The three states of matter... or four ?

[nonconformist]

How small can a rock be before it becomes a molecule ? In that case, SiO2 IIRC.

And glass is basically vitrified sand, and it's a solid. So you can get a solid from sand, yet sand itself is... nothing ?

A molecule is a molecule.
It is a molecule when you have one silicon atom, and two oxygen atoms bonded together.

 

[blackheart]

That's actually a myth. The glass is thicker at the bottom due to old glassmaking methods, not because it's actually flowing.

It isn't a myth. Certain types of glass (old old types) really do flow. The newer types of glass due to their crystalline lattice structure (or was it the other type?) don't flow.

 

[Perfection]

Rocks aren't made of molecules. They are made of either different ions bonded together or are really giant molecules. SiO2 is like a giant molecule.

 

[Perfection]

It isn't a myth. Certain types of glass (old old types) really do flow. The newer types of glass due to their crystalline lattice structure (or was it the other type?) don't flow.

Glasses don't have crystalline lattices. If they had them, they wouldn't be glass.

 

[The Last Conformist]

How small can a rock be before it becomes a molecule ? In that case, SiO2 IIRC.

Well, a typical sand grain is about 10^-3 m in diameter, while an atom is about 10^-10, a SiO2 block is a couple times that, so we'd have to chop the sand grain into something like 10^20 pieces to get down to molecules. I can't offhand say how many SiO2s you need to make up the crystaline structure of rock, but it's going to be a very very small number compared to 10^20.

Such minimal rocks wouldn't be sand, but ultrafine dust.

And glass is basically vitrified sand, and it's a solid. So you can get a solid from sand, yet sand itself is... nothing ?

Sand is a collection of solid particles that, in the making of glass, are melted, and then recrystalize as a single solid object.

It may help to think of two chunks of metal being melted, the melts mixed, and then cast into a single piece.

 

[blackheart]

Glasses don't have crystalline lattices. If they had them, they wouldn't be glass.

It had something to do with how its molecular structure was. But it seems SomethingWitty was right. Wiki confirms it.

http://en.wikipedia.org/wiki/Glass

 

[punkbass2000]

Some people have given some very good responses. I'd just like to point that like with any classification system, if you examine it enough there will be overlaps and grey areas. So take'em with a grain of salt.

 

[Masquerouge]

A molecule is a molecule.
It is a molecule when you have one silicon atom, and two oxygen atoms bonded together.

That's a tautology. I know it when I see it ?

When do a group of atom stop being a molecule and becomes a rock ? Is a molecule a solid ?

Ok, maybe I'm cutting hairs here, but apparently the macro properties of an element can offset its micro structure. Basically, ultra-small solids, when put together, behave not a whole lot differently than a liquid made of molecules, which kind of make sense, but also kind of blurs what a liquid is. After all, we could say of water that is if nothing but solid H2O molecules put together, the same way sand is mad of very small rocks put together...

 

[Masquerouge]

Some people have given some very good responses. I'd just like to point that like with any classification system, if you examine it enough there will be overlaps and grey areas. So take'em with a grain of salt.

Solid or liquid, the NaCl ?

Yeah, I had to make that one.

 

[nonconformist]

That's a tautology. I know it when I see it ?

When do a group of atom stop being a molecule and becomes a rock ? Is a molecule a solid ?

Ok, maybe I'm cutting hairs here, but apparently the macro properties of an element can offset its micro structure. Basically, ultra-small solids, when put together, behave not a whole lot differently than a liquid made of molecules, which kind of make sense, but also kind of blurs what a liquid is. After all, we could say of water that is if nothing but solid H2O molecules put together, the same way sand is mad of very small rocks put together...

I'm a chemist, so I think in chemiscal terms

A substance is merely a certain number of moles of atoms.

 

[nonconformist]

Solid or liquid, the NaCl ?

Yeah, I had to make that one.

NaCl is, IIRC, as a giant ionice lattice unable to become liquid, and cn only be dissolved aqueously.

 

[Masquerouge]

I'm a chemist, so I think in chemiscal terms

A substance is merely a certain number of moles of atoms.

Is there an "official" number ?

 

[nonconformist]

Is there an "official" number ?

At which it becoems a "rock"?
No, I don't think so, it's doubtful, scientists talk in masses, concentration, volume and moles.

 

[Masquerouge]

At which it becoems a "rock"?
No, I don't think so, it's doubtful, scientists talk in masses, concentration, volume and moles.

My guess was that it could be important to distinguish between a molecule and a rock (in that example), because obviously some very important physical properties apply to molecules only (i.e., a group of molecule can be a liquid) while other properties apply to groups of very small rocks (a group of very small rocks is apparently not a liquid).

So I thought there must be a point where you could say "this is a rock" or "this is a molecule", and thus deduce physical properties from there ("I can heat it and it will become gas" or "I can heat it and it will become glass")

Let's suppose for the sake of the argument that rocks are only made of the SiO2 molecule.
So are two SiO2 molecules lumped together a rock or a molecule ? What about 10 ? A mole ?

 

[punkbass2000]

Along a similar vein, when does an asteroid become a planet?

 

[Masquerouge]

Along a similar vein, when does an asteroid become a planet?

Don't try to thwart my molecule thread with your petty astrological matters, you Hubble wannabe !

Other than that... good point, good point.

 

[Perfection]

NaCl is, IIRC, as a giant ionice lattice unable to become liquid, and cn only be dissolved aqueously.

You can melt salts. They usually melt at a pretty high temperature, but they'll still melt.

Let's suppose for the sake of the argument that rocks are only made of the SiO2 molecule.
So are two SiO2 molecules lumped together a rock or a molecule ? What about 10 ? A mole ?

It's important to understand that quartz isn't made of SiO2 molecules. That is there are no small units of SiO2 that are repeated. rather there are networks of atoms where silicons atoms each are bonded to four oxygen atoms and each oxygen to two silicons. With very small numbers of atoms the network is incapable of making the structures associated with quartz. Larger numbers give it crystal unit scructures and all the properties we know and love.

 

[Perfection]

Along a similar vein, when does an asteroid become a planet?

I support the idea that for an object to be described as a "planet" it should be massive enough to clear out its orbital path of debris and not be in a population of similarly sized objects.

 

[Masquerouge]

It's important to understand that quartz isn't made of SiO2 molecules. That is there are no small units of SiO2 that are repeated. rather there are networks of atoms where silicons atoms each are bonded to four oxygen atoms and each oxygen to two silicons. With very small numbers of atoms the network is incapable of making the structures associated with quartz. Larger numbers give it crystal unit scructures and all the properties we know and love.

Eh. I do love cristals. They can be boring on a Saturday night, though.
But seriously, is there a way to determine that you're facing a crystal and not a molecule ?

 

[Perfection]

At the meso scale the determination is largely arbitrary. Certainly anything below the size of a unit lattice is not a cystal and anythign with a good sized mutliple is. In between though and you'd dealing with with properties of both.