Question

which statement identifies why oxygen is classified as a pure substance and air is classified as a mixture?
oxygen contains only one substance, but air contains several substances.
oxygen is more reactive than air.
oxygen goes under a phase change, whereas air is always a gas.
oxygen has a higher density than air.

Explanation:

To determine why oxygen is a pure substance and air is a mixture, we analyze the options:

• A pure substance has a uniform and definite composition (contains only one substance). A mixture has two or more substances.
• The green option states "Oxygen contains only one substance, but air contains several substances" which matches the definitions: oxygen (e.g., \(O_2\)) is a pure substance (element), air is a mixture of gases (like \(O_2\), \(N_2\), \(CO_2\), etc.).
• The blue option (reactivity) doesn't relate to pure substance/mixture classification.
• The orange option (phase change) is irrelevant to composition (the basis for pure substance/mixture).
• The teal option (density) is a physical property, not related to composition classification.

Answer:

The correct statement is the green - colored one: "Oxygen contains only one substance, but air contains several substances." (Assuming the green option is, for example, Option A, then A. Oxygen contains only one substance, but air contains several substances)