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[[File:Carbon-dioxide.svg|thumb|[[Stuctural formula]] of carbon dioxide. C is [[carbon]] and O is [[oxygen]]. The double lines represent the double [[chemical bond]] between the [[atom]]s.]]
[[File:Carbon-dioxide-3D-vdW.png|thumb|A picture to show simply how the atoms may fill space. The black is carbon and the red is oxygen.]]

'''Carbon dioxide''' (CO2) is a [[chemical compound]] and is acidic. It is a [[gas]] at [[room temperature]]. It is made of one [[carbon]] and two [[oxygen]] [[atom]]s. People and animals release carbon dioxide when they breathe out. Also, every time something organic is burnt (or a [[fire]] is made), it makes carbon dioxide. [[Plant]]s use carbon dioxide to make [[food]]. This process is called [[photosynthesis]].<ref>{{cite book|first=Ann|last=Fullick|title=Edexcel IGCSE Biology Revision Guide|url=https://archive.org/details/edexceligcsebiol0000full|publisher=Pearson Education|date=2011|isbn=9780435046767|page=40}}</ref> The properties of carbon dioxide were studied by the [[Scots|Scottish]] scientist [[Joseph Black]] in the 1750s.

Carbon dioxide is a [[greenhouse gas]].<ref>{{cite book|first=Steve|last=Woolley|title=Edexcel IGCSE Physics Revision Guide|url=https://archive.org/details/edexceligcsephys0000wool|publisher=Pearson Education|date=2011|isbn=9780435046736|page=[https://archive.org/details/edexceligcsephys0000wool/page/59 59]}}</ref> Greenhouse gases trap [[Thermal energy|heat energy]]. Greenhouse gases change the [[climate]] and [[weather]] on our [[planet]], [[Earth]]. This is called [[climate change]]. Greenhouse gases are a cause of [[global warming]], the rise of Earth surface [[temperature]].

Its concentration in Earth's atmosphere since late in the [[Precambrian]] was regulated by [[photosynthetic]] organisms and geological phenomena (mainly volcanos).

== Biological role ==
Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and [[amino acid]]s with [[oxygen]] as part of their [[metabolism]]. This is a process known as [[cellular respiration]]. This includes all [[animal]]s, many [[fungi]] and some [[bacteria]]. In higher animals, the carbon dioxide travels in the blood from the body's [[Tissue (biology)|tissues]] to the [[lungs]] where it is breathed out. Plants take in carbon dioxide from the [[atmosphere]] to use in [[photosynthesis]].

== Dry ice ==
'''Dry ice''', or [[solid]] carbon dioxide, is the [[solid|solid state]] of CO2 gas below -109.3 °F (-78.5[[°C]]). Dry ice does not occur naturally on earth but is man made. It is [[colorless]]. People use dry ice to make things cold, and to make drinks fizzy, kill [[Pocket gopher|gophers]], and freeze [[wart]]s. The [[vapor]] of dry ice causes [[suffocation]] and eventually, death. Caution and professional assistance is recommended whenever dry ice is in use.
[[File:Dry ice in water.JPG|thumb|left|Dry ice when put in water]]
At usual pressure it will not [[melt]] from a solid to a [[liquid]] but instead changes directly from a solid to a gas. This is called [[sublimation]]. It will change directly from a solid to a gas at any temperature higher than extremely cold temperatures. Dry ice sublimates at normal air temperature. Dry ice exposed to normal air gives off carbon dioxide gas that has no color. Carbon dioxide can be liquified at pressure above 5.1 [[Atmosphere (unit)|atmospheres]].

Carbon dioxide gas that comes off of dry ice is so cold that when it mixes with air it cools the water vapour in the air to fog, which looks like a thick white smoke. It is often used in the theater to create the appearance of [[fog]] or [[smoke]].

== Isolation and production ==
Chemists can get carbon dioxide from cooling air. They call this air [[distillation]]. This method is inefficient because a large amount of air must be refrigerated to extract a small amount of CO2. Chemists can also use several different [[chemical reaction]]s to separate carbon dioxide. Carbon dioxide is made in the reactions between most [[acid]]s and most [[metal]] carbonates. For example, the reaction between [[hydrochloric acid]] and calcium carbonate (limestone or chalk) makes carbon dioxide:

: <math>\mathrm{2 \ HCl + CaCO_3 \longrightarrow CaCl_2 + H_2CO_3}</math>

The [[carbonic acid]] (H2CO3) then [[wikt:decomposes|decomposes]] to water and CO2. Such reactions cause foaming or bubbling, or both. In industry, such reactions are used many times to neutralize waste acid streams.

