Let's imagine that I have a match in hand and nugget of coal on my desk. Then I light up the match and place it for few seconds near the coal so a tiny piece of nugget catches fire. Then another piece catches fire, then another and soon all the nugget is burnt down.

How did it happen? I gave the nugget just enough heat to burn the first piece. Where does come energy to burn the rest of nugget from?

  • $\begingroup$ When you light a nugget of coal, how does the flame persist even after you've extinguished the match? $\endgroup$ – lemon Feb 17 '15 at 18:12
  • $\begingroup$ @lemon Because I gave it some external energy which was equal to the enthalpy of combustion? But it was only equal to the enthalpy of a little piece of the nugget. I'm confused. $\endgroup$ – user46147 Feb 17 '15 at 18:20
  • $\begingroup$ @lemon pardon my stupid question, but my thermodynamics course was based mainly on memorizing very long derivations. Worst part of my life. $\endgroup$ – user46147 Feb 17 '15 at 18:38
  • $\begingroup$ The nuggets release heat + flammable gases when they burn, which allows other nuggets to catch fire. $\endgroup$ – pentane Feb 17 '15 at 19:11
  • $\begingroup$ @pentane But why do I need to deliver heat before the nugget releases more heat? I guess it is because of energy of bondings, but I would like to see more mathematical answer. $\endgroup$ – user46147 Feb 17 '15 at 19:19

It is called combustion, and it happens in materials which have a lower energy content when their component molecules join with the oxygen in the atmosphere, than when in a solid/liquid structure. When energy is given to start the fire the piece of coal burns and releases energy with excess enough to sustain the reaction and leave heat energy for use.

Combustion is a high-temperature exothermic chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.

  • $\begingroup$ But why do I need to give energy to start the fire, while even without my intervention the gaseous products have lower energy? Why isn't it spontaneous? And could it be understood only with classical thermodynamics, without molecular interpretation of matter? $\endgroup$ – user46147 Feb 17 '15 at 19:29
  • $\begingroup$ Oh! I found it in a thermodynamics textbook! Need of initial flame is not the matter of thermodynamics, but of reaction kinetics! But they didn't explained why and how. $\endgroup$ – user46147 Feb 17 '15 at 19:34
  • $\begingroup$ The joining of oxygen to carbon is not spontaneous at room temperatures, but at temperatures high enough to start breaking the chemical bonds, become gaseous to mix with oxygen. en.wikipedia.org/wiki/Spontaneous_combustion en.wikipedia.org/wiki/Fire_point $\endgroup$ – anna v Feb 17 '15 at 19:34
  • $\begingroup$ Now, with your help I can better understand what's in my handbook. Thanks! $\endgroup$ – user46147 Feb 17 '15 at 19:38

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