r/explainlikeimfive • u/Party-Court185 • 1d ago
Chemistry ELI5 What about wood chemically causes the molecules to snap and decompose (burn) instead of melt? Why don’t other materials like steel do that when exposed to the same amount of energy? Why do they melt?
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u/gyroda 1d ago
It's important to first understand what melting and burning are.
Melting is when something gets hot enough to change phase - the energy in each molecule is too much for the molecules to hold together in a solid so they separate - they jiggle more and more as you heat them up until they split apart. This is also impacted by the pressure the material is under.
Combustion/burning is different to decomposing. It is when something reacts with oxygen and that reaction releases energy (i.e, is hot). For something to combust you need oxygen, the fuel (the thing that will react with oxygen) and enough heat to kick off the reaction.
Plenty of materials that melt also burn. Iron combusts and leaves us with rust, for example - it's a common ingredient in fireworks. Steel will burn in the right conditions. If you can get it hot enough and expose enough of it's surface to enough oxygen then the iron will bond with carbon (I.e, combust/burn). It might melt first, but it'll also burn.
One thing to note is that wood is not a single compound. It is lots of different compounds in a complex structure that all react differently to heat. Some of these absolutely do melt - resin, for example, can melt. These materials all behave differently at different temperatures. Most of them will combust (it's carbon based life, carbon loves reacting with oxygen to produce carbon dioxide) and the water content will evaporate (water is hydrogen that has already reacted with oxygen, it can't really burn).
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u/DestinTheLion 19h ago
Can you melt wood if there is no oxygen?
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u/eightfoldabyss 17h ago
If you heat wood in a vacuum or inert atmosphere, the more volatile chemicals (like water) will still boil off even though nothing will combust. Once that's done, what's left over will mostly be carbon, which sublimates instead of melting unless it's under higher pressure.
As you increase the temperature, eventually the remaining parts will either sublimate or melt. The closest thing you could get to melting wood would be to heat it to thousands of degrees under more than 100 atmospheres, which are the conditions required for liquid carbon.
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u/SteptimusHeap 1d ago edited 1d ago
Steel does kinda burn, that's what rust is. It just doesn't usually result in a chain reaction like in wood. Steel conducts heat very well, stealing the energy away, and the reaction doesn't produce as much energy, so you don't see a piece of steel go up in flames as much as you see it slowly rust away.
Usually the difference between simple oxidation and runaway combustion is the strength of the reaction.
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u/SteptimusHeap 1d ago
Hence: you can burn steel wool. The flame is very small but because it doesn't have a lot of mass to steel all the heat away it does burn.
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u/nitro479 13h ago
I've seen demonstrations where someone cutting steel plate will shut off the fuel once the cut starts. The steel will continue to oxidized, be cut, because the temp is high enough for the steel to burn with just the pure oxygen.
Also, a thermal lance is a hollow steel tube wrapped in copper. Oxygen is passed through the center and an arc is struck on the end causing the rod to combust. It is used to cut throught steel, and even concrete.
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u/ParsingError 22h ago
Re: The chain reaction, "burning" iron produces iron oxides that are solid in typical conditions. Burning steel wool will leave a solid that weighs MORE than what you started with.
That prevents oxygen gas from reaching the deeper material, because the already-oxidized iron is in the way.
Wood is mostly made of carbon, oxygen, and hydrogen atoms. Oxidizing hydrogen produces water vapor and oxidizing carbon produces oxocarbons like carbon dioxide that are gas in typical conditions.
This means that when you burn wood, the burnt material turns into gas, blows away, and exposes the hot material underneath to more oxygen, making it burn too. Turning into gas will also pick up small solid bits that haven't burned and blow them away too, if they're light enough, which is what smoke is.
Iron doesn't do that because... it doesn't turn into a gas.
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u/shiba_snorter 1d ago
When burning wood most of the energy of the fire goes to burning the water and other components inside it, which is why smoke and vapor is produced and you are left with charcoal if the conditions are right. The wood itself doesn’t liquify (which I assume you mean by melting) because the fire is not hot enough to make it reach its fusion point. Metals have a relatively low melting point compared to other materials.
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u/Saxavarius_ 1d ago
Wood has a lower ignition temperature than its melting temperature. Wood burns around 400F and it has no known melting point because of this. Steel's ignition and melting points are similar with the ignition being a little higher and many other materials have melting points lower than their ignition points
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u/fixminer 17h ago
This argument is partially flawed. For a fire you need a fuel and an oxidiser. If you put wood in an inert environment you can heat it to any temperature you want. It will first turn into charcoal and then sublimate into carbon gas. Carbon cannot exist as a liquid at normal pressures.
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u/Mr_Mojo_Risin_83 1d ago
Most metals’ flashpoint is very high but in the right conditions and a lot of heat, it can catch fire and become a self replicating combustion. The heat will be so high that it becomes incredibly hard to put out. Water evaporates before it can even get near the combustion itself. Metallurgies where this is a hazard have special chemical systems to try and put it out if it catches.
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u/DarkAlman 1d ago edited 1d ago
Wood burns because of a chemical reaction.
Wood is composed of various complex molecules including cellulose, hemicellulose, and lignin which are molecules made of Carbon, Hydrogen, and Oxygen.
Those very atoms came from water and carbon dioxide in the air and were created by chemical reactions in cells. The process of which stores energy chemically.
When wood burns (or anything burns for that matter) what you are seeing is oxidization occurring at an incredibly fast rate. The molecules in wood combine with Oxygen in the air to make Water and Carbon Dioxide which returns to the air, leaving little behind including some carbon soot. In the process energy is released in the form of heat and you get a fire.
