What do you understand by a pure substance?

A pure substance is made of only one kind of particle with a fixed composition throughout.

It has definite physical and chemical properties like boiling point, melting point, and density. Pure substances include elements such as oxygen and gold, and compounds such as water (H₂O) or carbon dioxide (CO₂). 

They cannot be separated into simpler substances by physical means. In contrast, mixtures contain two or more substances physically combined. Purity ensures consistent quality and predictable reactions, which is why industries use distilled water or pure metals in production.