This sensation can also occur during an attempt to stifle a
burp after drinking a carbonated beverage. Amounts above 5,000 ppm are
considered very unhealthy, and those above about 50,000 ppm (equal to 5% by
volume) are considered dangerous to animal life.
temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3,
about 1.5 times that of
air. The carbon dioxide molecule (O=C=O)
contains two double bonds and has a linear shape. It has no
and as it is fully
oxidized, it is moderately
reactive and is non-flammable, but will
support the combustion of metals such as
At −78.51 °C
or −109.3 °F,
carbon dioxide changes directly from a solid phase to a gaseous phase through
sublimation, or from gaseous to solid
through deposition. Solid carbon dioxide is
normally called "dry
generic trademark. It was first observed in 1825 by the French chemist
Charles Thilorier. Dry ice is commonly used as
a cooling agent, and it is relatively inexpensive. A convenient property for
this purpose is that solid carbon dioxide sublimes directly into the gas phase
leaving no liquid. It can often be found in grocery stores and laboratories, and
it is also used in the shipping industry. The largest non-cooling use for dry
ice is blast cleaning.
Liquid carbon dioxide forms only at
above 5.1 atm; the triple point of carbon dioxide is about 518
at −56.6 °C (See phase diagram, above). The
critical point is 7.38 MPa at
An alternative form of solid carbon dioxide, an amorphous
glass-like form, is possible, although not at atmospheric pressure.
This form of glass, called
carbonia, was produced by
supercooling heated CO2 at extreme pressure (40–48
or about 400,000 atmospheres) in a
diamond anvil. This discovery confirmed the theory that carbon dioxide could
exist in a glass state similar to other members of its elemental family, like
glass) and germanium. Unlike silica and germania glasses,
however, carbonia glass is not stable at normal pressures and reverts back to
gas when pressure is released.
Supercritical carbon dioxide and
History of human understanding
Carbon dioxide was one of the first gases to be described as a substance
distinct from air. In the seventeenth century, the
Jan Baptist van Helmont observed that
when he burned
charcoal in a closed vessel, the mass of the resulting ash was much less
than that of the original charcoal. His interpretation was that the rest of the
charcoal had been transmuted into an invisible substance he termed a "gas" or
"wild spirit" (spiritus sylvestre).
The properties of carbon dioxide were studied more thoroughly in the 1750s by
the Scottish physician Joseph Black. He found that
(calcium carbonate) could be heated or treated
with acids to yield
a gas he called "fixed air." He observed that the fixed air was denser than air
and did not support either flame or animal life. Black also found that when
bubbled through an aqueous solution of lime (calcium
hydroxide), it would
precipitate calcium carbonate. He used
this phenomenon to illustrate that carbon dioxide is produced by animal
respiration and microbial fermentation. In 1772, English chemist
Joseph Priestley published a paper entitled
Impregnating Water with Fixed Air in which he described a process of
dripping sulfuric acid (or oil of vitriol as
Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the
gas to dissolve by agitating a bowl of water in contact with the gas.
Carbon dioxide was first liquefied (at elevated pressures) in 1823 by
Davy and Michael Faraday.
The earliest description of solid carbon dioxide was given by
Charles Thilorier, who in 1834 opened a
pressurized container of liquid carbon dioxide, only to find that the cooling
produced by the rapid evaporation of the liquid yielded a "snow" of solid CO2.
Isolation and production
Carbon dioxide may be obtained from air
distillation. However, this yields only very small quantities of CO2.
A large variety of chemical reactions yield carbon dioxide, such as the reaction
between most acids and most metal carbonates. For example, the reaction between
hydrochloric acid and calcium carbonate
(limestone or chalk) is depicted below:
2 HCl + CaCO3 → CaCl2 + H2CO3
The H2CO3 then decomposes to
water and CO2. Such reactions are accompanied by foaming or bubbling,
or both. In industry such reactions are widespread because they can be used to
neutralize waste acid streams.
