Naked Science Forum
General Science => General Science => Topic started by: Drogheda on 10/09/2013 07:06:20
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so it's been a while sense I took my science classes in college, the notes.... they are gone.
however one topic has come up and no matter how much I try I can't remember the exact name of this.... or even if I'm remembering correctly.
now the latent heat of phase change. 0 is ice, 100 is boiling above that is steam. I remember doing the lab were we would count off every 20 seconds and take measurements. the latent heat would hit 0 and stop for a while then go up to 100, stop for a bit and then you have a rolling boil. however right before the final phase something happened, it actually got cooler by X degree's, so the graph would have a little curve right before the next phase step.
the same was true (as I remember) for the opposite. when water finally froze it got just a tad warmer right before the phase change. if I remember the explanation it was that when the water was at 0 all of the atoms are at stand still but there are no real ways of them to lock together to form ice crystals (you can't move when you have no energy) so the water has to have some sort of energy catalyst in order to knock everything place.
now, am I just dreaming this or am I explaining (it was explained to me) wrong?
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Pretty slick observation, if your recollection was correct. And not a bad guess at the mechanism.
Not sure about the apparent temperature reversal before boiling - please check your notes and come back! - but life on earth is due to the anomalous behaviour of water close to 0 deg C. As you know, ice floats, so water is actually denser as a liquid than as a solid, and the maximum density occurs at 4 deg C. Now "normal" convection results from hot liquid or gas rising, so if you apply heat from the bottom of a vessel you will get automatic mixing, and if you stop applying heat the top level will be hotter than the bottom. But between 0 and 4 deg C, convection is reversed, so unless you stir the ice/water mixture fairly vigorously you will find the colder stuff at the top of the vessel.
If this didn't happen, the oceans would freeze from the bottom upwards, the top layer of water would prevent sunlight from melting the ice below, and we'd all be dead.
The release of heat energy on condensation and freezing accounts for the shape of clouds, air turbulence, hurricanes, thunderstorms, and climate change. The hydrogen bond, which is responsible for the extraordinary physics of water, also determines the ability of DNA to replicate. So if there was a Creator, all he had to do was to invent H2O, position a planet so that it could exist in all three states simultaneously, then sit back to watch the fun.
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... the water has to have some sort of energy catalyst in order to knock everything place.
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Supercooling is indeed feasible but quite difficult to achieve in a school laboratory. Superheating is also possible in the absence of boiling nuclei. And of course the addition of solutes will depress the freezing point and elevate the boiling point.
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I completely forgot about the 0-4 thing (I remember talking about it though, need to go refresh my memory). I freaking love science and it drives me nuts when there is a (laymans) thing that I just can't put my finger on. I do want to major physics, eventually.
but then what I'm saying is correct and there has to be somewhat of an energy catalyst. it grinds my gears because all the wiki's and this and that that get thrown at you never mention that, just that one way energy is gained and one way energy is lost. still any idea's on the name of that phenomenon?
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energy slows down as water freezes so the colder things are the less heat energy will come from ;D ;D ;D ;D ;D ;D ;D ;D
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water was at 0 all of the atoms are at stand still but there are no real ways of them to lock together to form ice crystals (you can't move when you have no energy) so the water has to have some sort of energy catalyst in order to knock everything place.
That is incorrect.
0°C = 273.15K
Whether in solid or liquid phase, there is still some movement of the atoms.
Movement doesn't stop until one reaches absolute zero -273.15°C or 0K. As one approaches even a few degrees above absolute zero, there no way not to have water in a solid chunk of ice.
As far as supercooled water, the latent heat of fusion of water is about 80 cal/g.
The specific heat of liquid water is about 1 cal/deg/g.
Perhaps I'll have to try a supercooled experiment. But, during the phase transition in the supercooled experiment, converting from liquid to solid would require the loss of the equivalent energy to raising the temperature by about 80°C. So, if you began with, say -10°C in supercooled water, then the temperature of the end product should jump to 0°C, and one should end up with slush rather than a solid block of ice.
I'm not sure what the best explanation for the latent heat of fusion and latent heat of vaporization. To melt ice, you must add 80 cals/gram to break the bonds in the ice. So, forming the ice, one would have to remove 80 cals/gram to achieve the state with restricted movement caused by the crystalline structure (restricted movement, not no movement).
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The latent heat of evaporation and the latent heat of fusion..
The properties of water are a really good example of how the shape of molecules work.. Think of the molecule of water.. 2 hydrogen atoms and an oxygen atom.. now consider the structure of the atoms.. one nucleus, can be considered as a positive 'ball' surrounded by electrons which can be considered as a negative 'ball'
A hydrogen atom has a single electron, an oxygen atom has 6 electrons..
(I dont want to go into how many electrons where and why as I dont really know WHY)
http://en.wikipedia.org/wiki/Hydrogen_bond
link provided as diagram went funny
As seen in the diagram the arrangement of the atoms give positive nodes on the outside of the oxygen atom, and the unaffected electrons give a negative charge on the other side.. This ionic charging gives rise to hydrogen bonding.. the breaking of which is the reason for the latent heat of fusion and evaporation.. (some are broken to melt the ice, most are broken to evaporate the water) (Note: this means the increased values for the latent heats for water.. compared to other chemicals)
Hydrogen bonding is why water exists as a liquid at relatively high temperatures compared ot other molecules with similar or higher molecular weights. It is also why alcohols such as methanol are liquid when they really should be gases. and why water forms a vapour when it should be a gas.
This obviously is a very simplistic model and description of what happens.. but it should suffice