What is the scientific definition of a pure substance?

A pure substance is defined scientifically as a material that contains only one type of particle. This means that it can be either a single element, which is made up of only one kind of atom, or a compound, which consists of two or more types of atoms chemically bonded together in a fixed ratio. The key characteristic of a pure substance is that it has consistent properties throughout, such as boiling point, melting point, and density, which distinguishes it from mixtures that can vary in composition and properties.

Choosing a single element or compound as the correct definition captures this essence accurately since both types fall under the category of pure substances. Mixtures, by contrast, involve combinations of different substances that retain their individual properties and can change their composition, which accounts for the inaccuracy of the other options.