The production of quicklime
(CaO) a chemical that has widespread use, from limestone by heating at about 850
°C also produces CO2:
CaCO3 → CaO + CO2
combustion of all carbon containing fuels, such as
gas), petroleum distillates (gasoline,
also of coal and wood, will yield carbon dioxide and, in most cases, water. As
an example the chemical reaction between methane and oxygen is given below.
CH4 + 2 O2 → CO2 + 2 H2O
Iron is reduced
from its own oxides with coke in a
furnace, producing pig iron and carbon dioxide:
2 Fe2O3 + 3 C → 4 Fe + 3 CO2
sugar to produce
carbon dioxide and ethanol, also known as alcohol, in the production of
wines, beers and other spirits, but also in the production of
2 CO2 + 2 C2H5OH
All aerobic organisms produce
CO2 when they oxidize
carbohydrates, fatty acids, and proteins in the mitochondria of
cells. The large number of reactions involved are exceedingly complex and not
described easily. Refer to (cellular respiration,
anaerobic respiration and
Photoautotrophs (i.e. plants,
cyanobacteria) use another modus operandi: Plants absorb
CO2 from the air, and, together with
water, react it to form carbohydrates:
nCO2 + nH2O → (CH2O)n
Carbon dioxide is soluble in
water, in which it spontaneously interconverts between CO2 and
acid). The relative concentrations of CO2, H2CO3,
and the deprotonated forms HCO3−
depend on the pH. In
neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates
(>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very
alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The
bicarbonate and carbonate forms are very soluble, such that air-equilibrated
ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg
of bicarbonate per liter.
Carbon dioxide bubbles in a soft drink.
Carbon dioxide is used by the food industry, the oil industry, and the chemical
is used in many consumer products that require pressurized gas because it is
inexpensive and nonflammable, and because it undergoes a phase transition from
gas to liquid at room temperature at an attainable pressure of approximately 60
(870 psi, 59 atm), allowing far more carbon dioxide to fit in a given container
than otherwise would. Life jackets often contain canisters of pressured carbon
dioxide for quick inflation. Aluminum capsules are also sold as supplies of
compressed gas for airguns,
markers, for inflating bicycle tires, and for making
seltzer. Rapid vaporization of liquid carbon
dioxide is used for blasting in coal mines. High concentrations of carbon
dioxide can also be used to kill pests, such as the Common Clothes Moth.
Carbon dioxide is used to produce
carbonated soft drinks and soda water.
Traditionally, the carbonation in beer and sparkling wine comes about through
natural fermentation, but some manufacturers carbonate these drinks
A candy called Pop Rocks is pressurized with carbon dioxide gas at
about 40 bar (600 psi). When placed in the mouth, it dissolves (just like other
hard candy) and releases the gas bubbles with an audible pop.
Leavening agents produce carbon dioxide to cause dough to rise.
Baker's yeast produces carbon dioxide by fermentation of sugars within the
dough, while chemical leaveners such as
baking powder and baking
soda release carbon dioxide when heated or if exposed to
Carbon dioxide is the most commonly used compressed gas for pneumatic systems in
portable pressure tools and combat robots.
Carbon dioxide extinguishes flames, and some
fire extinguishers, especially those designed
for electrical fires, contain liquid carbon dioxide under pressure. Carbon
dioxide has also been widely used as an extinguishing agent in fixed fire
protection systems for total flooding of a protected space, (National Fire
Protection Association Code 12). International Maritime Organisation standards
also recognise carbon dioxide systems for fire protection of ship holds and
engine rooms. Carbon dioxide based fire protection systems have been linked to
several deaths. A review of CO2 systems (Carbon Dioxide as a Fire Suppressant:
Examining the Risks, US EPA) identified 51 incidents between 1975 and the date
of the report, causing 72 deaths and 145 injuries.
