Quantitative versus qualitative change

[MikeNovack]

Except, in the water phase change examples, that ignores what is happening at the molecular level.

When, ice melts, not SIMPLY ice changing to water. We really mean, at the water ice interface, more molecules are leaving the crystal structure and joining the liquid (water is VERY polar, so water molecules link up with others*) than the number of water molecules hitting the crystal surface and taking their place in that structure. Similar when water boils. At the water vapor/water interface, more molecules leaving the water than joining it.

So while ice melts, there is still some freezing going on and when water boils, still some condensation going on. I'd say that makes both changes "quantitative". More molecules going one way than the other.

[Impenitent]

the total number of water molecules remains constant doesn't it?

-Imp

[MikeNovack]

the total number of water molecules remains constant doesn't it?

-Imp

?? ??? If by total number of water molecules you mean H2O whether solid, liquid, or gas, then yes. Of course we say H2O because of the very strong bond between the atoms of the molecule, but water in the liquid state is so "polar" that the bond BETWEEN water molecules is stronger than those holding many organic chemicals together and even some inorganic chemicals. In other words, water ACTS as if about H16O8. Normally we would not expect something as light as H2O (molecular weight only 18) to be a liquid at room temperature. We might think of water as ordinary but it has some rather "strange" properties.

[Impenitent]

the total number of water molecules remains constant doesn't it?

-Imp

?? ??? If by total number of water molecules you mean H2O whether solid, liquid, or gas, then yes. Of course we say H2O because of the very strong bond between the atoms of the molecule, but water in the liquid state is so "polar" that the bond BETWEEN water molecules is stronger than those holding many organic chemicals together and even some inorganic chemicals. In other words, water ACTS as if about H16O8. Normally we would not expect something as light as H2O (molecular weight only 18) to be a liquid at room temperature. We might think of water as ordinary but it has some rather "strange" properties.

you said "So while ice melts, there is still some freezing going on and when water boils, still some condensation going on. I'd say that makes both changes "quantitative". More molecules going one way than the other."

the amount of space that 1 mole of liquid water molecules occupies is different than the amount of space that 1 mole of water vapor occupies- which is different than the amount of space occupied by 1 mole of solid water molecules.

the space that is occupied changes, but the number of molecules (1 mole) remains constant...

(which is a long way of saying I think we agree- I think you meant quantitative in the number of molecules changing state not in total molecules)

-Imp

[MikeNovack]

the amount of space that 1 mole of liquid water molecules occupies is different than the amount of space that 1 mole of water vapor occupies- which is different than the amount of space occupied by 1 mole of solid water molecules.

Except while the volume occupied by one mole of molecules as water or ice is defined (as a function of temperature), the volume of a mole of water vapor is undefined (depends on pressure)

[cpuproc68]

So while ice melts, there is still some freezing going on and when water boils, still some condensation going on. I'd say that makes both changes "quantitative". More molecules going one way than the other.

I agree with you, Mike.

[cpuproc68]

Except, in the water phase change examples, that ignores what is happening at the molecular level.

When, ice melts, not SIMPLY ice changing to water. We really mean, at the water ice interface, more molecules are leaving the crystal structure and joining the liquid (water is VERY polar, so water molecules link up with others*) than the number of water molecules hitting the crystal surface and taking their place in that structure. Similar when water boils. At the water vapor/water interface, more molecules leaving the water than joining it.

So while ice melts, there is still some freezing going on and when water boils, still some condensation going on. I'd say that makes both changes "quantitative". More molecules going one way than the other.

Thank you for drawing attention to a very important aspect of the molecular analysis of the phenomena mentioned.