Ammonia gas (NH₃) is considered a single substance because it is composed of only one type of molecule, which is the ammonia molecule.

Here's why:

* Chemical Formula: Ammonia has a specific chemical formula (NH₃), meaning each molecule contains one nitrogen atom and three hydrogen atoms.

* Uniform Composition: All ammonia molecules have the same composition and structure.

* Homogeneous Mixture: Since it's made up of only one type of molecule, it's considered a homogeneous mixture – meaning its composition is uniform throughout.

To contrast:

* Air is a mixture of different gases like nitrogen, oxygen, and carbon dioxide.

* Saltwater is a mixture of salt dissolved in water.

In these examples, the substances have different components and are not considered single substances.