What characterizes a pure substance?

A pure substance is defined as having a uniform and definite composition. This means it contains only one type of material, which can either be an element or a compound. When we say that a pure substance "contains only one kind of compound," it indicates that it has a consistent arrangement of atoms and molecules throughout its entirety, reflecting its homogeneous nature.

For example, table salt (sodium chloride) as a pure compound consists only of sodium and chlorine in a fixed ratio of 1:1. Any sample of pure sodium chloride will have the same chemical properties and performance regardless of its source.

In contrast, a mixture, which is not a pure substance, can exhibit variable composition and can be easily separated into its components based on physical properties. This understanding reinforces why the identification of a pure substance relies on its singular classification and composition, distinguishing it from other forms of matter.