Carbon dioxide (CO2) is a chemical compound. It is a gas at room temperature. It is made of one carbon and two oxygen atoms. 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. Plants use carbon dioxide to make food. This process is called photosynthesis. The properties of carbon dioxide were studied by the Scottish scientist Joseph Black in the 1750s.
Carbon dioxide is a greenhouse gas. Greenhouse gases trap heat energy. Greenhouse gases change the climate and weather on our planet, Earth. This is called climate change. Most scientists think that greenhouse gases are causing global warming, the rise of Earth's surface temperature.
Biological role [change]
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 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 breathed out. In plants using photosynthesis, carbon dioxide is taken in from the atmosphere.
Dry ice [change]
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. We can use dry ice for making drinks fizzy, killing gophers, and freezing warts. The vapor of dry ice causes suffocation and eventually, death. Caution and professional assistance is recommended whenever dry ice is in use.
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 odor and no color.
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 [change]
Chemists get carbon dioxide from cooling air. They call this air distillation. But this method is inefficient. 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:
Quicklime (CaO), a chemical that has widespread use, can be made heating limestone to about 850 °C. This reaction also makes CO2:
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:
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. (They include cellular respiration, anaerobic respiration and photosynthesis). Photoautotrophs (i.e. plants, cyanobacteria) use another reaction: Plants absorb CO2 from the air, and, together with water, react it to form carbohydrates:
Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO2 and H2CO3 (carbonic acid). The relative concentrations of CO2, H2CO3, 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 liter.
Industrial production [change]
- 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, CaCO3, in the making of lime (Calcium oxide, CaO);
Chemical reaction [change]
Carbon dioxide can be created with a simple chemical reaction:
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