什麼是二氧化碳

一種無機化合物，無色而略帶刺鼻氣味和酸味的氣體。化學式CO2。地球大氣層次要組分（按體積約佔萬分之三）。在含碳物質燃燒、發酵和動物呼吸作用中形成。植物利用它進行碳水化合物的光合作用。大氣層中的二氧化碳能避免地球所接收的輻射能的一部分返回空間，因而產生所謂溫室效應。二氧化碳稍溶於水，生成弱酸性溶液。氨與二氧化碳在壓力作用下反應形成氨基甲酸銨，然後轉變成尿素。各種工業上應用的二氧化碳，是從煙道氣中回收或作爲製備氫氣（用於合成氨）的副產品，或從石灰以及其他來源回收得到。二氧化碳可用作冷凍劑，用於滅火器，充氣救生筏和救生衣、爆破採煤、泡沫橡膠和泡沫塑膠，它能促進溫室中植物生長，使動物在屠宰前喪失其活動能力，以及用於碳酸飲料中。點燃的鎂可在二氧化碳中繼續燃燒，但二氧化碳對大多數物質來說是不助燃的。人在二氧化碳濃度爲5%的空氣中長時間停留會昏迷或死亡。讓液態二氧化碳膨脹到大氣壓力時，則自行冷卻並有一部分凍結成乾冰。

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.