Here's why:

* Fixed Composition: Gaseous ammonia always has the same chemical formula (NH₃), meaning it contains one nitrogen atom and three hydrogen atoms per molecule.

* Uniform Properties: All samples of pure gaseous ammonia will have the same physical and chemical properties, regardless of their source.

However, it's important to note:

* Impurities: Ammonia gas can contain impurities, such as water vapor or other gases. This would make the sample a mixture, not a pure substance.

* Solutions: Ammonia can dissolve in water to form a solution called ammonium hydroxide (NH₄OH). This solution is no longer a pure substance, but rather a mixture.

Therefore, pure gaseous ammonia is a single chemical compound with a fixed composition and uniform properties, making it a pure substance.