Question

Why are compounds pure even if it can be separated into its constituent particles?

Answer 1

Pure Substance: The substances that are free from any kind of mixture and contain only one kind of particle are pure substances. Examples of pure substances include iron, aluminum, silver, and gold.
This one kind of particle doesn't means it must have only single type of element.
So two or more compounds or elements combine to form an impure substance.
A pure substance, basically composed of two or more elements and chemically combined in a fixed proportion is called a compound. Therefore, water is a compound. It has two elements, hydrogen and oxygen, combined in a fixed ratio.
​​​​​​Compound consist of two or more elements.It deals in the molecular level.
Definition of pure substance only means that it should contain only one type of substance and can be purified without any breaking of bond between the molecules and not necessary to have one type of element.