Question

7 which of the following statements explains why oxygen is classified as a pure substance
a oxygen is more reactive than air.
b oxygen has a higher density than air.
c oxygen undergoes phase changes, whereas air is always a gas.
d oxygen contains only one type of atom, but air contains several types of atoms.

Explanation:

A pure substance is made of only one type of particle (atom or molecule). Oxygen ($O_2$) consists of only oxygen atoms. Air is a mixture with nitrogen, oxygen, carbon dioxide, etc. (multiple atom types). Option A (reactivity) and B (density) are properties, not definitions of pure substances. Option C is incorrect (air can condense too). Only D relates to the composition (one atom type) defining a pure substance.

Answer:

D. Oxygen contains only one type of atom, but air contains several types of atoms.