What constitutes a pure substance?

A pure substance is defined as a material that consists of only one type of particle, whether those particles are atoms, molecules, or ions. This uniformity means that a pure substance has consistent physical and chemical properties throughout. For example, elements such as oxygen or gold are pure substances because they are composed entirely of one kind of atom. Similarly, compounds like water (H2O) or sodium chloride (NaCl) qualify as pure substances because they consist of molecules made from a specific combination of two or more types of atoms, but still only one type of molecule.

In contrast, a mixture, which would include any of the other options provided, comprises two or more different types of particles. Mixtures can be homogeneous (uniform throughout) like solutions, or heterogeneous (not uniform), but they necessarily contain multiple components, lacking the defining characteristics of a pure substance. Hence, a solution containing various compounds or any combination of particles would not meet the criteria for being a pure substance. This distinction is crucial in understanding the nature of materials in chemistry and their applications in various scientific fields.