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Comment by dekhn | Hacker News Reader

A long part of my scientific career has been unpacking simple statements about things like bike riding, and finding the underlying process is subtle, and reveals many interesting things.

When I was a kid, I had a teacher who said that cutting paper wasn't a chemical change- no bonds were broken. That seemed odd to me, but after I had a PhD, I understood a few things better. That when you cut paper, mostly hydrogen bonds- bonds, but a particularly weak form of bonds- are broken, but threre is enough kinetic energy to break some covalent bonds too. This turns out to have huge implications (besides the fact that the teacher was wrong).

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If you smash a salt crystal with a hammer, all the resulting pieces end up with equal charges of anions and cations, so were any ionic bonds really broken?

That covalent bonds are broken seems obvious if you ditch the paper and cut through a sheet of long-chain polyethylene. The scissors don't just stop if they're about to break a polymer chain.

I think even the salt crystal will have some ionic bonds broken- smashing it with a hammer should generate a bunch of kinetic energy which converts to heat, some of the heat will exceed the ionic bond strength, and break bonds.... right?

How tiny of a charge is so small as to be unnoticable?

Certainly, if an object millimeters across has a charge of 1 electron that isn't noticeable.

I'll bite.. what are the huge implications?

Could striking a match be viewed as cutting paper?

no, that's combustion, clearly a chemical reaction. Sure, you can use a laser to cut paper, but the teacher was talking about scissors.

And... what are the huge implications??

Come on dekhn, we want to know

OK, sure. One of the most interesting long-running issues in proteins is how they adopt their 3D structure and remain stable for such long periods.

Most people would say that covalent bonds hold the protein together and they dominate the overall structure- for example, you wouldn't ever see a strained covalent bond, so you can easily eliminate all structures with strained bonds. This was long an assumption. However, work I did at Google using Exacycle demonstrated that, in fact, large collections of hydrogen bonds can in fact work in concert to stabilize proteins with strained covalent bonds (http://onlinelibrary.wiley.com/doi/10.1002/pro.2389/full), which overturned the assumption, and made a modest contribution to the prediction of protein folds, and protein design.

Interesting, thanks!

Thanks!

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