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#16265 - 10/30/06 10:18 AM Why do things melt
Anonymous
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OK so I was wondering, why do things melt, I mean on the atomic level what interaction causes things to melt, and why are melting points on the periodic table so strange and seemingly random?

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#16266 - 10/30/06 10:43 AM Re: Why do things melt
samwik Offline
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Registered: 10/10/06
Posts: 1131
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Don't quote me, I should disclaim, because I may be mixing or making some things up here; but basically....

Heat is a manifestation of atomic or molecular vibration. There are various kinds of vibration (movement, as flying around; and various waverings/wigglings in place).

Anyway, as you add heat, the vibration levels increase, until the forces (bonds) holding particles together (making it solid) are overcome.

Stretching and rotational "fidgeting" vibrations will increase until a solid turns liquid; and translational (flying around) vibration is more associated with liquid to gas transition.

To summarize, adding heat (energy) overcomes the forces holding things together.

Hope this helps (see if this draws any "second that motion" comments or similar posts) or draws critiques revealing my lack....

Good luck,
~samwik

P.S. ...if you've the time, check out my "shower-curtain" question on the Not-Quite-Sci forum on this site. I still need more observations from folks.

~S smile
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#16267 - 10/30/06 10:48 AM Re: Why do things melt
samwik Offline
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Microwave ovens are an example of this. The microwaves are "tuned" to resonate with one of H2O's unique vibrational modes. I can't remember if it is one of the stretching or flexing modes, but you get the idea.

That's why many dry things (paper) don't heat up.

As to why the MP's are so whatever; I'd say each substance that melts results from a unique combination of atomic and molecular forces that hold it together. Hydrogen bonding is a very common one of these forces.

Corrections anyone??
...I'm just going on long ago memories.

~samwik
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Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.

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#16268 - 10/30/06 11:02 AM Re: Why do things melt
dr_rocket Offline
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Registered: 12/06/06
Posts: 196
Loc: Palo Alto, CA, USA
Hi Jappatat,

For the most part the heat of a solid object corresponds to the "in place" vibrations of the atoms and molecules that it is made of. These particles, if it were not for heat, would be in fixed position with respect to one another. This is because of the bonds between them, which impose solid structure on the object. When the object is heated the particles wiggle around these "fixed points." The more heat that is added the more violent the wiggle becomes. At some point the violence of the wiggle overcomes the bond strength between atoms and the object melts.

The melting points in the periodic table only seem to be random. Have a look at:

http://www.answers.com/topic/list-of-elements-by-melting-point

From this information you should see a bit of a pattern.

Enjoy,

Dr. R.

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#16269 - 10/30/06 11:07 AM Re: Why do things melt
samwik Offline
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Often a class of materials will exhibit increasing MP correlated with some other property which changes slightly with each particular material of the class. For instance:

An example would be the melting of fat.
Long chain fatty acids "lay side by side" and form a solid. If you add heat, they "wiggle" more and push each other away.

As the chain length increases, there is more tendancy to stick together, hence need more heat to wiggle apart.
Thus the MP increases as chain length increases and short chain fatty acids are liquid at room temperature (oils).

Hydrogenation of fats makes them "straighter" so they stick together better (stay more solid) at room temp.

corrections???
~samwik smile
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Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.

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#16270 - 10/30/06 01:55 PM Re: Why do things melt
Uncle Al Offline
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Registered: 10/17/04
Posts: 540
Loc: Southern California
Microwave ovens are tuned away from water resonances to allow deep penetration and gradual energy deposition with depth.

Melting occurs when the various binding energies of the solid's constitutent units are exceeded by enough of the tail of thermal energy. Discrete pure structures usually have narrow melting ranges unless they decompose first. (Consider incommesurrate melting for mullite.) Mixtures can have broad melting ranges for the variety of interactions possible. Polymers may have problems disentangling into a melt or fitting into a crystal lattice. Glycerin is a solid at ambient temp but it viciously undercools. Cast TNT must be thermally annealed or it changes shape and density over time as it settles into a more stable structure.

