The three states of matter... or four ?

[nonconformist]

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 ?

Two SiO2 Molecules are just 2 SiO2 molecules.
The state of a substance depends on their melting/boiling points, and intermolecular forces.

As for a mole, it's Avogadro's constant; in One Mole of SiO2, which weighs 60grammes, there are 6*10^23 molecules.

 

[The Last Conformist]

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).

The molecules in a liquid stick together in a way grains of dust do not do. The reason is, very simply, that molecules are small enough for attractive forces between their "surfaces" to matter, while the very small rocks are much too big for that.

 

[nonconformist]

The molecules in a liquid stick together in a way grains of dust do not do. The reason is, very simply, that molecules are small enough for attractive forces between their "surfaces" to matter, while the very small rocks are much too big for that.

And water is a particularly odd liquid.