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Question: Molecules of a solid :

(a) are always in a state of motion

(b) move only when heated

(c) move because they are loosely bound

(d) do not move at all

My attempt:

I could safely ignore (c), as molecules in a solid are tightly bound. My book says that molecules in a solid vibrate about their fixed position. But they also expand on heating. Due to this, I am totally confused whether the correct option (a) or (b) (or maybe (d)?)

I know the question I am asking is really basic, and maybe does not even belong here, but I searched many sites and none revealed satisfying information. And thus, I am looking for expert help. I even doubt if my book (from where this question is) is correct or not.

Thanks a lot.

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closed as off-topic by John Rennie, Dilaton, jinawee, tpg2114, Kyle Kanos Feb 22 '14 at 18:06

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  • $\begingroup$ Clue: the atoms in liquid helium move even at absolute zero due to zero point energy. $\endgroup$ – John Rennie Feb 22 '14 at 12:42
  • $\begingroup$ @JohnRennie So, it means that the answer is (a) ? Because if the liquid helium moves, then solids could also. I am sorry if I got it wrong because my physics level is really low. $\endgroup$ – Gaurang Tandon Feb 22 '14 at 12:46
  • $\begingroup$ Though your teacher might not know about zero point energy, in which case the answer would be (b). $\endgroup$ – John Rennie Feb 22 '14 at 12:49
  • $\begingroup$ @JohnRennie I really don't understand why the concept of zero point energy required in a class 8th book question. Could you please explain why is the answer (b) in a bit more plain language. Thanks. $\endgroup$ – Gaurang Tandon Feb 22 '14 at 12:56
  • $\begingroup$ The question is a little vague. I look at option (b), and assume that by "heated" the questioner means heated in an oven or a bunsen burner ... that is, starting at room temperature. From that I take the solid in option (a) as being at room temperature, and I think that's a reasonable assumption for an 8th grade science question. But as I said, the question is vague. Hope that helps a bit! $\endgroup$ – garyp Feb 22 '14 at 14:12
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Strictly speaking (a) is correct. But in cold temperature under the freezing point, the motion would be called "tiny vibration", and molecules cannot move freely as in liquid or gas state. If the "move" in (b) means free motion including replacement with neighboring molecules, (b) may be correct also. But the "move" includes tiny vibration, (b) is wrong. Tiny vibration occurs even though cooled to the absolute zero, minus 273 degrees celsius.

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  • $\begingroup$ Since it's a very small class question, of people who haven't studied such complex topics, I think I agree with you. $\endgroup$ – Gaurang Tandon Feb 22 '14 at 14:37
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The atoms are always moving as vibration. The molecules or formula units as such are almost always staying put - the definition of solid.

Consider alumina, quartz, diamond. Ionic, mixed, purely covalent. Hard, high melting temps. Single crystal diamond is a single molecule. Look up their crystal structures, their *.cif files. At the bottom of the file you will see the atom anisotropies. Everything is always vibrating in place from ambient thermal energy. Consider a molecular solid like benzil. Soft, mp 95 C, barely held together by van der Waals forces and pi-stacking. Same thing. In all cases, the formula units are rigidly in place as their atoms vibrate within their chemical bonds.

Buckminsterfullerene, ${C_{60}}$, is a very slippery ball. Despite its (lattice) high melting point, each molecule freely tumbles in plaice within the crystal lattice. This is seen by the disorder in its crystal structure (atomic positions cannot be resolved). More telling, a C-13 NMR of the solid (no magic angle spinning) shows sharp lines as a solution spectrum would.

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  • $\begingroup$ This, is, really very, very complicated ..... $\endgroup$ – Gaurang Tandon Feb 22 '14 at 16:53

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