Welcome to
Science a GoGo's
Discussion Forums
Please keep your postings on-topic or they will be moved to a galaxy far, far away.
Your use of this forum indicates your agreement to our terms of use.
So that we remain spam-free, please note that all posts by new users are moderated.


The Forums
General Science Talk        Not-Quite-Science        Climate Change Discussion        Physics Forum        Science Fiction

Who's Online Now
0 members (), 39 guests, and 2 robots.
Key: Admin, Global Mod, Mod
Latest Posts
Top Posters(30 Days)
Previous Thread
Next Thread
Print Thread
Page 1 of 2 1 2
#16265 10/30/06 04:18 PM
A
Anonymous
Unregistered
Anonymous
Unregistered
A
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?

.
#16266 10/30/06 04:43 PM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
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


Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.
#16267 10/30/06 04:48 PM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
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


Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.
#16268 10/30/06 05:02 PM
Joined: Dec 2006
Posts: 196
D
Member
Offline
Member
D
Joined: Dec 2006
Posts: 196
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.

#16269 10/30/06 05:07 PM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
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


Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.
#16270 10/30/06 07:55 PM
Joined: Oct 2004
Posts: 540
U
Superstar
Offline
Superstar
U
Joined: Oct 2004
Posts: 540
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.


Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf
#16271 10/31/06 02:06 AM
Joined: Mar 2006
Posts: 1,089
D
Megastar
Offline
Megastar
D
Joined: Mar 2006
Posts: 1,089
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


the more man learns, the more he realises, he really does not know anything.
#16272 10/31/06 05:18 AM
Joined: Oct 2004
Posts: 4,136
D
Megastar
Offline
Megastar
D
Joined: Oct 2004
Posts: 4,136
Weight of an atom is irrelevant. Changing the number of neutrons is of little or no consequence.


DA Morgan
#16273 10/31/06 06:19 PM
Joined: Oct 2004
Posts: 540
U
Superstar
Offline
Superstar
U
Joined: Oct 2004
Posts: 540
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.


Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf
#16274 10/31/06 07:03 PM
Joined: Mar 2006
Posts: 1,089
D
Megastar
Offline
Megastar
D
Joined: Mar 2006
Posts: 1,089
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.


the more man learns, the more he realises, he really does not know anything.
#16275 10/31/06 09:02 PM
Joined: Oct 2004
Posts: 4,136
D
Megastar
Offline
Megastar
D
Joined: Oct 2004
Posts: 4,136
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.


DA Morgan
#16276 10/31/06 09:09 PM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
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


Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.
#16277 11/01/06 12:36 AM
Joined: Oct 2004
Posts: 4,136
D
Megastar
Offline
Megastar
D
Joined: Oct 2004
Posts: 4,136
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.


DA Morgan
#16278 11/01/06 12:44 AM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
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


Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.
#16279 11/01/06 01:40 AM
Joined: Oct 2004
Posts: 4,136
D
Megastar
Offline
Megastar
D
Joined: Oct 2004
Posts: 4,136
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.


DA Morgan
#16280 11/01/06 01:49 AM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
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.
#16281 11/01/06 07:57 PM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
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.
#16282 11/02/06 06:29 AM
Joined: Mar 2006
Posts: 1,089
D
Megastar
Offline
Megastar
D
Joined: Mar 2006
Posts: 1,089
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.


the more man learns, the more he realises, he really does not know anything.
#16283 11/02/06 06:20 PM
Joined: Oct 2004
Posts: 4,136
D
Megastar
Offline
Megastar
D
Joined: Oct 2004
Posts: 4,136
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.


DA Morgan
#16284 11/02/06 06:23 PM
Joined: Oct 2006
Posts: 1,164
Megastar
Offline
Megastar
Joined: Oct 2006
Posts: 1,164
So, how do you boil water in a microwave? re: my correction above. smile


Pyrolysis creates reduced carbon! ...Time for the next step in our evolutionary symbiosis with fire.
Page 1 of 2 1 2

Link Copied to Clipboard
Newest Members
debbieevans, bkhj, jackk, Johnmattison, RacerGT
865 Registered Users
Sponsor

Science a GoGo's Home Page | Terms of Use | Privacy Policy | Contact UsokW
Features | News | Books | Physics | Space | Climate Change | Health | Technology | Natural World

Copyright © 1998 - 2016 Science a GoGo and its licensors. All rights reserved.

Powered by UBB.threads™ PHP Forum Software 7.7.5