It seems to me that a lot of solid materials like plastics and metal have a complex transition from solid to liquid. As you heat up metal, like iron, it gets softer and easier to shape before it finally liquefies.
But water ice is solid as a rock even 1 degree (Fahrenheit!) below freezing point and completely liquid 1 degree above it. Why is this?
This part, at least, is incorrect. Pure iron behaves like pure water, and has a defined melting point. Yes, iron and steel change their crystalline structure as they’re heated and hammered, becoming harder or softer at their edge and water doesn’t do that. But virtually all pure compounds have a defined melting points.
Likewise, some pure organic compounds, have defined melting points. For example, camphor, or naphthalene (one type of mothballs.) But if you melt them together, you might get a softening gloop. Same thing with plastics – they might not be one pure compound, but instead a collection of varying sized, long molecules. And that makes them melt differently.
Look at it this way. Iron has defined properties. Carbon has defined properties. Varying grades of steel, made of varying mixtures of the two can be heat hardened, or even made friable.
Sugar has a defined melting point. Water has a defined melting point. Sugar in water can be syrup, taffy, hard candy, burnt caramel, etc.
Now if you’ll excuse me, I’m going to get the heck out of this thread before it degrades into a ‘Glass is a super-cooled liquid’ thread.
Specifically, ordinary iron (alpha iron) changes into delta iron at 912 C, then into gamma iron at 1394 C, before it melts at 1538 C. There’s a phase diagram in the Wikipedia article: Iron - Wikipedia
My wag is that it is due to molecular properties.
Water is a polar molecule made up of two simple hydrogens bonded to one oxygen. It’s structure is defined by that shape, which is why it transitions to ice being less dense than water. Except there are several phases of ice, too. These are temperature and pressure dependent - just like iron.
Iron’s phases appear to be related to structural changes, which are also due to the amount of energy in the atoms, i.e. temperature and pressure. The shift in structure from Body Centered Cubic to Face Centered Cubic is one of the reasons iron is able to form steel - that shift allows gaps in the iron molecules that the carbon can slip into. Without that structural change, the iron would be too tightly packed for carbon to enter.
Someone who actually knows chemistry could probably give a better answer.
I think the phase change might be the wrong question - it seems to me that the actual question here is “why is water inflexible/brittlein its solid phase?”
The answer is that water does get softer as it comes closer to melting temperature–it is just we always experience ice near its melting temperature.
I wonder if the OP is actually trying to ask us to explain the plasticity and ductility of iron as it is heated versus the lack thereof in ice?
Yes, I wondered why iron and other materials become more plastic as they heat up, unlike ice, which remains solid right up to its melting point.
Based on the excellent responses received so far:
*ice actually does become softer as it heats up, I just haven’t been paying attention
*iron (and maybe other materials) go throught crystallization changes at different temperature, even while remaining solid
See for examples tin and cocoa butter.
Try reading a bit about plasticity in materials. That page is fairly technical, so ask for clarifications where necessary.
Here’s a particularly explanatory paragraph from the page on ductility:
This section on Ductile-Brittle Transition Temperature is also related.
I think the answer you’re looking for is water ice molecules don’t share a lot of valence electrons with each other, so they can’t move much with respect to each other before they break. Get enough load to move molecules, you separate molecules, not shift them about. Thus, ice is brittle.
It might be useful to distinguish between stiffness, strength and hardness/softness.
Most metals become a little less stiff as temperature increases. Here’s a helpful plot:
But when the OP describes iron as becoming “softer and easier to work,” the OP is clearly referring to yield strength, which falls a lot more dramatically with temperature than does stiffness:
Interestingly, many steels get stronger above room temperature before falling off again at much higher temperatures.
Hot iron, regardless of phase, isn’t much less stiff than cold iron. But it does yield (deform plastically) under much less stress than it did when it was cooler. Because the slope of the stress/strain curve is typically much shallower after yield, which feels “softer,” even though the pre-yield stiffness hasn’t changed much.
But “hardness” is actually a fairly technical concept that is strongly affected by the metal’s phase (not solid/liquid/gas phase). I don’t mind if people casually use “softer” to mean “weaker” or “less stiff,” but if we want to avoid conflating strength and stiffness, it’s helpful to clearly define what we mean by “softer.”
I recently overheard someone say that the World Trade Center towers collapsed on 9/11 “because the fire burned so hot it melted the beams.” They didn’t melt, exactly: as the steel beams got hotter, their yield strength decreased steadily until it was below the normal stress of holding the buildings up. Then the beams buckled and the whole runaway collapse cycle was set in motion. The beams were likely well below their melting temperature when the towers fell.
As a slight hijack and to further bake your noodle.
Water and several other substances at their proper temperature and pressure will enter a triple point state where they rapidly flash between between their solid, liquid, and gaseous states.
I would have thought that at the triple point these phases would be in equilibrium, and therefore no “flashing” would occur? In the linked video, you can tell they are pumping on the flask.