Carbon dioxide also finds use as an atmosphere for
although in the welding arc, it reacts to oxidize
most metals. Use in the automotive industry is common despite significant
evidence that welds made in carbon dioxide are brittler
than those made in more inert atmospheres, and that such weld joints deteriorate
over time because of the formation of carbonic acid. It is used as a welding gas
primarily because it is much less expensive than more inert gases such as
Liquid carbon dioxide is a good solvent for many lipophilic
organic compounds, and is used to remove
First, the green coffee beans are soaked in water. The beans are placed in the
top of a column seventy feet (21 m) high. Then super-pressurized carbon dioxide
in fluid form at about 93 degrees Celsius enters at the bottom of the column.
The caffeine diffuses out of the beans and into the carbon dioxide.
Pharmaceutical and other chemical processing
Carbon dioxide has begun to attract attention in the
pharmaceutical and other chemical processing industries as a less toxic
alternative to more traditional solvents such as
organochlorides. It's used by some
cleaners for this reason. (See
In the chemical industry, carbon dioxide is used for the production of
bicarbonates, and sodium salicylate.
Plants require carbon dioxide to conduct
photosynthesis, and greenhouses may enrich their atmospheres with additional
CO2 to boost plant growth, since its low present-day atmosphere
concentration is just above the "suffocation" level for green plants. A
photosynthesis-related drop in carbon dioxide concentration in a greenhouse
compartment can kill green plants. At high concentrations, carbon dioxide is
toxic to animal life, so raising the concentration to 10,000 ppm (1%) for
several hours can eliminate pests such as
spider mites in a greenhouse.
It has been proposed that carbon dioxide from power generation be bubbled into
ponds to grow algae that could then be converted into
Carbon dioxide is already increasingly used in greenhouses as the main carbon
Spirulina algae. In medicine, up to 5% carbon dioxide is added to pure
stimulation of breathing after apnea and to stabilize the O2/CO2
balance in blood.
A common type of industrial gas laser is the
carbon dioxide laser.
Polymers and plastics
Carbon dioxide can also be combined with
oxide from orange peels or other epoxides
to create polymers and plastics.
Carbon dioxide is used in
enhanced oil recovery where it is injected
into or adjacent to producing oil wells, usually under
supercritical conditions. It acts as both a
pressurizing agent and, when dissolved into the underground
crude oil, significantly reduces its viscosity,
enabling the oil to flow more rapidly through the earth to the removal well.
In mature oil fields, extensive pipe networks are used to carry the carbon
dioxide to the injection points.
Liquid and solid carbon dioxide are important
refrigerants, especially in the food industry, where they are employed
during the transportation and storage of ice cream and other frozen foods. Solid
carbon dioxide is called "dry ice" and is used for small shipments where
refrigeration equipment is not practical.
Liquid carbon dioxide (industry nomenclature R744 /
R-744) was used as a refrigerant prior to the discovery of
R-12. Its physical properties are highly
favorable for cooling, refrigeration, and heating purposes, having a high
volumetric cooling capacity. Due to their operation at pressures of up to 130
bars, CO2 systems require highly resistant components that have been
already developed to serial production in many sectors.
Its environmental advantages (GWP of 1, non-ozone depleting,
non-toxic, non-flammable) could make it the future working fluid to replace
current HFCs in cars, supermarkets, hot water heat pumps, among others. Some
applications: Coca-Cola has fielded CO2-based beverage coolers and
the US Army is interested in CO2 refrigeration
and heating technology.
By the end of 2007, the global car industry is expected to decide on the
next-generation refrigerant in car air conditioning. CO2 is one
discussed option.(see The Cool War)
Coal bed methane recovery
In enhanced coal bed methane
recovery, carbon dioxide is pumped into the coal seam to displace methane.
Carbon dioxide in the form of dry ice is often used in the wine
making process to cool down bunches of
after picking to help prevent spontaneous
fermentation by wild
advantage of using dry ice over regular water ice is that it cools the grapes
without adding any additional water that may decrease the
concentration in the grape must,
and therefore also decrease the alcohol concentration in the finished wine.
