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Commonly known as carbonic gas, it is also called carbonic anhydride; its chemical formula is CO2. It is colorless at room temperature, a slightly acidic gas with a density of 1.977 g·cm-3. The CO2 molecule has a linear structure of O=C=O, with a bond length of 116.3 pm; it is a non-polar molecule and is isoelectronic with N3-, N2O, NO2+, OCN-, etc. Liquid CO2 has a low density; when it expands under reduced pressure, a portion of it evaporates rapidly, absorbing a large amount of heat that causes another portion of the liquid CO2 to freeze into a white, snow-like solid, commonly known as dry ice. Dry ice is a molecular crystal; at -78.5°C, it sublimates directly without melting, and its evaporation is slow, allowing temperatures around -80°C to be achieved. It is commonly used as a refrigerant. CO2 has very stable chemical properties; it generally does not decompose to release oxygen, nor can it burn or support combustion, which is why it is a commonly used fire extinguishing agent. However, CO2 cannot be used to extinguish the fire of reactive metals (such as K, Na, Mg, Zn, etc.), because they can burn in CO2; for example, hot carbon can also take away some of the oxygen to produce carbon monoxide (CO). In excess air, the combustion of carbon and its compounds (such as wood, coal, petroleum and its products, natural gas, etc.) produces CO2. In addition, animal carcasses and decaying plants, as well as respiration, cracks in volcanic areas, and many springs also release CO2. In the laboratory, CO2 can be produced by reacting marble (or carbonates) with hydrochloric acid. The large amount of CO2 required in industry is obtained from the calcination of limestone or as a byproduct of alcohol fermentation; it can also be recovered from flue gas. The SO2 and H2S present in the CO2 gas can be removed using KMnO4 and Na2CO3 solutions. CO2 is slightly soluble in water; at room temperature, the concentration of a saturated CO2 solution is 0.03–0.04 mol·L-1. Most of the CO2 exists in the form of hydrated molecules, namely CO2·xH2O, with only a small portion converting to carbonic acid, H2CO3. Experiments have shown that when [CO2]/[H2CO3] = 600, the \"effective ionization constant\" of carbonic acid is K1 = 2×10-4; whereas if it is assumed that all CO2 dissolved in water converts into carbonic acid, the calculated \"apparent ionization constant\" is K1 = 4.3×10-7. Therefore, carbonic acid is a weak acid. The following equilibrium exists in aqueous solution: increasing the temperature shifts the equilibrium to the left, allowing all of the CO2 to be released. When the CO2 concentration in human inhalation reaches 5%, it can stimulate the respiratory center, doubling the volume of breathing. When there is inhibition of the respiratory center, a mixture of inhaled CO2 (5–7%) and oxygen (93–95%) can be used as an excitant. Green plants can synthesize organic substances through photosynthesis after absorbing CO2 and water. CO2 can also be used in various refreshing beverages. CO2 is used in many applications, including the production of soda ash and sugar, the quenching of steel castings, the synthesis of urea, the preparation of carbonates and lead white, as well as as an inert medium in chemical synthesis.