Which of the following is an example of a pure substance?

A pure substance is a material that has a consistent composition and distinct properties throughout. It is made up of only one type of particle. In the case of gold, it is considered a pure substance because it consists exclusively of gold atoms, which all have the same properties, such as density, melting point, and conductivity.

In contrast, saltwater, air, and chocolate milk are mixtures. Saltwater contains dissolved salt in water, which means it has more than one type of particle. Air is a mixture of various gases, primarily nitrogen and oxygen, among others. Chocolate milk contains a combination of milk, cocoa, sugar, and other ingredients, making it a heterogeneous mixture. Therefore, out of the provided options, gold is the only example that qualifies as a pure substance due to its uniform composition.