Why do some materials burn to ash rather than melting?

[MirceaKitsune]

Although this is something I should probably know as someone interested in theoretical physics, it's still a thing I never actually understood. So although a "n00b" question, I was wondering:

 

Why is it that fire or extreme heat will melt / evaporate some materials while turning others to ash? I understand melting and evaporating, caused by heat reducing the bonds between atoms and making the material softer. This happens with most things... water, metal, stone, so on. But wood for instance will not melt into a liquid. It turns into ash... and I never heard of ash turning into a liquid or gas at any temperature.

 

Why does this happen at an atomic scale? Also, what does eventually happen to a wooden log at extreme temperatures? Does the ash simply get smaller, or is there a point at which it turns from solid to liquid and from liquid to vapor too?

 

As one more curiosity, how does this relate to burning? Most materials which turn from solid to liquid never cause a flame... such as metal melting. Plastic and rubber for instance do melt temporarily, but they also burn with a flame. Wood burns with a flame too but doesn't melt, only becomes ashes.

[Endy0816]

Ash is a byproduct of a chemical reaction. Something melting is a phase change.

 

Take ice melting. It is still H₂O afterwards. It has changed phase but the chemical equation is still the same.

 

Compare that with burning Hydrogen(combining Hydrogen with Oxygen) and converting it into H₂O instead.

Edited April 25, 2014 by Endy0816

[`hýsøŕ]

If you could heat up wood in a vacuum, it would melt. usually it reaches the point where it can react with the air and ignite before it reaches the melting point, so it typically turns into ash first.

[Greg H.]

If you could heat up wood in a vacuum, it would melt. usually it reaches the point where it can react with the air and ignite before it reaches the melting point, so it typically turns into ash first.

What is the melting point of wood I wonder?

 

Edit:

 

According to Yale Scientific magazine you can't melt wood, even in a vacuum, due to its composition, unless you can somehow reduce the melting point of carbon to something below 3500 degrees.

 

http://www.yalescientific.org/2010/05/everyday-qa-can-you-melt-a-wooden-log/

 

 

 

What if you tried melting wood in a vacuum, where there is no oxygen to begin the oxidation process? This clever idea still fails. Although the water and volatile matter would evaporate in the vacuum, the long cellulose fibers in wood would strongly inhibit wood’s transition to the liquid state. Heat might break the carbonyl bonds in cellulose, leaving behind carbon in charcoal form or carbon dioxide.

Edited April 25, 2014 by Greg H.

[Fuzzwood]

At high temperatures it becomes easier for some compounds to release water. Cellulose based compounds (wood, plants) are reduced to carbon at this point (and some trace metals and non-metals of course) which is commonly known as ash.

[MirceaKitsune]

Alright... it just caugth me after a statement in another thread, as well as helpful replies here. Some materials (like wood) aren't made of a single element, but a variety of different atom types linked together... which is what I was failing to consider. Before heat gets to melt all those components as a whole, it disintegrates various atomic combinations first, right? That's why burning wood produces many things (smoke, ash) while also requiring oxygen for the reaction. All those individual components however should behave by the cassic model (solid - liquid - gas) if each is brought to the right temperature for that to happen.

[petrushka.googol]

Ash indicates a preponderance of carbon in the substrate.....

[John Cuthber]

Ash indicates a preponderance of carbon in the substrate.....

No it doesn't.

[Fuzzwood]

Ash is not just carbon, but a mixture of carbon and a lot of metal elements which may or may not have been oxidized.

 

People discovered long ago (and I am going on a tangent here) that the rain water coming down from a recently used cremation based offering table did a very good job washing clothes. Why? Simple: those metals and metal oxides form bases on contact with water, which in turn partly hydrolyze the fats coming off from whatever was burned. Tadaa, instant soap. Guess where the name potassium comes from: potASH.

Edited May 13, 2014 by Fuzzwood

[MirceaKitsune]

My curiosity was mostly to understand the process. If I understood correctly last time, the reason wood burns is because heat decomposes its various molecules first, while the atoms of each require much higher temperatures to melt or gas themselves. But at the very end, each element composing that wood as well as the ash it results acts like all elements... and has a gas liquid and solid temperature of its own. Hoping that is indeed correct and I didn't miss anything else.

[petrushka.googol]

No it doesn't.

 

Pls peruse :

 

https://in.answers.yahoo.com/question/index?qid=20081210210647AA00iPb

[John Cuthber]

 

Pls peruse :

 

https://in.answers.yahoo.com/question/index?qid=20081210210647AA00iPb

Yahoo is hardly a peer reviewed expert.

People leave quite a lot of ash when cremated. People are mainly water. They don't have a preponderance of carbon.

Magnesium, which contains no carbon, leaves lots of ash.

Diamonds, which are pure carbon leave no ash when burned.

