* Have a fixed and definite composition: A compound always contains the same elements in the same ratio by mass. For example, water (H₂O) always has two hydrogen atoms and one oxygen atom, regardless of its source.

* Have a unique set of properties: Compounds have distinct physical and chemical properties that differentiate them from their constituent elements. For example, table salt (NaCl) has a different melting point, boiling point, and reactivity than sodium (Na) or chlorine (Cl).

* Are homogeneous: Compounds are uniform throughout and have the same composition and properties in every part of the sample.

In contrast, mixtures:

* Do not have a fixed composition: Mixtures can be made with varying proportions of different components.

* Do not have unique properties: The properties of a mixture are a blend of the properties of its components.

* Can be heterogeneous: Mixtures can have different compositions and properties in different parts of the sample.

Therefore, compounds meet the criteria of a pure substance due to their fixed composition, unique properties, and homogeneity.