Choline chloride has mp=302 C, urea has mp=133 C. A 1:2 mixture freezes at 12 C. Transition metal oxides (e.g., rust) do not dissolve in ordinary solvents without reaction. They all dissolve in this stuff. Go figure.
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#16271 - 10/30/06 08:06 PM Re: Why do things melt
dehammer Offline
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Registered: 03/23/06
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the energy of an atom is determined by the amount of energy in its electron shell. the more energy it has the bigger those shells are. (each shell holds a certain amount of electons, but when the get more energtic they can jump to a larger shell). since the attraction of an atom is determined by

1)distance between them

2)weight of the atom

increasing the distance between the atoms, but increasing the size of the outer atomic shells reduces the attraction of the atoms. this allows more movement of the atoms.

the same is true of molucules that share electons
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#16272 - 10/30/06 11:18 PM Re: Why do things melt
DA Morgan Offline
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Weight of an atom is irrelevant. Changing the number of neutrons is of little or no consequence.
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#16273 - 10/31/06 12:19 PM Re: Why do things melt
Uncle Al Offline
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Melting point in C is equal to MW in daltons. Benzoic acid:

MW = 122.12
mp = 122.4 C

There are some exceptions to this rule.
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#16274 - 10/31/06 01:03 PM Re: Why do things melt
dehammer Offline
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Registered: 03/23/06
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Quote:
Originally posted by DA Morgan:
Weight of an atom is irrelevant. Changing the number of neutrons is of little or no consequence.
what does neutrons have anything to do with anything.

the weight and density of a group of atoms determine how much attraction those atoms have to each other atom. what do they teach in the schools from your part of the country. that is something we learned in jr high physics. The energy of their shells determine the temperature of that group of atoms. if the weight of a group of atoms does not increase, but the distance between the does, the density goes down. as the density goes down, the cohesivness of the atoms decreases, resulting in melting.
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#16275 - 10/31/06 03:02 PM Re: Why do things melt
DA Morgan Offline
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dehammer asks:
"the weight and density of a group of atoms determine how much attraction those atoms have to each other atom."

No it doesn't. The weight of an atom has nothing whatsoever to do with attraction. Weight is a measure of the number of protons and neutrons with neutrons weighing a bit more than protons.

Take, for example uranium. It has 92 protons. It can easily have (U238 - 92 = 146) neutrons. The majority of the atoms weight is irrelevant.

In short ... you are incorrect. Something you will no doubt refuse to acknowledge.
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#16276 - 10/31/06 03:09 PM Re: Why do things melt
samwik Offline
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Loc: Colorado
I wondered about the "weight" comment also, but couldn't be sure. I think the "size" is more commonly thought of as affecting bond strength. Probably at atomic/molecular scales, these distinctions (size, weight and density) are pretty fuzzy. Size or weight are still a function of particle numbers.

still?
~samwik
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Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.

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#16277 - 10/31/06 06:36 PM Re: Why do things melt
DA Morgan Offline
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One of the strongest bonds you will find in chemistry is lithium fluoride (LiF). Lithium is the lightest of the alkali metals and fluorine the lightest of the halides.

Uranium Iodide, on the other hand, is rather weakly bound as is dehammer's statement.

He should just acknowledge this and move on.
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#16278 - 10/31/06 06:44 PM Re: Why do things melt
samwik Offline
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Registered: 10/10/06
Posts: 1131
Loc: Colorado
size?

To be clearer, "size" would be atomic radius relative to others in it's class.
Orbital class?

Does that cover all the bases?

...how 'bout acids.LOL
~sorry bad pun

~samwik smile
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Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.

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#16279 - 10/31/06 07:40 PM Re: Why do things melt
DA Morgan Offline
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Loc: Seattle, WA
Not sure what you mean samwik.

