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  3. Fire can be thought of as any combustion process intense enough to emit light. It may be a quietly burning flame or the brillian

Fire can be thought of as any combustion process intense enough to emit light. It may be a quietly burning flame or the brillian

admin2009-04-27  83
问题     Fire can be thought of as any combustion process intense enough to emit light. It may be a quietly burning flame or the brilliant flash of an explosion.
    A typical combustion process is the burning of gasoline in an automobile engine. The vaporized fuel is mixed with air, compressed in the engine’s cylinder, and ignited by a spark. As the fuel flame up, the heat produced flows into the adjacent layer of unburned fuel and ignites it. In this way a zone of fire spreads throughout the fuel mixture is called a combustion wave.
    The speed at which such a combustion wave travels through a fuel mixture is called the burning velocity of the mixture. The burning velocity of a gas such as methane quietly burning in air is only about one foot per second. By comparison, the burning velocity of more reactive combinations such as the rocket Fuels, hydrogen and fluorine, can be hundreds of feet per second.
    If the fuel flows at the same speed as the combustion wave, the result is a stationary flame, like the one in your kitchen gas burner. In the kitchen burner a jet of gas mixed with airflows from the opening in the head of the burner. If the velocity of the fuel mixture flowing from the opening is greater than its burning velocity, the flame blows out.
    In jet engines speeding through the air at 500 to 600 miles per hour, the engine’s flame is sometimes blown out by the blast of air entering the combustion chamber at high speeds. Jet pilots call this condition "flameout".
    Combustion can sometimes occur very slowly. A familiar example of slow combustion is the drying of ordinary oil-based paint. In this chemical reaction, called oxidation, the oxygen in the air reacts with the drying oil in the paint to provide a tough film. The linseed oil molecules link together, forming an insoluble coating.
    How can the chemical reaction involved in such a quiet process as the drying of paint also produce spectacular flames and explosions? The main difference between the two is the temperature at which they occur.
    At lower temperatures the reaction must take place over a long time. The heat which is slowly produced is dissipated to the surroundings and does not speed up the reaction. When the heat produced by the low-temperature reaction is retained instead of being dissipated, the system breaks into flame.
    In a flame or explosion, the reactions are extremely fast. In many chemical processes, however, such a rapid oxidation process would be extremely destructive.
选项 A、Their burning velocities differ in rate.
B、Oxidation is a chemical reaction while fire is a physical reaction.
C、Oxidation does not create heat.
D、They occur at different temperatures.
答案D
解析 下面哪个选项陈述了氧化和燃烧的主要区别?本题考查对文章细节内容的理解。文章倒数第三段明确指出了燃烧与氧化的区别在于发生反应的温度不同。要读懂此段的第一句，找出主干，就可发现比较的内容。
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