Dry ice is also used during the cold soak phase of the wine making process to
keep grapes cool. The carbon dioxide gas that results from the sublimation of
the dry ice tends to settle to the bottom of tanks because it is heavier than
regular air. The settled carbon dioxide gas creates an hyoxic environment which
helps to prevent bacteria from growing on the grapes until it is time to start
the fermentation with the desired strain of yeast.
Carbon dioxide is also used to create a hypoxic environment for
carbonic maceration, the process used to
produce Beaujolais wine.
Carbon dioxide is sometimes used to top up wine bottles or other storage vessels
such as barrels to prevent oxidation, though it has the problem that it can
dissolve into the wine, making a previously still wine slightly fizzy. For this
reason, other gasses such as nitrogen or
preferred for this process by professional wine makers.
In the Earth's atmosphere
Carbon dioxide in earth's atmosphere is considered a
currently occurring at an average concentration of about 385 parts per million
by volume or 582 parts per million by mass.
The mass of the
Earth atmosphere is 5.14×1018 kg ,
so the total mass of atmospheric carbon dioxide is 3.0×1015 kg (3,000
gigatonnes). Atmospheric concentrations of carbon dioxide fluctuate slightly
with the change of the seasons, driven primarily by seasonal plant growth in the
Northern Hemisphere. Concentrations of
carbon dioxide fall during the northern spring and summer as plants consume the
gas, and rise during the northern autumn and winter as plants go dormant, die
and decay (see graph at right). Concentrations also vary considerably on a
regional basis: in urban areas it is generally higher and indoors it can reach
10 times the background atmospheric concentration.
Carbon dioxide is a greenhouse gas. See
greenhouse effect for more.
Yearly increase of atmospheric CO2
: In the 1960s, the average annual
increase was 37% of the 2000–2007 average.
Due to human activities such as the combustion of fossil
fuels and deforestation, the concentration of atmospheric
carbon dioxide has increased by about 35% since the beginning of the
age of industrialization.
In 1999, 2,244,804,000 (=~2.2×109) metric tons of CO2 were
produced in the U.S. as a result of electric energy generation. This is an
output rate of 0.6083 kg (1.341 pounds) per kWh.
Five hundred million years ago carbon dioxide was 20 times more prevalent than
today, decreasing to 4–5 times during the
period and then maintained a slow decline until the industrial revolution, with
a particularly swift reduction occurring 49 million
Up to 40% of the gas emitted by some
during subaerial volcanic eruptions is carbon dioxide.
According to the best estimates, volcanoes release about 130-230 million tonnes
(145-255 million tons) of CO2 into the atmosphere each year. Carbon
dioxide is also produced by hot springs such as those at the Bossoleto site near
Rapolano Terme in Tuscany, Italy. Here, in a bowl-shaped depression of about 100
m diameter, local concentrations of CO2 rise to above 75% overnight,
sufficient to kill insects and small animals, but warm rapidly when sunlit and
disperse by convection during the day
Locally high concentrations of CO2, produced by disturbance of deep
lake water saturated with CO2 are thought to have caused 37
fatalities at Lake Monoun,
1984 and 1700 casualties at Lake Nyos, Cameroon in 1986.
However, emissions of CO2 by human activities are currently more than
130 times greater than the quantity emitted by volcanoes, amounting to about 27
billion tonnes per year.
In the oceans
There is about 50 times as much carbon dissolved in the oceans in the form of CO2
and CO2 hydration products as exists in the atmosphere. The oceans
act as an enormous carbon sink, having "absorbed about one-third of all
human-generated CO2 emissions to date."
Generally, gas solubility decreases as water temperature increases. Accordingly
the ability of the oceans to absorb carbon dioxide from the atmosphere decreases
as ocean temperatures rise.
