Carbon dioxide

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids with oxygen as part of their metabolism. This is a process known as cellular respiration. This includes most 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 breathed out. Plants take in carbon dioxide from the atmosphere to use in photosynthesis.

Dry ice, or solid carbon dioxide, is the 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 gophers, and freeze warts. The vapor of dry ice causes suffocation and eventually, death. Caution and professional assistance are recommended whenever dry ice is in use.

 

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 atmospheres.

Carbon dioxide gas that comes off of dry ice is so cold that when it mixes with air it cools the water vapor 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.

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 reactions to separate carbon dioxide. Carbon dioxide is made in the reactions between most acids and most metal carbonates. For example, the reaction between hydrochloric acid and calcium carbonate (limestone or chalk) makes carbon dioxide:

${\displaystyle \mathrm {2\ HCl+CaCO_{3}\longrightarrow CaCl_{2}+H_{2}CO_{3}} }$ 

The carbonic acid (H₂CO₃) then 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:

${\displaystyle \mathrm {CaCO_{3}\longrightarrow CaO+CO_{2}} }$ 

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

${\displaystyle \mathrm {CH_{4}+2\ O_{2}\longrightarrow CO_{2}+2\ H_{2}O} }$ 

Carbon dioxide is made in steel mills. Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:^[3]

${\displaystyle \mathrm {Fe_{2}O_{3}+3\ CO\longrightarrow 2\ Fe+3\ CO_{2}} }$ 

Yeast metabolizes 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 bioethanol:

${\displaystyle \mathrm {C_{6}H_{12}O_{6}\longrightarrow 2\ CO_{2}+2\ C_{2}H_{5}OH} }$ 

All aerobic organisms produce CO
₂ 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. (They include cellular respiration, anaerobic respiration and photosynthesis). Photoautotrophs (i.e. plants, cyanobacteria) use another reaction: Plants absorb CO
₂ from the air, and, together with water, react it to form carbohydrates:

${\displaystyle \mathrm {nCO_{2}+nH_{2}O\longrightarrow (CH_{2}O)n+nO_{2}} }$ 

Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO₂ and H
₂CO
₃ (carbonic acid). The relative concentrations of CO
₂, H
₂CO
₃, and the deprotonated forms HCO⁻
₃ (bicarbonate) and CO²⁻
₃(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 litre.

Industrial carbon dioxide is produced mainly from six processes:^[4]

• By capturing natural carbon dioxide 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 fuels or wood;
• As a by-product of fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages;
• From thermal decomposition of limestone, CaCO
  ₃, in the making of lime (Calcium oxide, CaO);

Carbon dioxide can be created with a simple chemical reaction:

${\displaystyle \mathrm {C+O_{2}\longrightarrow CO_{2}} }$ 

carbon + oxygen → carbon dioxide