So, by simple observation, your unsupported assertion is wrong.

[Delta1212]

My curiosity was mostly to understand the process. If I understood correctly last time, the reason wood burns is because heat decomposes its various molecules first, while the atoms of each require much higher temperatures to melt or gas themselves. But at the very end, each element composing that wood as well as the ash it results acts like all elements... and has a gas liquid and solid temperature of its own. Hoping that is indeed correct and I didn't miss anything else.

That is, more or less, the reason wood will never melt. The reason it burns is that some of those chemicals undergo an oxidation reaction at high temperatures which gives off heat and light. That's what fire is.

[Fuzzwood]

Also keep in mind that the reaction gases (CO₂ and H₂O among others) are in fact comprised of several elements. In contrary to wood however, the atom bonds in these cases are a lot more stable than those found in wood. This is actually part of the reason why wood burns.

[petrushka.googol]

Yahoo is hardly a peer reviewed expert.

People leave quite a lot of ash when cremated. People are mainly water. They don't have a preponderance of carbon.

Magnesium, which contains no carbon, leaves lots of ash.

Diamonds, which are pure carbon leave no ash when burned.

So, by simple observation, your unsupported assertion is wrong.

 

Kindly refer -

 

http://www.huffingtonpost.com/2012/11/13/humans-carbon-chemist-molecular-bonds_n_2119037.html

 

This article endorses the view that carbon (whatever it's share may be in the total composition of elements) is by far the most important.

 

Actually my contention is that ash forming substances contain a preponderance of carbon because ash emissions originate from an inorganic part of the fuel, and the quantity of ash that is produced is small testifying to the abundance of carbon in the sample. (weight for weight).

Edited May 14, 2014 by petrushka.googol

[John Cuthber]

 

Kindly refer -

 

http://www.huffingtonpost.com/2012/11/13/humans-carbon-chemist-molecular-bonds_n_2119037.html

 

This article endorses the view that carbon (whatever it's share may be in the total composition of elements) is by far the most important.

 

Actually my contention is that ash forming substances contain a preponderance of carbon because ash emissions originate from an inorganic part of the fuel, and the quantity of ash that is produced is small testifying to the abundance of carbon in the sample. (weight for weight).

Why are you trying to argue with reality?

People contain more water than carbon.

That article doesn't mention ash and has nothing to do with the issue.

Diamonds burn to leave no ash.

Magnesium leaves a lot of ash even though it contains no carbon.

Your original assertion was simply wrong.

You are not going to get far trying to prove otherwise.

Incidentally, you contradict yourself when you say "my contention is that ash forming substances contain a preponderance of carbon because ash emissions originate from an inorganic part of the fuel,"

Carbon is organic so anything that is high in carbon is obviously low in inorganics.

As you say, the ash arises from that inorganic material.

So something which is high in carbon (and thus can't be high in inorganic material) can't produce much ash.

[Mordred]

carbon is not organic, it is an element. Carbon's high valence of 4 is simply highly useful in hydrocarbon chains (its readily combinable with other elements) But in and of itself it is not organic,. Also there are inorganic compounds with carbon

 

http://en.wikipedia.org/wiki/Compounds_of_carbon

Edited May 18, 2014 by Mordred

[hypervalent_iodine]

carbon is not organic, it is an element. Carbon's high valence of 4 is simply highly useful in hydrocarbon chains (its readily combinable with other elements) But in and of itself it is not organic,. Also there are inorganic compounds with carbon

 

http://en.wikipedia.org/wiki/Compounds_of_carbon

!

Moderator Note

This is incorrect within the context of John's comment and not exactly on topic.

 

I suppose a general reminder to everyone here to try and stick to the original topic is needed also, which seems more or less answered to me. Petrushka, if you wish to discuss your contention further, it's probably best that you open a separate thread on it.

[John Cuthber]

carbon is not organic, it is an element. Carbon's high valence of 4 is simply highly useful in hydrocarbon chains (its readily combinable with other elements) But in and of itself it is not organic,. Also there are inorganic compounds with carbon

 

http://en.wikipedia.org/wiki/Compounds_of_carbon

You may have noticed that the wiki page you cited explains that "The distinction between organic and inorganic compounds is only a matter of convention"

Also, there's this about inorganic fullerenes

http://en.wikipedia.org/wiki/Fullerene#Inorganic_fullerenes

which implies that the other fullerenes are organic. That's consistent with the chemistry of things like C₆₀.

So, at least that allotrope of carbon is organic so your blankets statement that "carbon is not organic" is false (as well as being irrelevant, in the context).

Edited May 19, 2014 by John Cuthber

[Mordred]

Yeah hypervalent_iodine already explained that in a private PM when I questioned him for clarity, interesting article on fullerines though. Thanks for that info

Edited May 19, 2014 by Mordred