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Oxygen, heat and fuel are essential for combustion. What counts as a fuel? Is a hydrocarbon essential for combustion? Is combustion, say, of magnesium really combustion? And what's the difference between ignition and combustion?

I'm new to chemistry. If you could explain the problems above that will be great.

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  • $\begingroup$ Combustion is a quite technological term (although the minimum technology required is known since the discovery of fire). At the moment I think it defines a reaction with the characteristics you know but the oxidant is air or oxygen. Still X burns in Y is OK as soon the reaction results in a flame. $\endgroup$
    – Alchimista
    Jun 21 at 8:25
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A combustion is a chemical reaction of something with oxygen, producing much heat. If it also produces light, it is a flame. Ignition is the operation required for starting a combustion. Using a match may do the job. Fuel is usually a mixture of hydrocarbons derived from oil or petroleum. But combustion of Magnesium is also a combustion, although magnesium is not usually defined as fuel. Another example : Hydrogen is a gas burning quite easily, and it is not a hydrocarbon.

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  • $\begingroup$ Does it really require oxygen ? I have seen titanium burn ( light . heat ) in bromine and reliable sources ( Timet engineer) tell me titanium will burn in a hydrogen atmosphere when the protective oxide has been removed $\endgroup$ Jun 20 at 16:07
  • $\begingroup$ You may burn hydrogen gas altogether with chlorine to yield HCl; and this product, dissolved in water, yields hydrochloric acid, performed at large scale (there are HCl ovens, shown, for example on wikipedia). The discern between combustion as a (typically exothermic) reaction of compound x and an oxidizer (often, but not limited to oxygen) and an oxidation (which refers to an increase to the [formal] oxidation number of compound x) - again not constrained to reactions with oxygen - perhaps is that the former uses at least 1 gaseous reagent. $\endgroup$
    – Buttonwood
    Jun 20 at 16:28
  • $\begingroup$ Magnesium (as a solid) is not a typical fuel, agreed on this. But there are examples of rockets with solid fuel of type «once lit, it burns without stop»; contrasting to those with liquid fuel which may be re-lit and easier moderated in terms of the mechanical momentum they yield. Though the use gasoline (or an other hydrocarbon, as asked by the OP) as propellant may be not very frequently seen. $\endgroup$
    – Buttonwood
    Jun 20 at 16:31
  • $\begingroup$ @Buttonwood. You are perfectly right. I agree with all you said. But. But the OP said he or she is new in chemistry. I have adapted my answer to his or her level. It is no use mentioning combustions in bromine or chlorine !! $\endgroup$
    – Maurice
    Jun 20 at 16:49
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Taking into account you just started studying chemistry, and to expand Maurice' answer, I would reply your question of

"What counts as a fuel?"

that this depends on the reaction conditions set (e.g., temperature, concentration of the reagents present, pressure).* Then, any compound may be considered as a fuel provided it

  • may be at all oxidized. This often is expressed in the release of energy which may be the emission of heat; yet emission of light, or the potential to perform mechanical work may be other forms. As burning logs of wood, more than one form (e.g., heat and light) may occur simultaneously. Chemists account for this release of energy (account similar like balancing a bank account) in terms of enthalpy, for example Gibbs free enthalphy. The more energy released, the better for compound x to be considered a fuel.

and

  • the oxidation proceeds rapidly. This is one of the reasons why hydrocarbons are used as fuel in combustion engines, because at set temperature and pressure in the cylinder, it is in a fraction of second that these react and release a lot of energy.

    A counter example would be the oxidation of a nail of iron at ambient conditions. This reaction equally releases energy, but this is way too slow to be useful in an industrial process (by today's standards). Thus, it is not considered as a fuel.

*) For reactions with gases, there is some conceptual overlap of "concentration of the reagents" and "pressure". You may return to this detail later in your study.

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