Most of the CO2 taken up by the ocean forms carbonic acid. Some is
consumed in photosynthesis by organisms in the water, and a small proportion of
that sinks and leaves the carbon cycle. There is considerable concern that as a
result of increased CO2 in the atmosphere the acidity of seawater has
been increasing and may adversely affect organisms living in the water. In
particular, with increasing acidity, the availability of carbonates for forming
Carbon dioxide is an end product in organisms that obtain energy from breaking
down sugars, fats and amino acids with
oxygen as part
metabolism, in a process known as
cellular respiration. This includes all
plants, animals, many fungi and some bacteria. In higher animals, the carbon
dioxide travels in the blood from the body's tissues to the lungs where it is
exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the
Role in photosynthesis
Plants remove carbon dioxide from the atmosphere by photosynthesis, also called
carbon assimilation, which uses light energy to produce organic plant
by combining carbon dioxide and water. Free oxygen is released as gas from the
decomposition of water molecules, while the hydrogen is split into its protons
and electrons and used to generate chemical energy via
photophosphorylation. This energy is
required for the fixation of carbon dioxide in the
cycle to form sugars. These sugars can then be used for growth within the
plant through respiration.
Even when vented, carbon dioxide must be introduced into greenhouses to maintain
plant growth, as the concentration of carbon dioxide can fall during daylight
hours to as low as 200 ppm. Plants can potentially grow up to 50 percent faster
in concentrations of 1,000 ppm CO2 when compared with ambient
Plants also emit CO2 during respiration, so it is only during growth
stages that plants are net absorbers. For example a growing forest will absorb
many tons of CO2 each year, however a mature forest will produce as
much CO2 from respiration and decomposition of dead specimens (e.g.
fallen branches) as used in biosynthesis in growing plants.
Regardless of this, mature forests are still valuable
sinks, helping maintain balance in the Earth's atmosphere. Additionally, and
crucially to life on earth, phytoplankton photosynthesis absorbs dissolved CO2
in the upper ocean and thereby promotes the absorption of CO2 from
Carbon dioxide content in fresh air (averaged between sea-level and 10 hPa
level, i.e. about 30 km altitude) varies between 0.036% (360 ppm) and 0.039%
(390 ppm), depending on the location (see graphical map of CO2).
According to the Australian Maritime Safety Authority, "Prolonged exposure to
moderate concentrations can cause acidosis and adverse effects on calcium
phosphorus metabolism resulting in increased calcium deposits in soft tissue.
Carbon dioxide is toxic to the heart and causes diminished contractile force. At
concentrations of three per cent by volume in air, it is mildly narcotic and
causes increased blood pressure and pulse rate, and causes reduced hearing. At
concentrations of about five per cent by volume it causes stimulation of the
respiratory centre, dizziness, confusion and difficulty in breathing accompanied
by headache and shortness of breath. At about eight per cent concentration it
causes headache, sweating, dim vision, tremor and loss of consciousness after
exposure for between five and ten minutes." 
A natural disaster linked to CO2 intoxication occurred during the
limnic eruptions in the CO2-rich lakes of
in the Okun range of North-West Cameroon: the gas was brutally expelled from the
mountain lakes and leaked into the surrounding valleys, killing most animal
forms. During the Lake Nyos tragedy of 1986, 1700 villagers and 3500 livestock
Due to the health risks associated with carbon dioxide exposure, the U.S.
Occupational Safety and Health Administration says that average exposure for
healthy adults during an eight-hour work day should not exceed 5,000 ppm (0.5%).
The maximum safe level for infants, children, the elderly and individuals with
cardio-pulmonary health issues is significantly less. For short-term (under ten
minutes) exposure, the U.S. National Institute for Occupational Safety and
Health (NIOSH) and American Conference of Government Industrial Hygienists
(ACGIH) limit is 30,000 ppm (3%). NIOSH also states that carbon dioxide
concentrations exceeding 4% are immediately dangerous to life and health.
