If a reduction in heat is the reduction in movement of molecules, or is it?, why then does it cause things to change characteristics (e.g., “frozen” or damaged, etc.)? Is it because as motion slows down molecules actually move differently? I.e. reorganize?
Most things that are harmed by cold are harmed because of what water does when it’s cold - it gets bigger and it gets pointier and it gets sharp. For living things, this means the water starts poking holes in its cells and stuff leaks out, and as more water freezes, blood vessels and bigger organs start to get holes in them.
We’re all essentially big sacks of water, so there’s a lot of it to harm us when it gets big and pointy.
Perhaps someone else can explain why metals and other substances get brittle and more prone to breakage when they get very cold.
I am not a physicist, but it’s essentially the same reason as with water. A great many things we think of as solid are only semi-solid. Glass in a window pane, for instance, over time will actually become measurably thicker at the bottom of the pane than at the top as gravity works its wondrous magic on the molecules. Things which become easier to damage when they get cold usually do so because their structures (often crystalline or semi-crystalline) become more rigid and less able to flex, however minutely.
The molecules in a solid are definitely organized differently than the molecules in a liquid, so yes. And some solids will even have different organizations at different temperatures and pressures. As I recall, there are something like 8 or 9 different structures for ice (plain old solid water), depending on conditions.
I’m pretty sure the “glass is really a liquid” thing has been debunked, much to the chagrin of fourth grade teachers everywhere. Old glass is wavy because of the manufacturing process, not because it’s moving. I’ve got glass in my windows from 1928, and it’s still even and smooth. “Old” glass 50 years ago was from 18something, when sheetglass was made differently.
It’s usually the cooling that wrecks things, rather than the cold. This piece on Physical Parameters of Cooling in Cryonics details many of the problems involved in freezing things intact. The plot of “Cell survival as a function of cooling rate” sums things up nicely:
Big sacks of dirty water.
This is a complicated issue. Solid materials break in cold temperature (or due to temperature cycling between relatively large thermal states) due to brittle fracture, but the actual mechanism for fracture can be any of a number of possibilities or a combination thereof. In metallic materials and crystalline nonmetal (like diamond) the atoms are typically arranged in geometic lattices, which then form grains that are globbed together at boundaries to make the apparently solid material. When the material is cooled, the grains can shrink (thermal shrinkage), or the strength of their connection to each other can change (phase change); the macro (observable) effect is the material becomes less tough (resistant to damage or impact), and then shrinkage itself creates internal stresses that can locally exceed the nominal material strength. Combined with external loads that are sized to the nominal material properties without taking into account reductions from low temperature can cause the metal to fail suddenly.
With materials like ceramics or glass which have an amorphous (i.e. no overarching) structure, and while they can be very hard, they’re typically not very tough or have great tensile strength unless combined with other materials in a composite structure. They also don’t expand or contract very evenly unless carefully tempered (like Pyrex), so they tend to crack easily.
With polymers that are formed from long, twisted organic molecules, the strength of the bonds can change, and the thermal stresses and stretch out the molecule chains, making them more prone to cracking. These properties vary wildly with different types and combinations of polymers, and as I know next to nothing about polymer material science, that’s all I’ll say on that topic.
The claim that glass or other amorphous solids “flow” is a myth. The flow rate of glass at standard temperature and pressure is so slight to be negligable even on timescales of millions of years. “Semi-cyrstalline” is a meaningless, or at least uselessly vague, description; solid materials are either formed in lattices, or they’re not, although as noted above the lattices may form grains within a more amorphous matrix. In metal alloys this is called microstructure of intermetallic compounds, and in fact this is often done in very deliberately and carefully; for instance control of the eutectoid reaction to get dispersion strengthening by forming a microstructure of pearlite embedded in ferrite, resulting in good ductility (corresponding to high impact toughness and low brittleness) combined with high strength. The study of all of this is called metallurgy, and it is (to me) hideously complicated enough to make quantum chromodynamics seem simple in contrast.
Stranger
This is old, but here and here is some brief information on different phases of ice. Fortunately, none of these exotic phases is stable at temperatures and pressures found anywhere on Earth, so the cataclysmic ending to Kurt Vonnegut’s Cat’s Cradle is still just fiction.
Stranger
The other ‘proof’ offered is often ‘look at these old windows, the glass is thicker at the bottom than the top’. Yes it is. Because they choose to install the glass with the strongest part at the bottom :smack:
To simplify some of what has been said here. We teach the kiddies that there are three phases solid-liquid-gas. This is easy but simplified.
Solids can go through phase changes to other solids. Sometimes there are many phase changes for one compound. When a compound goes to through a phase change it’s characteristics change.
When the o-rings on the space shuttle Challenger got too cold, they were no longer effective as o-rings. This caused the Challenger accident.
I stand corrected.
These are called second order phase transitions. The fundamental characteristic that changes is the free energy, which impacts how much it can deform in elastic or ductile mode, and how linear the behavior is.
Pedantic nitpick, but the lack of resiliency in the Challenger SRB o-rings isn’t strictly due to a phase change as such, but rather the non-linearity in the behavior of polymers over temperature ranges. In my (admittedly limited) understanding, polymers (except for cystalline polymers) can’t be readily classified into discrete phase changes on the microstructure level, and both plastic and elastic deformation, along with viscous flow can all occur simultaneously.
Stranger
Hey, I wanted to simplify things.
Like I said, it was an utterly pedantic nitpick, not an overall criticsm of your otherwise excellent illustration of how cold affects materials. But while in some materials the phase transitions are distinct and readily quantifiable, in others phase changes are kind of arbitrary and indistinct. What actually happens on the molecular level is even more confusion, espically with materials like polymer chains that behave in difficult to model ways. I prefer to stick with nice, mostly linear materials like steel, or fundamental particles which have clearly defined levels of uncertainty.
Stranger
Stranger On A Train - if you don’t mind my asking… what do you do? I love reading your posts, and I’m just curious to know what your background is. I’m going to guess that you’re an Engineer of some sort…?
I have nothing to actually add to this thread 
Stranger is a 35 year-old Mechanical Engineer with minors in mathematics and physics. His hobbies are hating his job and liking Julianne Moore (and Scarlett Johansson when she’s a brunette). In high school he was voted most likely to get stuck in a tuba.
I think I got most of that right 
True except for the word “most” in the first sentence.
Consider this: a human at a uniform 10[sup]o[/sup]C is just as dead as a human at a uniform 0[sup]o[/sup]C.
Rather, a great many living things have optimum temperatures in which their physiological processes operate. Even without any formation of ice at all, proteins assume a different, non-functional shape, enzymes lose efficiency and eventually stop working altogether, and as Squink pointed out, molecules and fluids start to move in ways that upset homeostasis - all without any ice formation at all. Keeping things in their operating temperature range is essential even if you don’t take them so cool that you form ice.
More on your body’s reaction to cold here:
Yeah, that’s mostly correct, except that I like Scarlett Johansson as a blonde, too. Heck, I’m pretty sure I’d like her bald, but the whole [post=6161638]Natalie Portman debacle[/post] has made me wary of committing to that. I think it’s a shame she got in with the likes of Woody Allen, but I know that can’t last so I’m just biding my time. Huh, I never realized I was so obsessed with her, which is odd considering I only have two films with her in them in my collection. Oh well, I guess it’s at least marginally more plausible than being obsessed with Jodie Foster, plus without the whole “psychotic wouldbe Presidential assassin” asthetic to it it.
Now when do you bring the bachelorettes, Chuck?
Stranger