Here's why:

* Pure substances have a fixed composition, meaning they are made up of only one type of atom or molecule. This means they have the same properties throughout.

* Examples:

* Elements like gold (Au) or oxygen (O) are pure substances.

* Compounds like water (H₂O) or table salt (NaCl) are also pure substances because they consist of a specific ratio of elements bonded together.

In contrast, mixtures are combinations of two or more pure substances that are not chemically bonded. They can have varying compositions and properties.

Let me know if you'd like to know more about pure substances or mixtures!