The radius of an atom changes as you excite the electrons to higher states. But then chemical reactions, breaking down compounds, tend to double in speed for every 10 degrees Celsius making things more and more unstable.

So, again, I'm not quite sure what you intend with your statement.
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#16280 - 10/31/06 07:49 PM Re: Why do things melt
samwik Offline
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Registered: 10/10/06
Posts: 1131
Loc: Colorado
For elements in a given class (horizontally) (similar orbital structure), the more filled the shell, the more strongly it bonds.
NO, that's not it; ummm the farther to the right (on the period.table) the stronger the bonding?
Something like that. ... and at a given temp.

I was addressing dehammer's comment about weight, saying he meant size, but then I couldn't quite remember how size affected things either, so I tried to talk it out, but....
as i say; something like that? Anyone??
~samwik
_________________________
Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.

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#16281 - 11/01/06 01:57 PM Re: Why do things melt
samwik Offline
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Registered: 10/10/06
Posts: 1131
Loc: Colorado
Correction to my above post of 10/30/06, 11:48am:
first paragraph should have * at end to direct to P.S.

*
P.S.
re: correction posted by Uncle Al
Wow, lots of stuff out there about this (google ?microwave oven? water or ?microwave heating? water). ?There are textbooks which support the "tuning fork" or "resonance" theory of microwave ovens. Some people say they are right and some say they are wrong!? -from How Things Work?.
So, even controversy in physics over this. But you get the idea? if you get things moving (add energy), bonds are overcome and things fall apart (melt).
~S (computer balky today)
_________________________
Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.

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#16282 - 11/02/06 12:29 AM Re: Why do things melt
dehammer Offline
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Registered: 03/23/06
Posts: 1089
Quote:
Originally posted by DA Morgan:
One of the strongest bonds you will find in chemistry is lithium fluoride (LiF). Lithium is the lightest of the alkali metals and fluorine the lightest of the halides.

Uranium Iodide, on the other hand, is rather weakly bound as is dehammer's statement.

He should just acknowledge this and move on.
your talking compounds. the bonds of compounds are different than the ones that hold two of the same type atom or compounds togethers.

perhaps weight was the wrong word, but its close. everything has some attraction to everything else. i dont know the scientific terms for this, but its what causes particles in space to pull towards each other if they are in simular postions.

the same thing happens on a atomic level. what counters this is the energy of the election shell. the more energy there is, the higher the shell those electons inhabit. the larger the shell, the farther apart the nucluses are. as with all things that are attracted in such a manner, the attraction is the square root of the distance. IF the distance between the nucliuses doubles, the attraction is reduced to the square root.

while at rest (low energy), the atoms may form crystals, at higher energy, they are too far apart to hold their form. Thus the shapes melt.

of course since it would destroy the universe for you to be wrong, i dont expect to to accept this.
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#16283 - 11/02/06 12:20 PM Re: Why do things melt
DA Morgan Offline
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Registered: 10/17/04
Posts: 4136
Loc: Seattle, WA
dehammer wrote:
"perhaps weight was the wrong word, but its close. everything has some attraction to everything else. i dont know the scientific terms for this, but its what causes particles in space to pull towards each other if they are in simular postions."

gravity. ;-)

"of course since it would destroy the universe for you to be wrong, i dont expect to to accept this."

If you knew something of chemistry and I didn't I'd gladly acknowledge my error: That is not the case.

What you wrote is incorrect. And trying to say you were talking about something else, for example, are two hydrogen atoms bound together as H2 less strongly than two lead atoms due to their weight or mass is just digging a deeper hole.

You are standing in quicksand. Acknowledge your error and move on.
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#16284 - 11/02/06 12:23 PM Re: Why do things melt
samwik Offline
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Registered: 10/10/06
Posts: 1131
Loc: Colorado
So, how do you boil water in a microwave? re: my correction above. smile
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Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.

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