Adaptation to increased levels of CO2 occurs in humans. Continuous
inhalation of CO2 can be tolerated at three percent inspired
concentrations for at least one month and four percent inspired concentrations
for over a week. It was suggested that 2.0 percent inspired concentrations could
be used for closed air spaces (ex.
since the adaptation is physiological and reversible. Decrement in performance
or in normal physical activity does not happen at this level.
These figures are valid for pure carbon dioxide. In indoor spaces occupied by
people the carbon dioxide concentration will reach higher levels than in pure
outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more
than 20% of occupants, and the discomfort will increase with increasing CO2
concentration. The discomfort will be caused by various gases coming from human
respiration and perspiration, and not by CO2 alone. At 2,000 ppm the
majority of occupants will feel a significant degree of discomfort, and many
will develop nausea and headaches. The CO2 concentration between 300
and 2,500 ppm is used as an indicator of indoor air quality.
Acute carbon dioxide toxicity is sometimes known by the names given to it by
blackdamp (also called choke damp or stythe).
Miners would try to alert themselves to dangerous levels
of carbon dioxide in a mine shaft by bringing a caged canary with them as they
worked. The canary would inevitably die before CO2 reached levels
toxic to people. (The canary would also indicate dangerous levels of methane and
other gases by the same principle.)
Carbon dioxide ppm levels (CDPL) are a surrogate for measuring indoor pollutants
that may cause occupants to grow drowsy, get headaches, or function at lower
activity levels. To eliminate most Indoor Air Quality complaints, total indoor
CDPL must be reduced to below 600. NIOSH considers
that indoor air concentrations that exceed 1,000 are a marker suggesting
inadequate ventilation. ASHRAE
recommends they not exceed 1,000 inside a space.
Arterial blood gas
CO2 is carried in blood in three different ways. (The exact
percentages vary depending whether it is arterial or venous blood).
- Most of it (about 70% – 80%) is converted to
bicarbonate ions HCO3− by the enzyme
carbonic anhydrase in the red blood cells,
by the reaction CO2 + H2O → H2CO3 →
H+ + HCO3−.
the main oxygen-carrying molecule in
blood cells, carries both oxygen and carbon dioxide. However, the CO2
bound to hemoglobin does not bind to the same site as oxygen. Instead, it
combines with the N-terminal groups on the four globin chains. However, because
of allosteric effects on the hemoglobin
molecule, the binding of CO2 decreases the amount of oxygen that is
bound for a given partial pressure of oxygen. The decreased binding to carbon
dioxide in the blood due to increased oxygen levels is known as the
Haldane Effect, and is important in the transport
of carbon dioxide from the tissues to the lungs. Conversely, a rise in the
partial pressure of CO2 or a lower pH will cause offloading of oxygen
from hemoglobin, which is known as the Bohr
Carbon dioxide is one of the mediators of local
autoregulation of blood supply. If its levels are high, the
capillaries expand to allow a greater blood flow to
Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate
influences the level of CO2 in their blood. Breathing that is too
slow or shallow causes
respiratory acidosis, while breathing that
is too rapid leads to hyperventilation, which may cause
Although the body requires oxygen for metabolism, low oxygen levels do not
stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide
levels. As a result, breathing low-pressure air or a gas mixture with no oxygen
at all (such as pure nitrogen) can lead to loss of consciousness without ever
experiencing air hunger. This is especially perilous for
high-altitude fighter pilots. It is also why flight attendants instruct
passengers, in case of loss of cabin pressure, to apply the oxygen mask to
themselves first before helping others — otherwise one risks going unconscious.
Typically the gas we exhale is about 4% to 5% carbon dioxide and 4% to 5%
less oxygen than was inhaled.
According to a study by the
United States Department of
Agriculture, an average person's respiration generates approximately 450
liters (roughly 900 grams) of carbon dioxide per day.
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