Hello,
I have been pondering about this little tid-bit of information i witnessed for a few weeks now. Why, when you burn things such as paper, do the edges curl? And Those little plastic wrapers around the straws on juiceboxes, when you touch them with a lit cigarette, they curl. Why?
Thanks.
KaoS
whistles
That is a humdinger of a good question, and I have no idea. If I had to guess, it is because things that burn are changing chemically, which usually means there is a change in density. The change is not uniform, so the material will curl away from the less dense side and toward the more dense side.
Not sure if that is really what is happening. Me thinks this would make a good question for Cecil.
Burning a material can be a fairly complex serious of both chemical and physical changes, so there is no general answer one can give for all materials and objects. Dr. Lao’s answer is good in that the “curling” is caused by a change occurring on one side of an object and not the other.
In many cases the change can be physical. The little plastic juice box straw wrappers are an example of curling occurring primarily by a physical change. The plastic melts when heated. Once molten the plastic material will be drawn together by surface forces. Since these changes don’t occur uniformly, the wrapper curls.
Well, hrmm. these are definately good answers, but i don;t really understand the one point that you made after Dr. Lao’s. Anyways, i think we should get Cecil in here, that would be fun. 
Thanks guys
Shoot, My first sentence in that last post should read “series” not “serious of both…”.
The point i was trying to make is a material can curl by physical changes in addition to the chemical changes suggested by Dr. Lao. When the molten plastic material coalesces, or pulls in on itself, that’s a physical change and it can result in much more deformation than something like a change in density. Nothing is changing chemically; the material is just picking itself up and moving.
I’m having more difficulty coming up with a good explanation of paper curling. The question can be simplified by just considering what happens to paper when it’s heated (not necessarily directly in the flame). After the paper has been in the flame itself, all that’s left is ash, so it’s tough to describe the changes in shape during that transition. So if one were to focus on the possible changes paper undergoes when heated in air, we may get closer to an answer.
The simple answer as to why paper curls is that the changes (whatever they are) are not occurring uniformly (changes rarely do). If changes happen on one side more than the other side the paper would curl.
I’m a little unclear on exactly what these change are. I usually think of combustion as a gas phase reaction, but i don’t think the paper vaporizes before it burns, and i’m not sure exactly how a solid burns. Basically carbon and hydrogen in the paper combine with oxygen, but i’m not sure how this might translate into a change in density (or some other change that would result in deformation). I’ve cooked up a couple schemes but, they’re probably wrong so I don’t think i’ll post’em.
I don’t think it’s time defer to Cecil just yet, I’d give the dopers a little more time. Heck it’s Saturday night most of our fellow dopers are probably out partying. Give them a little while. I betcha someone with some crazy degree in combustion science or the like will show up and clear the whole thing up.
So, basically what your saying is that, when the paper is heated, it changes, and obviously change is not uniform, so one side is less/more dense then the other and it curls to that side…kind of like osmosis i guess. Trying to equal it out, i suppose. Or maybe i am completely wrong.
Anyways, thanks for the help guys, i will kep this thread going as long as i can, to hopefully get a reaction from cecil. He he. Anyways, thank you agian.
hmmm… no not like osmosis. density is not trying to be equaled out. (at least not to any substantial degree.)
Density is mass/volume. One way that an object can change density is by changing volume (or size). If a given amount of material expands it becomes less dense; if it contracts it becomes more dense. If this expansion or contraction happened more on one side of an object than the other (and the object was flexible) the object would curl. (I’m having trouble thinking of a good example of this right now. sorry.)
This is just one example of how paper might curl when it’s being burnt. It’s also likely that the mass is changing and that complicates the density = mass/volume relationship a bit. But, that doesn’t really matter; what’s important is that whatever the changes are that are occurring, it’s likely that they involve changes in volume on very small local areas. When all these local small size changes are added up, one side of the paper is larger than the other side and the paper curls. It can be tough to image one side of a piece of paper being larger than the other, but think of a race track: on the straight away both sides are equal, but in the curves the inside track is shorter than the outside track. The paper changes from being like a straight away to being like a curved section and in the process, curls.
At least I think that how it goes.
hmm, i think i follow you with the density thing, but, how come, like you said, when things burn, they expand, and lower the density, so it would make that one side weaker, so, it would most likely curl towards the flame, BUT paper curls away from the flame…ponder about that and see what u come up with.
Thanks