二氧化碳(carbon dioxide)
一种无机化合物，无色而略带刺鼻气味和酸味的气体。化学式CO2。地球大气层次要组分（按体积约占万分之三）。在含碳物质燃烧、发酵和动物呼吸作用中形成。植物利用它进行碳水化合物的光合作用。大气层中的二氧化碳能避免地球所接收的辐射能的一部分返回空间，因而产生所谓温室效应。二氧化碳稍溶於水，生成弱酸性溶液。氨与二氧化碳在压力作用下反应形成氨基甲酸铵，然後转变成尿素。各种工业上应用的二氧化碳，是从烟道气中回收或作为制备氢气（用於合成氨）的副产品，或从石灰以及其他来源回收得到。二氧化碳可用作冷冻剂，用於灭火器，充气救生筏和救生衣、爆破采煤、泡沫橡胶和泡沫塑胶，它能促进温室中植物生长，使动物在屠宰前丧失其活动能力，以及用於碳酸饮料中。点燃的镁可在二氧化碳中继续燃烧，但二氧化碳对大多数物质来说是不助燃的。人在二氧化碳浓度为5%的空气中长时间停留会昏迷或死亡。让液态二氧化碳膨胀到大气压力时，则自行冷却并有一部分冻结成乾冰。
English version：
carbon dioxide
Inorganic compound, a colorless gas with a faint, sharp odor and a sour taste when dissolved in water, chemical formula CO2. About 0.03% of air by volume, it is produced when carbon-containing materials burn completely and from fermentation and animal respiration. Plants use CO2 in photosynthesis to make carbohydrates. In the atmosphere, CO2 keeps some of the sun's energy received by the earth from radiating into space (see greenhouse effect). In water, it forms a solution of a weak acid, carbonic acid (H2CO3). Its reaction with ammonia is the first step in synthesizing urea. An important industrial material, CO2 is recovered from sources including flue gases, the process that produces hydrogen, and limekilns. It is used as a refrigerant, chemical intermediate, and inert atmosphere; in fire extinguishers, foaming rubber and plastics, carbonated beverages (see carbonation), and aerosol sprays; in water treatment, welding, and cloud seeding; and for promoting plant growth in greenhouses. Under pressure it becomes a liquid, the form most used in industry. If the liquid is allowed to expand, it cools and partially freezes to the solid form, dry ice.