Hi! Regarding Special:Diff/1033545034: I have to admit that it's hard to tell with sufficent certainty which is the correct relation as currently the entire classification of chemical substances is rather messy. I may have just hidden parts of the confusion. Generally it would make sense that chemical compound (Q11173) is a subclass of substance, the latter in turn being a subclass of concrete object of which instances are not substance classes like germanium dioxide (Q419133) (as opposed, say, particular chunk of this substance displayed on a photo in that item). However for some reason chemical compound (Q11173) is currently also set a metaclass (through "chemical component" item), as if it was a group or class of chemical substances (Q17339814). This is certainly wrong as it should not be a class of concrete objects and class of classes at the same.
Indeed there are lots of items (about 164k) directly set as chemical compound instances. Lots of it seem to be set by bots. This might have been wrong in the first place as it's unclear if "chemical compound" was ever supposed to be a metaclass and not class of concrete objects.
However, there are also lots of class item that are set as subclass of (subclasses of) "chemical compound" item. I'm having trouble to get the exact counts without query timing out, but for instance there are currently about 54k items that are a subclass of subclass of subclass of chemical compound (Q11173). Probably many items are incorrently set as instance of and subclass of chemical compound at the same time, too. So I doubt that using chemical compound (Q11173) in either relation is established really.
For the sake of practicality and simplicity I'd suggest correcting root items like chemical compound (Q11173) so that they were explicitly set as classes of concrete objects. Otherwise, if it was a metaclass, then what were true instances of concrete objects with particular chemical composition (substances) instances of. (In long term it might be reasonable to have separate items from substances and relevant molecular entities.)