What is a pure substance defined as?

A pure substance is defined as a material that is entirely composed of identical particles. This means that all the particles in the substance are uniform in composition and properties. For example, elements like gold or compounds like water are both considered pure substances, as they consist solely of one type of atom or molecule without any impurities or variations.

In contrast, a substance made of various types of particles does not meet the criteria for purity, as it inherently involves a mixture of different components. Similarly, a mixture of different compounds or a solution with variable composition also involves multiple types of particles or compounds, which cannot be classified as pure substances. Therefore, the defining characteristic of a pure substance is its homogeneity, where all the particles are identical, ensuring consistent properties throughout.