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I have been looking at thermodynamics and I tried to find the answer on the internet but nothing of relevance came up and even this site did not have the answer.

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  • $\begingroup$ Is it flammable if it burns without a flame? $\endgroup$
    – user137289
    Commented Jul 24, 2019 at 19:55
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    $\begingroup$ @Manav I'm really confused about this question. How do you know that something is "flammable if it burns without flame" but don't know if "burning always produces a flame"? It seems like by saying Yes to Pieter you're suggesting that you already know the answer. $\endgroup$
    – JMac
    Commented Jul 24, 2019 at 20:04
  • $\begingroup$ What about substances (like hydrogen, methanol) burning with invisible flame? $\endgroup$ Commented Jul 24, 2019 at 20:06
  • $\begingroup$ You need to define your usage of "burning" and "flame" here. $\endgroup$ Commented Jul 24, 2019 at 20:26
  • $\begingroup$ Given your response to Jmac’s answer, what are you really adking? $\endgroup$
    – user207455
    Commented Jul 24, 2019 at 21:13

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Does burning always produce a flame and if so why?

No. And the reason generally involves the character of the fuel (chemical and physical properties) as well as the availability of sufficient oxidizer (commonly oxygen).

Combustion (the more technical term for burning) does not necessarily involve a flame. Three broad categories (there are several intermediary) are: Smoldering combustion, glowing combustion, and flaming combustion.

Smoldering combustion is the slow, lower temperature, flameless form of combustion. We see this in the end stages of fires where materials can smolder for long periods of time without being noticed. Smoldering combustion involves solid fuel materials such as coal, wood, cellulose, tobacco, cotton and some synthetic polymers. It occurs primarily in the interior of porous combustible materials. A familiar example is a lit cigarette sitting in an ashtray.

Glowing combustion, a.k.a surface burning, is a reaction between oxygen (or other oxidizer) at the surface of a solid fuel in which heat and light is produced, but no flame. In contrast to flaming combustion fuel oxidations occurs in the solid phase of the fuel, rather than the gas phase. A familiar example is when a cigarette glows when puffed.

It should be noted that both smoldering and glowing combustion can rapidly transition to flaming combustion, particularly when given an enriched supply of oxygen. An example of intentionally doing so is the use of a bellows to increase the rate of combustible of a fire. Bedding fires due to smoking in bed involving mattresses made before newer safety standards were promulgated is an example. Smoldering mattresses can suddenly burst into flames due to aeration (resulting from the movement of the sleeper).

Flaming combustion occurs in the gaseous phase of fuels. Generally, flaming combustion occurs most readily in gaseous fuels, followed by liquids and lastly solids. Flaming combustion occurs easiest in gaseous fuels, simply because the fuel is already in the gaseous phase.

Liquid fuels are classified or grouped as being either flammable or combustible, but both can involve flaming combustion once ignited. The main difference is the temperature at which the rate of vaporization above the liquid is sufficient to be ignited. For flammable fuels ignition can occur at or below normal working temperatures. A common example is gasoline. Combustible liquids have higher temperature flashpoints. The actual categorizations can vary, but typically begin over 100 F. They generally require some preheating or atomizing in order to obtain a sufficient rate of vaporization to ignite. A common example is kerosene.

Solid fuels (with some exceptions) are generally the most difficult to ignite and flame. Generally this is because they need to be heated and decomposed (process called pyrolysis) in order to product volatile gases. Generally, flaming occurs in the vapor phase and not the solid itself.

There's a lot more to this stuff than I have covered in this limited forum.

Hope it helps.

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  • $\begingroup$ First of all thank you for all the pointers but can you please explain to me the difference between glow and flame because as I see it a glow is a small flame. Am I right or not? $\endgroup$
    – Manav
    Commented Jul 25, 2019 at 10:17
  • $\begingroup$ @Manav I'm afraid that's not correct. Consider the various stages of a wood fire in a fireplace. At the peak of the fire flames are present above the surface of the wood. That is flaming combustion. The flame is the visible, gaseous part of combustion. As the fire dies down and the flames first disappear you see glowing embers remaining on the surface of the wood. These aren't "flames". The combustion is now glowing combustion. Eventually the glowing embers disappear, but there is still combustion (an exothermic reaction) occurring but at lower temperatures. This is smoldering combustion. $\endgroup$
    – Bob D
    Commented Jul 25, 2019 at 11:41
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The Wikipedia definition of burning is "combustion, a high-temperature reaction between a fuel and an oxidant". However, rust is a type of oxidation that occurs much more slowly and, while it gives off heat it does so over a longer period of time, so it does not produce the instant high heat. Consider steel wool, if the wires are thin enough, they can be lit to produce combustion, or you can get them wet and leave them in your sink for a few days and you will see they have rusted. Both processes produce iron oxide although the impurities will likely be different. So while burning is considered to be combustion, you can have oxidation at a slower rate without flame.

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  • $\begingroup$ Also steel wool does not really have a flame when burning. It is the solid iron that glows. $\endgroup$
    – user137289
    Commented Jul 24, 2019 at 21:51
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You can have combustion without a flame. If, for example, you have a very low pressure of methane (say 1e-3 mbar) in an oxygen-containing atmosphere and heat it up to the methane autoignition temperature (550 C or so), you will be capable of triggering the combustion of the methane (transforming it into CO2 and water). But if the pressure is low enough, the energy released by this reaction will not be enough to ionize or heat the surrounding gas substantially to produce a flame.

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"Burning" is really combustion (reaction with $\mathrm{O}_2$). Above a certain temperature, flammable substances will spontaneously react with oxygen (autoignition). Once a substance begins to burn, it will release an amount of heat that depends on the difference in energy stored in the bonds before and after reaction with oxygen. For the burning to be sustained, this amount of energy must be great enough to maintain the temperature above the threshold for reaction. This will occur more readily for substances with high vapor pressures, as the reaction will occur more rapidly and the heat will not have as much time to dissipate. Too much vapor pressure, then, will lead to an accelerated release of heat, driving the temperature higher.

In a combustion reaction, the visible flame is the result of ionization driven by high temperatures. I don't have an example on hand, but it's conceivable that a substance with a low enough autoignition temperature, combined with a suitable vapor pressure and a small release of heat, could result in a sustained "flame" with no visible fire. Too low of a vapor pressure, and the substance would not react quickly enough to sustain the reaction. Too high of a vapor pressure, and the substance would quickly become hot enough to cause a visible flame.

To achieve combustion with no visible flame, removing impurities from the system, such as trace amounts of metallic elements, would help. Providing a way of dissipating the heat/restricting the available oxygen, such as a flowing stream of inert gas, would also help to prevent the temperature from rising too much.

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  • $\begingroup$ Most of the flames that one sees is glowing soot. $\endgroup$
    – user137289
    Commented Jul 24, 2019 at 21:08
  • $\begingroup$ Thank you for pointing this out to me. I didn't realize blackbody radiation played a part in flames and was fascinated to learn as much. I should have researched it more rather than relying on what I thought I knew. $\endgroup$
    – user138962
    Commented Jul 28, 2019 at 2:21

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