定义Definition
二氧化碳是化学式为 CO₂ 的直线形分子，常温下为无色无味气体，密度比空气大。它是含碳物质燃烧和生物呼吸的产物，是光合作用的原料，也是最主要的人为温室气体。固态二氧化碳称为干冰。
Carbon dioxide, CO₂, is a linear molecule and at room temperature a colourless, odourless gas denser than air. It is produced by burning carbon-containing materials and by respiration, consumed by photosynthesis, and is the most important human-made greenhouse gas. Solid CO₂ is called dry ice.
结构与性质Structure and properties
碳原子 sp 杂化，两个 C=O 键长 116 pm，分子呈直线形，两个键偶极相互抵消，偶极矩为零，但有较大的四极矩。CO₂ 在常压下不存在液态，−78.5 °C 直接升华；三相点为 −56.6 °C、518 kPa，临界点 31.0 °C、7.38 MPa，超临界 CO₂ 是绿色萃取溶剂。
Carbon is sp hybridised; the two C=O bonds are 116 pm and the molecule is linear, so the bond dipoles cancel and the dipole moment is zero, though it has a large quadrupole moment. At atmospheric pressure CO₂ has no liquid phase and sublimes at −78.5 °C; the triple point is −56.6 °C and 518 kPa, and the critical point 31.0 °C and 7.38 MPa. Supercritical CO₂ is a green extraction solvent.
发现史History
17 世纪海尔蒙特发现木炭燃烧和发酵产生一种“野气”。1750 年代布莱克通过加热石灰石研究了这种“固定空气”，证明它比空气重、能使石灰水变浑浊，并存在于呼气中。1823 年法拉第首次将其液化。
In the seventeenth century van Helmont found that burning charcoal and fermentation release a 'gas sylvestre'. In the 1750s Black studied this 'fixed air' from heated limestone, showing it is heavier than air, turns limewater milky and is present in exhaled breath. Faraday first liquefied it in 1823.
环境与用途Environment and uses
大气 CO₂ 浓度从工业革命前的约 280 ppm 上升到 2020 年代的 420 ppm 以上，是全球变暖的主要原因，并使海洋酸化。CO₂ 用于碳酸饮料、灭火器、干冰制冷、超临界萃取咖啡因、焊接保护气以及合成尿素和甲醇；碳捕集与封存是减排技术之一。
Atmospheric CO₂ has risen from about 280 ppm before the industrial revolution to over 420 ppm in the 2020s, the main driver of global warming, and it is acidifying the oceans. CO₂ is used in fizzy drinks, fire extinguishers, dry-ice cooling, supercritical decaffeination, welding shielding gas, and the synthesis of urea and methanol; carbon capture and storage is one mitigation technology.
反应Reactions
CO₂ 是酸性氧化物，一般不助燃。
CO₂ is an acidic oxide and generally does not support combustion.
使澄清石灰水变浑浊，检验 CO₂
Turns limewater milky, the test for CO₂
继续通入，沉淀溶解（溶洞形成原理）
More CO₂ redissolves the precipitate, as in cave formation
镁能在 CO₂ 中燃烧
Magnesium burns in CO₂
光合作用
Photosynthesis
化学性质Chemical properties
与碱反应：少量 CO₂ 生成碳酸盐，过量生成碳酸氢盐。
With bases, a little CO₂ gives carbonate and excess gives bicarbonate.
与 NH₃ 高压反应生成尿素：2 NH₃ + CO₂ → CO(NH₂)₂ + H₂O。
Reacts with ammonia under pressure to give urea: 2 NH₃ + CO₂ → CO(NH₂)₂ + H₂O.
高浓度可致窒息：空气中超过约 10% 会使人失去意识。
High concentrations asphyxiate: above about 10% in air causes unconsciousness.
年表Timeline
- 约 1640
海尔蒙特描述“野气”
Van Helmont describes gas sylvestre
- 1754
布莱克研究“固定空气”
Black studies fixed air
- 1823
法拉第液化 CO₂
Faraday liquefies CO₂
- 1958
基林开始在莫纳罗亚连续监测大气 CO₂
Keeling begins continuous CO₂ measurements at Mauna Loa