What is an example of a pure substance?

A pure substance is defined as a material that has a consistent composition and distinct set of properties throughout. Table salt, or sodium chloride (NaCl), is an example of a pure substance because it consists entirely of one type of compound, with each unit containing the same ratio of sodium and chloride ions.

In contrast, the other options contain mixtures of different components. Chicken noodle soup includes various ingredients such as chicken, noodles, vegetables, and broth, which contribute to its varied composition. Milk is also a heterogeneous mixture that contains water, fats, proteins, and lactose among other elements, meaning it does not have uniform composition throughout. Air is a mixture of gases, primarily nitrogen and oxygen, along with trace amounts of other gases, making it not a pure substance either. Thus, table salt stands out as the only option that fits the definition of a pure substance.