Understand the difference between a pure substance and a mixture. A pure substance has a fixed composition and distinct chemical properties, while a mixture contains two or more substances physically combined, each retaining its own properties.
Examine each option: Carbon dioxide (CO2) is a compound with a fixed chemical formula, so it is a pure substance.
Distilled water (H2O) is chemically pure water, meaning it contains only H2O molecules, so it is also a pure substance.
Sodium chloride (NaCl) is an ionic compound with a fixed ratio of sodium and chloride ions, making it a pure substance.
Air is a combination of different gases (mainly nitrogen, oxygen, and small amounts of other gases) physically mixed together without chemical bonding, so it is a mixture.