Here's why:

* Mixtures are formed by physically combining two or more substances that retain their individual properties. They can be separated by physical means.

* Compounds are formed by chemically combining two or more elements in a fixed ratio. Their properties are different from the original elements, and they can only be separated by chemical means.

Ammonia is formed by the chemical combination of nitrogen (N) and hydrogen (H) in a fixed ratio of 1:3. It has unique properties distinct from nitrogen and hydrogen, and cannot be separated into its components by simple physical methods.