# What is chemical decomposition in the context of a crystal or amorphous solid?

The breakdown of a single entity (normal molecule, reaction intermediate, etc.) into two or more fragments.

That is a very general definition, but I am not sure if it is supposed to be this inclusive. For example, think of a diamond crystal. It is a single entity. Now if chemical decomposition occurs, it can break down into multiple fragments. These fragments are not defined more closely, so it could be that the fragments are single C atoms, polyatomic C molecules... or even diamond fragments again.

In this sense, simply cutting or polishing a diamond would be a special case of chemical decomposition. This seems quite absurd to me. So the question is: Is chemical decomposition really that broad, or is there some better definition that is more commonly used in chemistry? It seems to me that the intuitive definition of chemical decomposition would be that a molecule is broken into smaller fragments, while a crystal or an amorphous solid would need to be broken into non-crystalline (non-amorphous) fragments in order for it to be considered chemical decomposition. Otherwise it's just cutting!

The question is relevant to me because I am writing my PhD thesis on laser-induced breakdown spectroscopy. The "breakdown" in the name of the technique refers to a chemical breakdown, which is the same as chemical decomposition. Usually, solid samples are analyzed, in which case some material from the sample surface is removed by laser ablation. Laser ablation is basically the same as laser-induced breakdown, if you look at the definitions. So there is some confusion on the terminology in general, and I want to unravel it as good as possible in my theory chapter.

Thank you very much for your input!

(Note: By the way, some non-IUPAC definitions I have found for chemical decomposition also confusingly define it as relating to compounds. As far as I know, the definition of a compound excludes crystals and amorphous solids, but it also excludes homonuclear molecules. So this definition seems way too exclusive, because it makes sense that decomposition can also occur for homonuclear molecules.)

• It's decomposition if a relevant part of the sample (e.g. the one you actually analyse) is not the same substance as before. Polishing a diamond, you still have a diamond, plus some dust you don't care about much. – Karl Jan 7 at 9:46

• This depends on what mixture you are talking about. A mechanical mix of both salts would indeed have 2 phases. However, a system of molten salts with an arbitrarily chosen composition $\ce{Na_xK_{1-x}Cl}$ ($0\leq x\leq 1$) is homogeneous all right (single phase, see e.g. this NaCl-KCl phase diagram). – andselisk Jan 5 at 13:00