[[Quicklime]] (CaO), a chemical that has widespread use, can be made heating limestone to about 850 °C. This reaction also makes CO2:

: <math>\mathrm{CaCO_3 \longrightarrow CaO + CO_2}</math>

Carbon dioxide is also made in the [[combustion]] of all carbon-containing [[fuel]]s, such as [[methane]] ([[natural gas]]), petroleum distillates ([[gasoline]], [[Diesel fuel|diesel]], [[kerosene]], [[propane]]), coal or wood. In most cases, water is also released. As an example the chemical reaction between methane and oxygen is:

: <math>\mathrm{CH_4 + 2 \ O_2 \longrightarrow CO_2 + 2 \ H_2O}</math>

Carbon dioxide is made in [[steel mill]]s. [[Iron]] is reduced from its oxides with [[coke (fuel)|coke]] in a [[blast furnace]], producing [[pig iron]] and carbon dioxide:<ref>
{{Cite book
| last = Strassburger
| first = Julius H.
| title = Blast Furnace: Theory and Practice
| publisher = American Institute of Mining, Metallurgical, and Petroleum Engineers
| place = New York
| year = 1969
| isbn = 9780677104201
}}
</ref>

: <math>\mathrm{Fe_2O_3 + 3 \ CO \longrightarrow 2 \ Fe + 3 \ CO_2}</math>

[[Yeast]] metabolizes [[sugar]] to produce carbon dioxide and [[ethanol]], also known as [[alcohol]], in the production of [[wine]]s, [[beer]]s and other [[Liquor|spirits]], but also in the production of bioethanol:

: <math>\mathrm{C_6H_{12} O_6 \longrightarrow 2 \ CO_2 + 2 \ C_2H_5OH}</math>

All [[cellular respiration|aerobic]] organisms produce {{chem|CO|2}} when they oxidize [[carbohydrate]]s, [[fatty acid]]s, and proteins in the mitochondria of cells. The large number of reactions involved are exceedingly complex and not described easily. (They include [[cellular respiration]], [[anaerobic respiration]] and [[photosynthesis]]). Photoautotrophs (i.e. plants, [[cyanobacteria]]) use another reaction: Plants absorb {{chem|CO|2}} from the air, and, together with water, react it to form carbohydrates:

: <math>\mathrm{nCO_2 + nH_2O \longrightarrow (CH_2O)n + nO_2}</math>

Carbon dioxide is [[soluble]] in water, in which it spontaneously interconverts between CO2 and {{chem|H|2|CO|3}} ([[carbonic acid]]). The relative concentrations of {{chem|CO|2|, H|2|CO|3}}, and the deprotonated forms {{chem|HCO|3|−}} ([[bicarbonate]]) and {{chem|CO|3|2−}}([[carbonate]]) depend on the acidity ([[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. So, air-equilibrated [[ocean]] water (mildly alkaline with typical pH = 8.2–8.5) contains about 120 mg of bicarbonate per liter.

=== Industrial production ===
Industrial carbon dioxide is produced mainly from six [[process]]es:<ref name="kirk">{{cite encyclopedia|title=Carbon Dioxide|first=Ronald|last = Pierantozzi|encyclopedia=Kirk-Othmer Encyclopedia of Chemical Technology|publisher=John Wiley & Sons, Inc.|year=2001|doi =10.1002/0471238961.0301180216090518.a01.pub2}}</ref>
* By [[wikt:capturing|capturing]] natural carbon dioxide [[spring (hydrosphere)|springs]] where it is produced by the action of acidified water on [[limestone]] or [[dolomite]].
* As a by-product of [[hydrogen]] production plants, where [[methane]] is converted to CO2;
* From combustion of [[fossil fuel]]s or [[wood]];
* As a by-product of [[fermentation]] of [[sugar]] in the [[brewing]] of [[beer]], [[whisky]] and other [[alcohol]]ic [[beverage]]s;
* From thermal decomposition of limestone, {{chem|CaCO|3}}, in the making of lime ([[Calcium oxide]], {{chem|CaO}});

== Chemical reaction ==
Carbon dioxide can be created with a simple [[chemical reaction]]:
: <math>\mathrm{C + O_2 \longrightarrow CO_2}</math>
: [[carbon]] + [[oxygen]] → carbon dioxide 

== References ==
{{commons category|Carbon dioxide}}
{{reflist}}

[[Category:Organic compounds]]
[[Category:Biochemistry]]
[[Category:Greenhouse gases]]
[[Category:Oxides]]

Carbon dioxide - Revision history

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