Carbon dioxide

What is carbon dioxide and how is it discovered?

Joseph Black, a Scottish chemist and physician, first identified carbon dioxide in the 1750s. At room temperatures (20-25 oC), carbon dioxide is an odourless, colourless gas, which is faintly acidic and non-flammable.
Carbon dioxide is a molecule with the molecular formula CO2. The linear molecule consists of a carbon atom that is doubly bonded to two oxygen atoms, O=C=O.
Although carbon dioxide mainly consists in the gaseous form, it also has a solid and a liquid form. It can only be solid when temperatures are below -78 oC. Liquid carbon dioxide mainly exists when carbon dioxide is dissolved in water. Carbon dioxide is only water-soluble, when pressure is maintained. After pressure drops the CO2 gas will try to escape to air. This event is characterised by the CO2 bubbles forming into water.

CO2-molecule

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Properties of carbon dioxide

There are several physical and chemical properties, which belong to carbon dioxide.
Here we will sum them up in a table.

Property

Value

Molecular weight

44.01

Specific gravity

1.53 at 21 oC

Critical density

468 kg/m3

Concentration in air

370,3 * 107 ppm

Stability

High

Liquid

Pressure < 415.8 kPa

Solid

Temperature < -78 oC

Henry constant for solubility

298.15 mol/ kg * bar

Water solubility

0.9 vol/vol at 20 oC

Where on earth do we find carbon dioxide?

Carbon dioxide can be found mainly in air, but also in water as a part of the carbon cycle. We can show you how the carbon cycle works, by means of an explanation and a schematic representation. --> Move to the Carbon Cycle.

Applications of carbon dioxide by humans

Humans use carbon dioxide in many different ways. The most familiar example is its use in soft drinks and beer, to make them fizzy. Carbon dioxide released by baking powder or yeast makes cake batter rise.
Some fire extinguishers use carbon dioxide because it is denser than air. Carbon dioxide can blanket a fire, because of its heaviness. It prevents oxygen from getting to the fire and as a result, the burning material is deprived of the oxygen it needs to continue burning.
Carbon dioxide is also used in a technology called supercritical fluid extraction that is used to decaffeinate coffee. The solid form of carbon dioxide, commonly known as Dry Ice, is used in theatres to create stage fogs and make things like "magic potions" bubble.

The part carbon dioxide plays in environmental processes

Carbon dioxide is one of the most abundant gasses in the atmosphere. Carbon dioxide plays an important part in vital plant and animal process, such as photosynthesis and respiration. These processes will be briefly explained here.

Green plants convert carbon dioxide and water into food compounds, such as glucose, and oxygen. This process is called photosynthesis.

The reaction of photosynthesis is as follows:
6 CO2 + 6 H2O --> C6H12O6 + 6 O2

Plants and animals, in turn, convert the food compounds by combining it with oxygen to release energy for growth and other life activities. This is the respiration process, the reverse of photosynthesis.

The respiration reaction is as follows:
C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O

Photosynthesis and respiration play an important role in the carbon cycle and are at equilibrium with one another.
Photosynthesis dominates during the warmer part of the year and respiration dominates during the colder part of the year. However, both processes occur the entire year. Overall, then, carbon dioxide in the atmosphere decreases during the growing season and increases during the rest of the year.
Because the seasons in the northern and southern hemispheres are opposite, carbon dioxide in the atmosphere is increasing in the north while decreasing in the south, and vice versa. The cycle is more clearly present in the northern hemisphere; because it has relatively more land mass and terrestrial vegetation. Oceans dominate the southern hemisphere.

Influence of carbon dioxide on alkalinity

Carbon dioxide can change the pH of water. This is how it works:

Carbon dioxide dissolves slightly in water to form a weak acid called carbonic acid, H2CO3, according to the following reaction:
CO2 + H2O --> H2CO3

After that, carbonic acid reacts slightly and reversibly in water to form a hydronium cation, H3O+, and the bicarbonate ion, HCO3-, according to the following reaction:
H2CO3 + H2O --> HCO3- + H3O+

This chemical behaviour explains why water, which normally has a neutral pH of 7 has an acidic pH of approximately 5.5 when it has been exposed to air.

Carbon dioxide emissions by humans

Due to human activities, the amount of CO2 released into the atmosphere has been rising extensively during the last 150 years. As a result, it has exceeded the amount sequestered in biomass, the oceans, and other sinks.
There has been a climb in carbon dioxide concentrations in the atmosphere of about 280 ppm in 1850 to 364 ppm in 1998, mainly due to human activities during and after the industrial revolution, which began in 1850.
Humans have been increasing the amount of carbon dioxide in air by burning of fossil fuels, by producing cement and by carrying out land clearing and forest combustion. About 22% of the current atmospheric CO2 concentrations exist due to these human activities, considered that there is no change in natural amounts of carbon dioxide. We will take a closer look at these effects in the next paragraph.

