de Lufz 發問於 科學及數學化學 · 1 十年 前

What is a flame?Please help! This is my homework!!

Please! Please help!

1.What is a flame?

2.What affects the colour of flame?

3.Why burning different substances will have different colours of flames?

4.What are the differences between flames in nuclear reactions and that in chemical reactions?

Help! Thanks a lot!!

2 個解答

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  • 1 十年 前
    最佳解答

    Flame introduction

    A flame is the product of a highly exothermic reaction (for example, combustion) a self-sustaining oxidation reaction. In other words, a flame could be said to be the visible (light emitting) part of a fire.

    The color and temperature of a flame are dependent on the type of fuel involved in the combustion. For example, when a lighter is held to a candle. This applied heat causes the fuel molecules in the wick to vaporize, in this state they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent flame. The high temperature of the flame tears apart the vaporized fuel molecules, forming various incomplete combustion products and free radicals, these products then react with the oxidizer involved in the reaction. Sufficient energy in the flame will excite the electrons in the pre-combusted products, which results in the emission of visible light (see spectrum below for an explanation of which specific radical species produce which specific colors). As the combustion temperature of a flame increases, (if the flame contains small particles of unburnt carbon or other material) so does the average energy of the electromagnetic radiation given off by the flame (see blackbody).

    Other oxidizers besides oxygen can be used to produce a flame. Hydrogen burning in chlorine produces a flame and in the process emits gaseous hydrogen chloride (HCl) as the combustion product. Another of many possible chemical combinations is hydrazine and nitrogen tetroxide which is hypergolic and commonly used in rocket engines.

    Flames, or at least portions of them, are often considered to be partial plasmas or a partially ionized gas, but in more general terms a flame is a region of exothermic chemical reaction of high enough temperature to emit visible light. A central region enclosed by such a reaction (but which does not necessarily participate in the reaction itself) is often also considered part of the flame.

    There are different methods of distributing the required components of combustion to a flame. In a diffusion flame, oxygen and fuel diffuse into each other; where they meet the flame occurs. In a premixed flame, the oxygen and fuel are premixed beforehand, which results in a different type of flame. Candle flames (a diffusion flame) operate through evaporation of the fuel which rises in a laminar flow of hot gas which then mixes with surrounding oxygen and combusts.

    Flame color

    Flame color depends on several factors. Typically the most important being: blackbody radiation and spectral band emission with both spectral line emission and spectral line absorption playing smaller roles. In the most common type of flame, hydrocarbon flames, the most important factor which determines color is oxygen supply and the extent of fuel oxygen "pre-mixture", which determines the rate of combustion and thus the temperature and reaction paths, different color hues are thereby produced.

    圖片參考:http://upload.wikimedia.org/wikipedia/commons/thum...

    Image of a flame test for sodium. Note that the yellow color in this gas flame does not arise from the blackbody emission of soot particles (as the flame is clearly a blue premixed complete combustion flame) but instead comes from the spectral line emission of sodium atoms, specifically the very intense "sodium D lines".

  • 1 十年 前

    i agree with him. you didnt ans the 4th qs.

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