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Flash point ,autoignition temp

  1. Aug 22, 2012 #1
    Please see their definitions:
    Flash point
    The flash point of a volatile material is the lowest temperature at which it can
    vaporize to form an ignitable mixture in air. (i.e. the lowest temp to start a fire)
    At the flash point, the vapor may cease to burn when the source of ignition is removed.

    Autoignition temperature
    The autoignition temperature or kindling point of a substance is the lowest
    temperature at which it will spontaneously ignite in a normal atmosphere
    without an external source of ignition, such as a flame or spark.

    In fact my question is not directly related to the above information,

    Can anyone tell me why a fire is needed to "start another fire" at the flash pt but not at the autoignition temp?
    Let's take an example,
    You have a lighter,you generate a flame and you can "transfer" it onto a candle wick or to a gas stove if the fuels stored in the stove doesn't combust.

    In this case the conditions to start a fire are reached(fuel,O2,temperature reaches flash point,according to the fire triangle)

    Now,imagine there isn't any source of ignition(sparks or flame),but the
    ambient temp is very high(lower than the autoignition temp and higher than the flash point),the candle wax will only melt but it will not burn,similar case in the gas stove,that is nothing will burn.

    What is the use of fire?Why does a fire is needed to start another fire at flash point?
     
  2. jcsd
  3. Aug 23, 2012 #2

    chemisttree

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    That is because of how it is used. Flash point is for the most part a safety measurement. At what temperature will something combust in the presence of an ignition source? Very important from a safety point of view. This is mainly a function of the vapor pressure of the substance but it is also related to the compound's inherent stability. Some compounds are very easy to ignite over a wide range of concentrations in the air. They are much more dangerous than something that only can ignite at fairly high concentrations and over a narrow concentration range. The best way to measure this hazard is to expose the sample to a flame at varying temperatures. The hazard is controlled by limiting ignition sources and ventilation.

    Autoignition is also related to safety in a different way. Autoignition is related mostly to a compound's inherent stability against combustion. Even if ignition sources are carefully controlled there is a point (temperature) at which the sample will burn. This hazard is usually controlled by limiting temperature exposure. In a fire senario, this hazard is addressed by spraying water onto containers to keep them cool although that doesn't always work.
     
  4. Aug 23, 2012 #3
    What do you know about activation energy?
     
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