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Author Topic: Why did microwaving my exam papers make them catch fire?  (Read 3705 times)

Offline InTheHeat

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A few weeks ago, some exam papers, I was supposed to mark, were left under a dripping tap and about half the paper got damp. I figured to put it in a microwave to boil off the water. The paper started to catch fire and the scorch marks were where the droplets of water originally were. The water was normal tap water.

I realise this is similar to the detonating cloth except this was not salty water.

Could the explanation be superheating the water above the flashpoint of the paper? If it matters, I live and work in Kuwait. The tap water is drinkable and at a slightly higher standard than UK tap water (regarding filtration, ion content and bacterial content)
« Last Edit: 14/06/2012 08:28:23 by chris »


 

Offline InTheHeat

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Re: Paper in a microwave oven.
« Reply #1 on: 08/06/2012 22:14:20 »
Wow!! Not one person wants to take this on?

I think you guys need to stop pontificating about your 'pie in the sky' new theories and get back to Earth. If you can't handle microwaves, how the hell do you think you can handle tensor analysis (a necessary precursor for general relativity and all ideas beyond)??
 

Offline syhprum

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Re: Paper in a microwave oven.
« Reply #2 on: 09/06/2012 01:10:24 »
I ain't rocket science water molecules interact strongly with the 2.4GHz of the cooker.
 

Offline InTheHeat

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Re: Paper in a microwave oven.
« Reply #3 on: 09/06/2012 09:23:58 »
Mmm... paper catching fire is the issue not the water heating up. In case you were unaware, paper does not usually contain water and one would reasonably expect to be able to boil the water off the paper without the paper catching fire.

It isn't rocket science to understand that the boiling point of water and the flashpoint of paper are not the same.
 

Offline Bored chemist

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Re: Paper in a microwave oven.
« Reply #4 on: 09/06/2012 13:24:01 »
"In case you were unaware, paper does not usually contain water"
Oh yes it does.
 

Offline InTheHeat

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Re: Paper in a microwave oven.
« Reply #5 on: 09/06/2012 14:01:25 »
I would assume it doesn't due to the fact that paper doesn't heat up in a microwave. Am I wrong?

I get that some oils and chemicals are vibrated so maybe coloured dry paper may not fare so well. I was more interested in why the scorch marks on the paper were where the water drops were which only heats to 100C.

If you would like to explain this then please do, if not then refrain from snarky comments
 

Offline Bored chemist

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Re: Paper in a microwave oven.
« Reply #6 on: 09/06/2012 15:33:08 »
Yes, you are wrong.
Googling "water content of paper" gets about 27,000 hits.
And, if you want to avoid sarky comments perhaps you shouldn't have started with what might be seen as one.
" The tap water is drinkable and at a slightly higher standard than UK tap water "
From a scientific point of view, that statement is meaningless because the dissolved solids content of water in the UK is so variable.
So, at best it comes across as "we are better than you" .
 

Offline InTheHeat

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Re: Paper in a microwave oven.
« Reply #7 on: 09/06/2012 17:51:33 »
Oh I'm sorry, you think I am Kuwaiti??

I am British teacher in Kuwait, so 'we' are better than 'you' doesn't really mean anything to me. Better keep that hidden racism under check, you never know who's listening.....

Sorry, but the UK does have minimum quality standards (137 000 000 hits). Kuwait usually follows UK standards but it is slightly tighter here. I hope your patriotism is now satisfied. Believe it not, rather than fending off closet rascists, I was actually looking for a reason why paper ignites and not why it gets hot but if you find this too difficult to address than please continue pontificating about unrelated issues
 

Offline Bored chemist

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Re: Paper in a microwave oven.
« Reply #8 on: 09/06/2012 19:32:08 »
It's not racism to point out that " slightly higher standard than UK tap water" is, at best,  meaningless and potentially insulting.
There was, when all is said and done, no reason to mention it at all. Of course, what I said would all still be true if I were a Kuwaiti living in England wouldn't it?
Actually, I hadn't initially noticed that you were posting from Kuwait. I had assumed that you were American, which kicks the allegation of racism into the long grass.
Anyway,
whatever the initial quality of the water, it will leach salts and traces of sugars from the paper and so become impure.
(See, I told you it was meaningless).
As they dry you will get relatively concentrated solutions of material which will consequently have a high boiling point. Once they get hot enough (and particularly if they happen to be acidic) they will start to char the paper.
Once any of the paper is converted to carbon which absorbs microwaves rather well, it will get hotter and hotter.
« Last Edit: 09/06/2012 21:04:28 by Bored chemist »
 

Offline Geezer

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Re: Paper in a microwave oven.
« Reply #9 on: 09/06/2012 22:40:23 »
In The Heat,

Please read the AUP, and stop pushing your luck.

Thanks.

Mod.
 

Offline evan_au

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Re: Paper in a microwave oven.
« Reply #10 on: 10/06/2012 04:21:49 »
Some more information, please InTheHeat:
Q1: Were the pages separated in the microwave, or all in a pile? [If they were in a pile, the heat & steam may not have been able to escape, and may have continued heating far beyond 100C. Whether the contained steam could then reach 451 Fahrenheit/232C necessary for paper to burst into flames would be the subsequent question...]

Q2: Were the droplets sharply outlined, or had they spread out beyond the initial spots? [This might provide some insight about whether it was caused by water in droplet form on the paper, or had it wicked out into a wet patch, or perhaps spread as steam?]

Q3: Were there any staples in the pages? [Staples act as an antenna for the microwaves, and can produce sparks, especially if two staples are close together. This might explain the papers catching fire, and scorch marks near staples, but not scorch marks near water droplets.]

[Q4, for interest: What did the students say, when they got their papers back?]
 

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Re: Paper in a microwave oven.
« Reply #10 on: 10/06/2012 04:21:49 »

 

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