Final answer:
Among the options given, air cannot be classified as a substance because it is a mixture of gases with variable composition, unlike table salt, nitrogen, and gold, which have constant compositions and are either elements or compounds.
Step-by-step explanation:
The question is asking which of the following cannot be classified as a substance: a) table salt, b) air, c) nitrogen, or d) gold. To answer this, we need to understand what a substance is. A substance is a form of matter with a constant chemical composition and characteristic properties. It can be an element or a compound, but not a homogeneous or heterogeneous mixture. Among the options given:
- Table salt (sodium chloride) is a compound, thus it is a substance.
- Nitrogen is an element, making it a substance as well.
- Gold is also an element, so it is a substance.
- Air, however, is a mixture of many gases, primarily nitrogen, oxygen, and other trace gases, and therefore cannot be considered a substance since its composition can vary.
Based on these classifications, air is the one that cannot be classified as a substance.