Question

is quartz(SiO2) a substance or mixture?

Answer 1

That's a wee bit tricky. The answer you probably want is that it's a substance, because it's all made of SiO2. A mixture would have several different compounds in it, each with a different chemical formula.

However...if you are talking about actual quartz, the mineral, the problem is that it's a complex substance, with many different kinds of rings and pyramids and chains and so forth, mostly made out of SiO4-4 tetrahedra joined together by bridging oxygen atoms. Much like graphite, actually. You can't really call this conglomeration a mixture, because pretty much everything is bonded to everything else, while in a mixture the various chemical components don't have chemical bond to each other. But on the other hand, it's hard to call it just one compound, too, given all the various forms that are in it. Minerals can be this way. Another good example is the iron ore magnetite, which is made of O-2 anions, Fe+2 cations, AND Fe+3 cations. It can be represented as FeO*Fe2O3, but that doesn't quite get at it, since I don't think there are large separate islands of FeO and Fe2O3.

It's not super likely an introductory chemical course would go into this kind of real-world complexity -- the answer that's almost certainly wanted is that it's a substance, Khattam-Shud. But I thought you might be interested.

Answer 2

By "much like graphite" I mean that graphite, too, is made out of a large variety of rings and chains and whatnot, not that graphite is made of SiO4-4 pyramids!

Answer 3

It is a compound