A pure substance is a material that has a consistent composition and defined properties throughout. Among the options given:
- Air: A mixture of gases, primarily nitrogen and oxygen, along with other gases.
- Nitrogen: A pure element and a pure substance, since it consists only of nitrogen molecules.
- Apple juice: A mixture that contains water, sugars, acids, and other compounds.
- Tap water: Generally considered a mixture because it can contain various dissolved minerals and contaminants.
Therefore, the pure substance among the options provided is nitrogen.