1
$\begingroup$

I've got a question regarding the microscopic difference between liquids and solids. I know that there exists a precise phase transition between solids and liquids, so some discrete parameter must change in the transition. I'm letting the molecules under examination be covalent, so we can't assume that a crystal lattice is formed in the solid case.

(1) Are different forces dominant in between solid and liquid molecules? By "forces" I mean ionic bonds, dipole-dipole interactions, London forces, etcetera.

(2) What is the difference in the geometric structure of the molecules' interactions? Solids are fixed and gases are mobile, but what precisely are liquids? Solid structures with enough degrees of freedom to pivot fluidly? Do liquids generally have the same structure (simple cubic, hexagonal, etcetera) as their solid counterparts when they do interact?

I guess my primary confusion is that the distance between molecules in a material (solid or liquid) should intuitively be a continuous parameter dependent on the density of particles and their temperature, not a discrete parameter taking the values present in the "solid" and "liquid" states. Why the discrete nature?

$\endgroup$

closed as too broad by Klaus-Dieter Warzecha, M.A.R., ron, Todd Minehardt, Jon Custer Jul 12 '16 at 14:33

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • 2
    $\begingroup$ Unfortunately it seems your question is too broad, or more precisely there are various questions in your post which are broad by themselves. Also I have no idea how you can know the types of crystal lattices, but don't know that liquids can't have one. $\endgroup$ – Mithoron Jul 12 '16 at 0:57
2
$\begingroup$

Let's break this down, and keeping it to simple covalent compounds...

Are different forces dominant in between solid and liquid molecules? By "forces" I mean ionic bonds, dipole-dipole interactions, London forces, etcetera.

All those forces (if applicable) will still exert their expected effect and there is no 'dominant force' which increases in strength when temperature rises. Just that when we raise the temperature, the molecule may have enough kinetic energy to overcome London Dispersion but not ionic forces, etc.

What is the difference in the geometric structure of the molecules' interactions? Solids are fixed and gases are mobile, but what precisely are liquids? Solid structures with enough degrees of freedom to pivot fluidly? Do liquids generally have the same structure (simple cubic, hexagonal, etcetera) as their solid counterparts when they do interact?

So let's take a look at a typical phase transition diagram, that of water:

Phase Transition Diagram of Water

At the flat stages, water is changing discreetly and exists either as solid (rigid lattice) or liquid (completely formless and structure-less). And if you look at ice melting, that is exactly what you see and experience.

But the source of this question is probably the fact that you've also seen certain solids soften under heat without melting completely. These covalent materials are called thermoplastic and have a property called the "Glass Transition Temperature". The Stiffness-vs Temperature graph looks like this:

Glass Transition for Rubber

In this state, the melting solid is able to overcome the forces that hold it to a lattice, but is not able to overcome the forces that hold the molecules together and so the molecules cannot 'flow'. The reason this exists for some compounds and not others is largely a property of how their molecules hold each other together.

$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.