What defines a pure substance in chemistry?

A pure substance is defined as a material that has a uniform and definite composition. Specifically, it cannot be decomposed into simpler substances by chemical reactions, meaning it consists entirely of one type of particle, which can be either an element or a compound. This characteristic distinguishes pure substances from mixtures, which can be separated into their components using physical methods.

In the case of an element, it is made up of only one type of atom, and in the case of a compound, it consists of two or more different types of atoms bonded together in a fixed ratio. Therefore, the inability to be broken down into simpler substances by chemical means highlights the stability and uniformity of pure substances.

This understanding shows that pure substances can exist in various physical forms (solid, liquid, or gas), which makes the assertion that they are always gaseous incorrect. Also, a pure substance cannot consist of two or more different kinds of atoms without being a compound since a pure substance by definition is uniform in its composition. Hence, recognizing these attributes reinforces the distinction of pure substances in chemistry.