[thetada]

Hello,

I'm trying to find out a term, if it exists. A chemical change is when bonds are broken and new ones made, leading to the creation of a new substance. A physical change is when matter changes state. But what's the term for a change where the molecular structure stays the same, but the manner in which the particles change their arrangement completely changes the properties. Candy floss, for example, has the same molecular structure as sucrose but owing to its amorphous structure, has markedly different properties. Is there a term for this kind of change? I want to call it an "arrangemental change" but I suspect someone else has already coined an accepted term.

thanks

[Rutherford]

I think it's anisotropy.

[thetada]

A friend just told me - it's polymorphism. In this specific example. Anisotropy is to do with how materials can have different properties depending on which of its axes you consider. Wood, for example, will resist tensile strength alot better against the grain than with the grain (thank you Wikipedia).

[fledarmus]

Confirming thetada's answer - it is polymorphism. See http://en.wikipedia.org/wiki/Polymorphism_(materials_science)

Different solid structures including different crystal structures and possible amorphous structures can be a real challenge in drug discovery. Several isomorphs may be possible for the same compound, and each will have its own solubility and stability characteristics. A change from one crystalline form to another may be the difference between a dose which is well absorbed and one which is poorly absorbed, leading to underdosing or overdosing on the drug.

[juanrga]

Hello,

I'm trying to find out a term, if it exists. A chemical change is when bonds are broken and new ones made, leading to the creation of a new substance. A physical change is when matter changes state. But what's the term for a change where the molecular structure stays the same, but the manner in which the particles change their arrangement completely changes the properties. Candy floss, for example, has the same molecular structure as sucrose but owing to its amorphous structure, has markedly different properties. Is there a term for this kind of change? I want to call it an "arrangemental change" but I suspect someone else has already coined an accepted term.

thanks

First, electron transfer processes are considered as among the simplest chemical processes [1]. What bonds are broken and made during the next transfer?



Additionally, the evaporation of water implies a change of state but also implies the breaking of bonds (many hydrogen bonds between molecules at liquid must be broken). The conclusion here is that the distinction 'physical' vs 'chemical' processes is very fuzzy.

Second, the terminology that you must be looking for is phase reaction or phase transition. A typical phase reaction is

CaCO₃(s,calcite) CaCO₃(s,aragonite)

Phase transitions are studied as any chemical reaction. For example, the Gibbs reaction energy at 25ºC for the above process is :delta: G_rº = 1393 J mol^-1

[1] The chemist Rudolph Arthur Marcus received the 1992 Nobel Prize in Chemistry for his theory of electron transfer.

[thetada]

Yes, it's one of many watered-down truths I'm required to teach my students. Thanks for your comment about phase transition.