temperature and the meta model

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by emanuel</i>
<br />What is it exactly that makes different molecules melt/vaporize at different temperatures?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Speed of molecule vibrations.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Another way to phrase the question is what is it about molecules that keep them vibrating close together (solid) rather than slipping away from nearby molecules (liquid) or flying away (gas)? Is there a bond between molecules that is strengthened or weakened by temperature?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">Whatever "stickiness" molecules normally have, there exists a vibration speed (temperature) high enough that the stickiness bond is broken. Then molecules float around and mix with other molecules, making the structure a liquid. If the vibrations become fast enough, the molecules "take off" and fly away, which makes them a gas.

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Is temperature nothing more than the strengthening/weakening of this bond? What is the bond?<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">The bond is the force between atoms that binds them together in a molecule. [electricity and magnetism] Temperature is a measure of mean molecular vibration speed, which is always trying to break the bond. At low vibration speeds, the bond holds and you have a solid. At higher speeds, you have a liquid or a gas, as already described. -|Tom|-

emanuel, I have wondered about these same things and in researching thermal properties I come to the conclusion no one knows the answers to most of the questions you are asking about here. It seems things just are as they are and no one knows why. The melt point of solids is a real mystery-helium melts at 3 kelvin under a few pounds of pressure but without pressure solid helium never forms. And hydrogen melts at 20 kelvin in vacuum. Go figure.

Tom,

Are you really saying that the bond that holds molecules together (with other molecules in a solid state) is the same as the bond that holds atoms together (in a molecule)? If I remember correctly, the latter involves ionic and covalent bonds, sharing of electrons, etc (it's been a long time for me here). But what I am asking about is what holds molecules together into solids. In a block of ice, for example, the water molecules don't share electrons do they?

And please forgive me if my questions aren't phrased properly. It may be the case that I am just not understanding something very basic. Or maybe Jim, you are right, and I am just asking some deeper question that nobody knows the answer to. Thanks for your thoughts, too.

Emanuel

[emanuel] "It may be the case that I am just not understanding something very basic."

On the supposition that this is the case, here is the short version ...

Atoms stick to each other because of their electrical force fields. And those same atoms move around relative to each other because of the heat energy they contain. How these sets of forces balance determins the state (solid, liquid, gas) of a clump of atoms.

In solids the motion caused by heat is small compared to the electrical forces and as a result the atoms have a strong tendency to stay in the same place relative to their neighbors. Atoms in a solid are generally visualized as being in contact with each other, and in more or less constant contact with the same neighbors.

As heat energy is added (thus raising the temperature) the atoms move faster and farther from their neighbors. At some temperature point (called the melting point) the motion is large enough to allow atoms to move about relative to their neighbors. The solid becomes a liquid. Atoms in a liquid are generally visualized as being in contact with each other, but seldom with the <u>same</u> neighbors.

As more heat energy is added another temperature point (called the boiling point) is reached. Now the heat energy is large compared to the energy of the electrical fields and individual atoms can move fast and far relative to their neighbors. The liquid becomes a gas. Atoms in a gas are generally visualized as not being in contact with each other except for brief periods when they collide.

Regards,
LB

(If that was too basic, or not basic enough, let me know.)

And yes, the bond between molecules works the same way. It is (mostly) a balance between electrical force fields and heat energy.