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[–] xyzzy 0 points 20 points (+20|-0) ago 

The higher the temperature, the more movement in molecules and the more movement there is the less stable is the "grid" of molecules on a solid.

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[–] goatboy 0 points 8 points (+8|-0) ago 

Not all materials, just most common materials behave that way. However, the aerospace industry regularly works with exotic chromium based alloys that actually become stronger and more rigid the hotter they become. The blades inside an SR-71 Blackbird's jet engine for example are made from an alloy that doesn't event start to get stiff until they're past a temperature that would melt aluminum.

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[–] Eualos 0 points 2 points (+2|-0) ago 

However, they will eventually become soft again once the proper temperature is reached.

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[–] joerebellion 0 points 6 points (+6|-0) ago 

As an answer to your secondary question - things with proteins in them - animal products etc.

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[–] Eualos 0 points 1 points (+1|-0) ago 

The words you're looking for are "disulfide bonds" things like tires that have been vulcanized exhibit this property.

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[–] RicardoCabesa 0 points 1 points (+1|-0) ago 

This doesn't seem right to me. Maybe there is something I am missing. Meat is certainly more pliable if it is warm than if it is frozen. Fat too. Even bone, but it has less pliabilty over all.

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[–] joerebellion 0 points 0 points (+0|-0) ago 

yeah, it's hard to generalize with something as broad as 'proteins' i know, and not my area of speciality, but something like egg white is an example of something that goes from soft to hard.

http://pubs.acs.org/doi/pdf/10.1021/jf00102a004

the answer about adding energy to chemical bonds is an accurate picture of the macro situation - my point is more of an aside

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[–] Eualos 0 points 2 points (+2|-0) ago 

Thermal sets are what you're referring to. Like tires. They don't get more bendy and flexible, they just degrade. The reason everything else does is because they approach their liquid states and their individual pieces which aren't attached to each other begin to flow around each other. They only stayed rigid next to each other because they didn't have enough energy to break the forces that cause their mutual attraction.

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[–] The_Cat 0 points 0 points (+0|-0) ago  (edited ago)

All solids are made up of a "grid" of atoms keeping each other in place. The potential between two atoms looks somewhat like this. How to interpret this? You have a potential well, in which the atom likes to sit at the bottom. This determines the interatomic distance at absolute zero.

As the temperature rises, all the atoms start vibrating around their equilibrium position. This means that in that figure, the atom is basically "rolling" back and forth in that little valley there. The higher the temperature, the further it goes up the hill before rolling back again. Now if that valley was perfectly symmetric, you wouldn't be able to notice anything, because all atoms would on average still be at the same position as they were before. But these potentials aren't symmetric, the attraction becomes gradually weaker as they drift apart and the repulsion becomes extremely strong as they're pushed together. This means that on average, the atoms will end up sitting slightly further apart at higher temperatures, this causes thermal expansion.

Now, the second effect of this is not only that the atoms are sitting further apart from each other, but also that the effective attraction they feel to all their neighbors is slightly weakened. As you can see on the left hand side as opposed to the right, at high temperatures that valley isn't that steep anymore, and pushing the material makes it rather easy for the atoms to slip further out from each other. That's why the material not only expands, but also becomes softer.

Of course, for real materials there's a lot more to consider than just the one-dimensional interaction between two atoms. There's the specific chemical bonds at play, the three-dimensional structure of the grid, how further out atoms still influence each other even if they're not neighbors, etc... For example, due to some specific quirks in the crystal structure, some materials, such as silicon, even exhibit thermal shrinking at some temperatures. But for most regular crystalline solids, the explanation above is fairly correct.

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[–] mrrabe 1 points 0 points (+1|-1) ago 

Here's a philosophic way of looking at it. Heat disintegrates, cold integrates. Heat anything enough and it turns from solid to liquid to gas. Cool any gas enough and it condenses to liquid then solid. Matter has a spectrum of solidity, and bendy is located between solid and liquid.

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[–] greycloud 1 points 0 points (+1|-1) ago 

warm atoms have their electrons pushed out to higher energy states (further from the nucleus). this in effect creates a larger field, the larger field is weaker as the probability of the electron being in the right spot to hold the bond is decreased. at certain temperatures the bond will break and the atoms will break apart (vaporization point). it is possible in solutions/alloys and the like where multiple materials are near each other that they are set up in a way so that as the bond between two atoms/molecules is weakened, it rebinds to a different atom/molecule more favorably. but if you keep heating it up, eventually this bond will break as well. at 4 trillion kelvin (temperature) atoms themselves break apart from the heat even protons/neutrons are so energetic as to break away from each other (and even the quarks that make up the protons and neutrons break away from each other breaking the strong force).

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[–] The_Cat 0 points 0 points (+0|-0) ago 

You could hardly be more wrong. This has nothing to do with atomic excitations.

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[–] YoHomie 1 points -1 points (+0|-1) ago  (edited ago)

My pecker experiences the opposite effect.

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[–] Empiricist 0 points 0 points (+0|-0) ago 

That's only in a limited temperature range, just stick it in an acetylene torch and you'll see how it get's weaker as it carbonizes.