if I burn 5 pounds of paper, will the ashes weigh 5 pounds, and if not, why?
The ashes will not weigh five pounds.
The ashes plus the smoke that was released plus the energy that was released during the burning will be exactly five pounds.
In case you think that smoke does not weigh anything since it rises up you are right but we are are talking about mass which is different.
A large part of the mass of paper is carbon (mostly cellulose). Upon burning, carbon will combine with oxygen to form CO2, which is a gas. Smoke does rise up because it is carried up by hot air, not because it is weightless.
The weight of the energy is a rounding error. The ashes plus the smoke (and hot non-sooty gases) will mass exactly five pounds plus the mass of the oxygen that it took to burn it. This is non-trivial; f’instance, twelve pounds of carbon burn down to 44 pounds of CO[sub]2[/sub], two pounds of hydrogen to 18 pounds of H[sub]2[/sub]O.
When you burn paper, you take all of the materials that the paper is made out of (mostly processed cellulose which is a combination of carbon, hydrogen, and oxygen compounds) and combine them with oxygen in the air. The result is carbon dioxide, water vapor, and a few other bits of stuff that becomes the small particles that you see as smoke. The vast majority of the weight of the paper ends up as carbon dioxide, which floats away into the air if you don’t do anything to trap it (there goes your carbon footprint).
If you were to add up the weight of the gases released and all of the particulates that float away, plus the weight of the ash, then all of this would total up to 5 pounds. ETA: Actually, I’m wrong. As Malacandra said while I was typing, there’s the weight of the oxygen that gets used in the combustion you need to add in as well. Oops.
Since most of your weight becomes carbon dioxide, the actual ash you get at the end will be only a tiny fraction of the weight of the original paper.
Your second paragraph is badly wrong. :dubious:
Yep. As soon as I read your post I realized I left something important out. :smack:
Right. To get 5lbs you would need to subtract the weight of the atmospheric oxygen consumed in the burning.
engineer_comp_geek is also wrong to say that “the vast majority of the weight of the paper ends up as carbon dioxide.” Quite a lot of it ends up as water vapor too. There is a fair bit of both hydrogen and oxygen in the cellulose, and all the hydrogen and some of the oxygen will wind up as water.
I wonder, though, if the OP is being serious.
Roughly speaking, cellulose is polymerised C[sub]6[/sub]H[sub]12[/sub]O[sub]6[/sub] and I think it’s been established * that when sugar burns the oxygen from it ends up in the carbon dioxide, not the water, so engineer_comp_geek would still be right on that point.
- (You can use heavy oxygen, oxygen-18, in the glucose molecule - or burn it in heavy oxygen instead - and see where the heavy oxygen ends up.)
Paper is made of trees, and trees are made mostly of air and water. Neither of these are left after burning.
By “trees are made of air and water” I mean that most of the mass of a tree comes from what it gets out of air and water, namely, carbon dioxide and water per se.
I’ve heard that quote, “90% of a tree is air”, but did you ever try to hit a golf ball through one? Every stinkin’ time I hit the part made out of wood. Statistically improbable, but true.
By volume. But not by mass.
So if I understand correctly:
Let’s say I had a hermetically-sealed container containing paper, an oxygen source, radio receiver, and igniter. I use a precision balance to measure the mass of the container. I press the button on my transmitter. The receiver inside the container sends a high voltage to the igniter and lights the paper on fire. The paper inside the container completely burns, and the walls of the container get hot as a result. After letting the container cool down, I (again) measure the mass of the container. Will the mass be the same?
Sure will be, by any reasonable standard of accuracy. I’ve done similar procedures with a bomb calorimeter to measure heating values of oils and pulping liquors. Technically heat lost to the environment relates back to mass by the old standby, E=mc^2, but you’d never notice on any balance you’d use for this.
Um…what? Things don’t stop having weight just because they’re being moved around. If I jump up and down on a scale, the needle will move, but that’s just because it’s not a perfect measuring device. I still weight the same thing throughout the jump.
I’m not sure if you guys are talking about the same thing.
90+% of a tree is air (by mass OR volume), if you mean in the sense that it’s built of carbon, hydrogen and oxygen that ultimately, came out of the air (Hydrogen and Oxygen in the form of rainwater - taken up in liquid form by the roots, but rain comes from the atmosphere)
But when you say ‘by volume, not by mass’, I think you might mean that 90+% of ‘solid’ matter is in fact empty space (because atoms themselves are mostly empty space). However, the empty space in this case is not ‘air’. It’s empty space.
No mass is converted to energy by combustion.
Question has been answered. I heat with wood and wanted to note how impressively small is the weight of the ashes compared to that of the wood burned to produce them.
Hell, I’m not even talking about the same thing. In my reference to a tree being 90% air, I mean the total volume occupied by the tree being mostly air. Grind a tree down to a fine powder and you’d fit it into a tenth of the volume easily. My frustration at not being able to hit a golf ball through it is a small joke, a blimp is 99% helium by volume, and I can’t hit a golf ball through one of those either.
Now wood of course, even a solid 2x4, has a lot of air in it contained in vessel elements. It varies with dryness and species, but a southern hardwood at 40% moisture is 20% air by volume. Intuitively, you know it’s true, the specific gravity of cellulose is 1.5, yet most trees float just fine. That’s a separate issue.
So. . .wait. . .you’re saying matter cannot be created or destroyed?:eek:
OK. Not sure I quite agree with those numbers though. Dry Scots Pine (not even a very dense wood) is about 0.5g/cm[sup]3[/sup], diamond is about 3.5g/cm[sup]3[/sup]. Balsa might be 90% air, but it seems as though few other woods will be 90% air, and hardly any living trees will.