* Pure substance: A pure substance is composed of only one type of molecule or atom.

* Homogeneous mixture: A homogeneous mixture has a uniform composition throughout (like saltwater).

* Heterogeneous mixture: A heterogeneous mixture has a non-uniform composition (like sand and water).

C2H2 is made up of only one type of molecule, containing two carbon atoms and two hydrogen atoms bonded together. It has a consistent composition regardless of where you sample it.