1Kg of wood

[Beswad]

If I set 1kg of wood on fire what is the resultant weight after it finishes burning?

[jake-the-peg]

According to this

http://cemariposa.ucdavis.edu/newsletterfiles/Backyard_H orticulture1965.pdf

4000lbs of wood will produce 50lbs of ash for softwood or 160lbs of hardwood ash.

I'd guess that that is assuming that the wood is dry

[Flurfl]

Well technically if you weighed all the products of the combustion (mostly CO2 and H2O) it would actually be heavier than 1kg due to the addition of oxygen as the wood burns

[jake-the-peg]

I think technically it would be more massive as the resultant gasses would occupy more volume they displace air and are bouyed up giving them less net weight

[Flurfl]

Well yes it would be more massive, but it would also weigh more. It may be less dense than the wood alone at STP but not lighter.

Weight is purely the gravitational force acting on a mass irrespective of any other concerns, so if something is more massive than something else being subjected the same acceleration it weighs more. Helium still has a weight even though it is lighter than air.

[jake-the-peg]

I thought you'd say that which is why I said net weight. Seeing as you can't burn the wood without the surrounding air the concept of the absolute weight in a vacuum is hardly useful in this context.

But I think we're getting into symantics really 

[Flurfl]

Well yes we are arguing semantics, but semantics are important when you are talking about science. You need to know that everybody is talking about the same thing.

Firstly there is no concept absolute weight. There is only weight which I already defined.

Secondly while I know what you mean by net weight what you are actually talking about is the resultant force of the vector addition of weight and lift or Buoyancy. (net weight only has a definition in the packaging industry)

Now the actual weight of the combustion products is extremely useful. It tells us that something has been added to the components of the wood during it's combustion and how much of that something. But you are suggesting that the lower density of the products of will mitigate this addition in weight through buoyancy in Air. I don't think so as none of the products are lighter than air, but I'll run the numbers.

[Flurfl]

If we say the dried firewood consists of 20% water to begin with, this has to evaporated before the wood will burn leaving us with 800g of actual wood. This consists of about 50% Carbon 6% Hydrogen and 42% Oxygen in various organic compounds (mostly cellulose) and small quantities of other compounds. These break down under heat to form volatiles and char (Carbon), the volatiles combust (in an ideal situation) to form CO2 and H2O while the char forms CO2 (I'll ignore CO for the moment). So for our combustion we'll need Oxygen in the amount of twice the number of Carbon atoms and half the number of Hydrogen atoms. That works out to 1070g of Oxygen for the Carbon and 380g for the Hydrogen. We already have about 340 g of Oxygen in the wood so the rest will come from the air. Once this is all burnt we should then end up our 0.2 kg of water that was in the wood to begin with, 1.47 kg of CO2, 0.43 kg of H2O from the combustion as well as small amount (20g or so) of other compounds (Nitrogen and Sulphur compounds and ash) which gives us about 2.12 kg of total combustion products. Our original 1 kg of wood had a density of about 0.7. The water we've now got has a density of 1 and CO2 has a density of .002 while air has a density of .0012. Volume = mass / density  which gives our wood a Volume of 1.43 L while the CO2 has a Volume of 735 L and the H2O is 0.630 L. Our wood therefore displaced  1.72 g of Air while our CO2 and H2O displace 882 g of air. This displacement is equivalent to the buoyancy force. Therefore our 1kg of wood would read as 998.28 g on scales in air while our 2.12kg of combustion products would read as 1238 g in air and hence are still heavier than our wood.

[Flurfl]

On the other hand If we harness the physical (as opposed to the chemical) products of the combustion, which is primarily heat, then you will get a lift force in air. 1kg of firewood should give you 13 MJ of heat energy and if turned into hot air 1MJ of energy gives about 26 N of lifting force. Therefore We'll have 338 N of lift in total. If you had a hot air balloon at equlibrium (not rising or sinking) then added your 1kg (9.8 N weight force) of burning wood it will float away. How long it stays up depends on the amount of heat loss the balloon suffers.

[jake-the-peg]

I agree that symantecs are important but when the question is imprecise you have to make some assumptions.  I thought I'd make the assumption that Starscream was asking the question from a practical perspective and not just trying to be smart and seeing if we understood that mass is conserved. After all as trick questions go that'd have been pretty weak!

Nice calculations though - what do you make of the observation I quoted at the top that hardwood creates over 3 times the mass of ash per Kg of wood.

I guess that would imply a proportionally lower water content - I haven't tried it but it seems quite a large difference to me.

[Flurfl]

The difference, as you say, would be a lower moisture level but also denser wood, some hardwoods will sink in water. I could also imagine that as hardwoods are slower growing they will build up more compounds over time (metal salts etc.) that they can't excrete. The figures I used were for a generic "firewood", You'll get different answers for different species.

Actually I wasn't thinking it was a trick question,  I thought it important that starscream (and other readers) understand Conservation of Mass. Given that some people will hear about mass to energy conversion in nuclear reactions (E=mc squared and all that) and assume that the same happens in chemical reactions.

(As an aside: Symantec make software, semantic is the meaning of words)