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 Originally Posted by Renton
Can you concisely explain states of matter and how boiling and freezing points work? Like taking water for example, it's freezing point is 0 degrees celsius. Does that mean that any water that reaches below 0 is automatically ice? Similarly is any water that reaches the boiling point steam instantly?
Preface:
I fear that the word concisely is going to give me some trouble. I am pressed to fully explain P-T diagrams (much easier to point at as a result than to teach you to make) as well as a thorough explanation of entropy, enthalpy, internal energy, and more. These concepts take a few hours (if not weeks) to wrap your head around. They are forms of energy which matter utilizes. If I don't explain them and account for them, then you'll never be happy with my description of phases of matter. Without them, I can't really teach you about steam tables... I can only do some hand waving and "trust me" teaching... and an understanding of steam tables is what you're really driving at.
Renton, I strongly encourage you to buy a Thermodynamics book. I feel confident that you'll be flipping the pages and reading it out of personal enrichment and curiosity of the subject matter. It'd be well worth the purchase price.
[/preface]
Freezing & boiling:
We touched on this before when you asked me about water's freezing point in #157. Here's my initial reply in #159.
I think the one thing I want you to know about this is that a freezing or melting point requires BOTH a temperature AND a pressure.
So the freezing point of water is 0 C at 1 atm (or 1 times the average pressure of the atmosphere at sea level, 14.7 psi or 101 kPa). If the pressure goes up or down, the freezing point will change, too.
The same is true for boiling point. At high altitudes, most recipes offer substitute cooking instructions. This is mostly due to the decreased atmospheric pressure causing water (and everything else) to boil at lower temperatures.
OK, so let's talk about the "all at once" part of your question. Let's start with a picture. When I was a freshman in HS, my Physical Science class did an experiment with ice and water.
We put ice into a beaker and put that beaker on a hot plate. We measured the temperature of the ice and then turned on the hot plate. We stirred the ice and monitored the temperature of the ice/water in the beaker until it was boiling for a few minutes.
Here's an image that shows the theoretical results:
The horizontal axis is time. The vertical axis is the temperature of the ice/water in the beaker. The rate of energy produced by the heat plate and absorbed by the beaker is assumed to be constant.
You can see that while the matter is changing phases, it doesn't change temperature. This is true as the ice melts and again as the water evaporates.
OK, so why?
Temperature is just a fancy way of measuring and talking about vibrations on the molecular and smaller scales. Temperature is vibration. Something cold is vibrating less than something hot.* Heat is the flow of temperature, or the translation (movement) of vibrations (temperature). In order to change temperature, one vibrating thing has to bump into another vibrating thing. They exchange vibrations and the result is that both particles have an amount of vibration that is closer to the average of their vibrations...  that sounds so dense....
How about: 2 particles bump into each other and are more similar to each other in temperature afterward than before. The total vibrational energy is the same, but the low energy one gained some and the high energy one donated some.
OK, so now we understand that temperature is really really just kinetic energy in the form of oscillations on the micro scale of molecules and smaller, and that temperature is passed by collisions. We also know that the collisions tend toward an equilibrium between the involved things.** It is also true that temperature can be gained or lost by emitting/absorbing photons, and maybe even other mechanisms that I'm not thinking of at this moment, but the primary mode of thermal exchange in this example is collisions.
OK, we're getting close.
The inter-molecular bonds which hold the water molecules in a bond with their neighbors have a maximum range. Well, the attractive force decreases with distance, and at a certain distance is negligible. The magnetic dipole moment of H2O is the primary contributor to this bonding in the phase of liquid water. (It is the hydrogen bonds which dominate in ice; there are no dominating bonds in steam.) The dipole moment of each molecule makes it want to align with its neighbors, as, by doing so, it is in a lower energy state than if it weren't aligned.
Basically the material, in this case fluid, bond is like tying a thin thread to hold something together. If it's pulled on hard enough, it will break. When the temperature of a particle increases to a state where it is vibrating beyond the range of the bonding force, then that particle will break loose of it's bond. In this case, a water molecule absorbs a bit of energy, causing it to vibrate slightly more than it was before. However, just before, it was vibrating as much as it could while still maintaining the fluid bond. Once the energy was absorbed, it could no longer maintain the fluid bond, so it evaporated. In evaporating, it carries energy away with it, cooling the matter it leaves behind ever so slightly. (Sweat glands FTW)
This must happen to every molecule individually, so the total amount of heat energy needed to make this happen would have to come instantaneously for this to happen instantaneously.
***
Linking to some higher concepts:
*If that something is charged, like, say, a proton or electron, and it is vibrating, then it's creating waves in the electromagnetic fields, which are photons... blackbody radiation! If it has non-zero temperature, then it radiates!
**If the vibrations of 2 interacting particles approaches the average vibration between the 2 particles, then no finite number of interactions could ever bring a vibrating particle to be completely still... I.e. nothing can be cooled to absolute zero in any finite number of steps.
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