Environmental problems - the greenhouse effect

The troposphere is the lower part of the atmosphere, of about 10-15 kilometres thick. Within the troposphere there are gasses called greenhouse gasses. When sunlight reaches the earth, some of it is converted to heat. Greenhouse gasses absorb some of the heat and trap it near the earth's surface, so that the earth is warmed up. This process, commonly known as the greenhouse effect, has been discovered many years ago and was later confirmed by means of laboratory experiments and atmospheric measurements.
Life as we know it exists only because of this natural greenhouse effect, because this process regulates the earth's temperature. When the greenhouse effect would not exist, the whole earth would be covered in ice.
The amount of heat trapped in the troposphere determines the temperature on earth. The amount of heat in the troposphere depends on concentrations of atmospheric greenhouse gasses and the amount of time these gasses remain in the atmosphere. The most important greenhouse gasses are carbon dioxide, CFC's (Chlor-Fluoro-Carbons), nitrogen oxides and methane.

Since the industrial revolution in 1850 began, human processes have been causing emissions of greenhouse gasses, such as CFC's and carbon dioxide. This has caused an environmental problem: the amounts of greenhouse gasses grew so extensively, that the earth's climate is changing because the temperatures are rising. This unnatural addition to the greenhouse effect is known as global warming. It is suspected that global warming may cause increases in storm activity, Melting of ice caps on the poles, which will cause flooding of the inhabited continents, and other environmental problems.

Together with hydrogen, carbon dioxide is the main greenhouse gas. However, hydrogen is not emitted during industrial processes. Humans do not contribute to the hydrogen amount in the air, this is only changing naturally during the hydrological cycle, and as a result it is not a cause of global warming.
Increasing carbon dioxide emissions cause about 50-60% of the global warming. Carbon dioxide emissions have risen from 280 ppm in 1850 to 364 ppm in the 1990s.

In the previous paragraph various human activities that contribute to the emission of carbon dioxide gas have been mentioned. Of these activities fossil fuel combustion for energy generation causes about 70-75% of the carbon dioxide emissions, being the main source of carbon dioxide emissions. The remaining 20-25% of the emissions are caused by land clearing and burning and by emission from motor vehicle exhausts.
Most carbon dioxide emissions derive from industrial processes in developed countries, such as in the United States and in Europe. However, carbon dioxide emissions from developing countries are rising. In this century, carbon dioxide emissions are expected to double and they are expected to continue to rise and cause problems after that.
Carbon dioxide remains in the troposphere about fifty up to two hundred years.

The first person who predicted that emissions of carbon dioxide from the burning of fossil fuels and other burning processes would cause global warming was Svante Arrhenius, who published the paper "On the influence of carbonic acid in the air upon the temperature of the ground" in 1896.
In the beginning of the 1930 it was confirmed that atmospheric carbon dioxide was actually increasing. In the late 1950s when highly accurate measurement techniques were developed, even more confirmation was found. By the 1990s, the global warming theory was widely accepted, although not by everyone. Whether global warming is truly caused by increasing carbon dioxide in the atmosphere, is still debated.

Rising carbon dioxide concentrations in air in the past decades

The Kyoto treaty

World leaders gathered in Kyoto, Japan, in December 1997 to consider a world treaty restricting emissions of greenhouse gases, mainly of carbon dioxide, that are thought to cause global warming. Unfortunately, while the Kyoto treaties have worked for a while America is now trying to evade them.

Carbon dioxide and health

Carbon dioxide is essential for internal respiration in a human body. Internal respiration is a process, by which oxygen is transported to body tissues and carbon dioxide is carried away from them.
Carbon dioxide is a guardian of the pH of the blood, which is essential for survival.
The buffer system in which carbon dioxide plays an important role is called the carbonate buffer. It is made up of bicarbonate ions and dissolved carbon dioxide, with carbonic acid. The carbonic acid can neutralize hydroxide ions, which would increase the pH of the blood when added. The bicarbonate ion can neutralize hydrogen ions, which would cause a decrease in the pH of the blood when added. Both increasing and decreasing pH is life threatening.

Apart from being an essential buffer in the human system, carbon dioxide is also known to cause health effects when the concentrations exceed a certain limit.

The primary health dangers of carbon dioxide are:
- Asphyxiation. Caused by the release of carbon dioxide in a confined or unventilated area. This can lower the concentration of oxygen to a level that is immediately dangerous for human health.
- Frostbite. Solid carbon dioxide is always below -78 oC at regular atmospheric pressure, regardless of the air temperature. Handling this material for more than a second or two without proper protection can cause serious blisters, and other unwanted effects. Carbon dioxide gas released from a steel cylinder, such as a fire extinguisher, causes similar effects.
- Kidney damage or coma. This is caused by a disturbance in chemical equilibrium of the carbonate buffer. When carbon dioxide concentrations increase or decrease, causing the equilibrium to be disturbed, a life threatening situation may occur.
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Resources:

http://www.oism.org/pproject/s33p36.htm
http://cdiac.ornl.gov/pns/faq.html
http://www.ilpi.com/msds/ref/carbondioxide.html
Living in the Environment, a book by G. Tyler Miller



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