Here's why:

* Chemical Formula: Saltpeter, also known as potassium nitrate (KNO3), has a specific chemical formula. This means it's made up of a fixed ratio of potassium, nitrogen, and oxygen atoms.

* Consistent Properties: All samples of potassium nitrate will have the same chemical and physical properties, such as melting point, solubility, and density.

* No Variable Composition: Unlike mixtures, which can have varying amounts of different components, saltpeter always has the same composition.

Therefore, saltpeter is a pure substance, specifically a compound.