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Also known as carbon monoxide(II), with the chemical formula CO. It is a colorless gas with a density of 1.250 g·cm-3, slightly lighter than air. It has a structural formula of C≡O, featuring a triple bond; it is isoelectronic with N2 and CN-. The distance between the nuclei, dc-O, is 112.8 pm. It possesses a low dipole moment (μ=0.118 D) as well as a high dissociation energy of 1071.6 kJ·mol-1. CO has a very low melting and boiling point; it liquefies at –192°C and solidifies at –205°C. CO dissolves very little in water and does not undergo any chemical reaction with it, but it is soluble in ethanol. Under normal conditions, CO is inert; at high temperatures it exhibits reducing properties and can reduce many metal oxides to metals, making it widely used in the metallurgical industry. When burned in air, CO produces a pale blue flame, releases a large amount of heat, and is converted into carbon dioxide; it is a commonly used gaseous fuel. In the presence of sunlight or activated carbon (a catalyst), CO reacts directly with chlorine to produce phosgene, the highly toxic gas COCl2. Phosgene is an important industrial product used in the manufacture of many organic and inorganic substances, such as dyes. When heated, CO can be oxidized by sulfur to form thionyl COS. For inert transition metal compounds, such as palladium chloride PdCl2, CO can also reduce them to form black metallic palladium even in aqueous solution. This reaction is very noticeable even at trace amounts of CO, and thus it can be used to detect the presence of CO. CO is an inert or neutral oxide, but under high temperature and pressure it can still react with caustic alkalis to form formates; therefore, CO can be considered, in a formal sense, the anhydride of formic acid. Under certain conditions, the non-bonding electrons of the carbon atom in the CO molecule can be donated to an electron pair acceptor to form a complex; for example, CO reacts with the electron-deficient compound diborane B2H6 to form BH3CO. Under certain conditions, CO can also directly combine with certain transition metals such as nickel and iron to form a series of metal carbonyl complexes, such as tetracarbonylnickel Ni(CO)4 and pentacarbonyliron Fe(CO)5. In solution, CO can also undergo a combination reaction with certain metal ions to form complex compounds. For example, a concentrated hydrochloric acid solution or an ammonia solution of copper(I) chloride, CuCl, can absorb CO to form CuCl·CO. This property can be utilized to remove small amounts of CO from the raw gases used in ammonia synthesis, as well as to determine the CO content in gas analyses. CO is the main component of producer gas and water gas. A small amount of CO can be produced in the laboratory by reacting concentrated sulfuric acid with formic acid or oxalic acid. The product obtained from oxalic acid is impure and contains CO2, which can be removed using a sodium hydroxide solution or lime water. At relatively low temperatures, carbon burns to produce CO2, whereas CO can hardly be formed even in conditions of low oxygen supply. However, at high temperatures of 400–500°C, carbon begins to react with the generated CO2 to form CO; this reaction occurs on a large scale during the blast furnace process and in gas generators. CO is highly toxic; what makes it particularly dangerous is that it is colorless and odorless, allowing people to be poisoned without realizing it. As little as 1/800 of the volume of CO in the air can be fatal within half an hour, which is what is known as carbon monoxide poisoning. The toxicity of CO lies in its ability to form complexes with red blood cells in the blood, thereby preventing hemoglobin from carrying oxygen and leading to death. The reducibility of CO can also be used to remove SO2 from flue gas. It is also used for the pretreatment of iron ore in steelmaking and other smelting processes. Under high temperature and pressure, CO can be catalytically combined with hydrogen to produce methanol, which is one of the main methods for manufacturing methanol on an industrial scale. CO is also an intermediate in various organic reactions, such as carbonylation, where it can convert unsaturated hydrocarbons into aldehydes. Aldehydes can be converted into alcohols through catalytic hydrogenation or into acids through oxidation. When CO reacts with acetylene, it produces acrylic acid, which is an important raw material in organic synthesis.