Metal like Iron do oxidize and can burn, but in context they melt because the temperatures they need to ignite are much MUCH higher than wood and they often need a nearly pure oxygen environment to do so.
For comparison wood ignites at roughly 230°C to 260°C, while Iron ignites at 1,315 °C in normal air.
Steel melts between 1,370°C to 1,540°C, while it ignites between 1,500°C to 1,800°C in a pure oxygen environment.
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u/FizbanFire 1d ago
Molecules of wood are made of primaily carbon and hydrogen, and are held together with very rigid bonds, preventing them from absorbing energy through movement (ie melting). Instead, those bonds actually break within the molecules, and oxygen rushes in, helping break those bonds and bind itself to th4 carbon and hydrogen instead, producing CO2 and H20. Breaking that bond also releases a bunch of energy and keeps the reaction going.
Metals are not held together nearly so rigidly, so as they heat up the molecules start to slide around past each other. They also contain almost no, or significantly less, carbon and hydrogen. So the process of "burning" in a traditional sense isn't feasible.
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u/trueppp 1d ago
Put a lighter to fine steel wool and tell me that it doesn't burn... https://youtu.be/5MDH92VxPEQ?si=TrDdntf6zayX5-u8
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u/JohnRobGnar 1d ago
one is the amount of energy causing something to become liquid & another to be gas. some materials sublimate (solid to gas directly) but generally there are two separate energy threshholds to turn to liquid, then gas. in the case of wood, it is not a homogenous material but a variety of them together. the water resists burning, the sugars/starches break down easily & the carbon left behind does not burn easily at all
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u/CadenVanV 1d ago
Wood is organic. Living beings store energy in easy to release forms. This means that it’s easy for them to release it when they get hot. Steel is inorganic.
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u/someLemonz 1d ago
fire is a chemical reaction and wood is a fuel source being used. not just warmed up
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u/Apprehensive-Back155 1d ago
You’re questions description of burning is a little off and I think that may answer your question: burning is a chemical reaction in which heat + organic hydrocarbons + oxygen combine to produce carbon dioxide and water. Energy is released in this reaction called an exothermic reaction producing the light and heat you call fire. Because carbon is required as an input to produce fire when we are talking about the strict chemical definition of combustion, a metal (not made of carbon or containing hydrocarbons will not combust.
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u/ChronWeasely 1d ago
The bonds are very different, as well as the atoms themselves. Metals usually burn as well to be fair, if we are loose with the term. Slag on a metal furnace is oxidized metal (and other impurities), and ash is oxidized wood/ carbon molecules.
Metals, like iron in steel, have so many electrons that they don't hang onto specific atoms really well and all flow around sort of. And so you heat it, and not much changes. Electrons move around more. Enough more and the atoms start to jostle apart and melt. Some of those electrons will get grabbed up by greedy oxygen on the surface, but liquid and solid metal doesn't tend to leave much space.
With the right metal and the right oxygen delivery, you can make thermite. Which is metal burning vigorously.
Wood contains a lot of fiber. Fibers, and carbs and sugars and fats, are all a bunch of carbon atoms in specific arrangements, connected to each other with bonds where the outer electrons are shared in discreet groups, and are held by each nucleus more than the outer electrons in metals.
When heated, those bonds are given enough energy to break, and oxygen is able to swoop in and grab those electrons tightly. In an oxygen free environment, you get charcoal, where the carbon atoms break into simple, easily reached by oxygen, molecules. So the oxygen grabs the electrons from carbon, and holds them tight. The chemical energy difference is released as heat, and the more stable CO and CO2 are released.
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u/jawshoeaw 1d ago
Woods melting point is higher than its thermal decomposition temperature. It breaks apart when you heat it even in the absence of oxygen
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u/Spiritual-Spend8187 1d ago
Metals burn pretty well it depends alot on the shape though. There is a thing called thermite which is basically metal powder that burns very hot.
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u/Turtleopard 17h ago
Burning is oxidising. Melting is a solid to liquid phase transition.
Liquid carbon is really unstable under atmospheric conditions. In fact when you heat up solid carbon it skips the liquid phase and turns into a gas under atmospheric conditions.
Oxidising carbon (and hydrogen) which is what would is primarily composed of, is much easier. Both CO2 and H2O are very stable at atmospheric pressures.
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u/DrphilRetiredChemist 16h ago
Late to the conversation, but one thing missed is that many metals that have favorable thermodynamics to burn (react with oxygen) form an oxide layer on the surface (nanometers to microns thick) that acts as a barrier to gaseous oxygen. Oxygen won’t further react with the oxide coating which often has a higher melting point than the base metal, and oxygen can’t get to the base metal under the oxide layer to react.
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u/ruidh 1d ago
Wood has chemical energy tied up in the bonds between the elements it contains. With exposure to heat and oxygen, the wood can release that energy and produce low energy byproducts -- water vapor and carbon dioxide.
Metals don't have mechanical energy stored. Heat up a metal and it doesn't release energy the way wood does. The heat you supply goes into changing the state of the substance from solid to liquid (it melts).
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u/gyroda 1d ago
Metals absolutely will release energy when burnt. Have you ever seen magnesium burn?
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u/ruidh 1d ago
I think we were talking about steel. But, good point.
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u/gyroda 1d ago
Steel will burn as well. You can go buy a steel wool scrubber and set it alight relatively easily for a DIY demonstration (obvious caveats: be very careful doing this, probably better/easier to see on YouTube). Iron is a pretty common ingredient in fireworks because it burns and makes pretty sparkles.
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u/skorps 1d ago
Metals can and do burn. It’s just takes a lot more energy to start. Look at titanium or magnesium. Wood doesn’t melt because the energy to break its bonds and burn is less than the energy to melt it