In introductory general chemistry textbooks, a chemical substance is commonly defined as ‘any material with a definite chemical composition.’ This definition encompasses both pure chemical elements and pure chemical compounds. However, there are exceptions to this strict definition; a pure substance can alternatively be described as a form of matter possessing both a definite composition and distinct properties. The chemical substance index compiled by CAS also includes several alloys of uncertain composition. Non-stoichiometric compounds, a unique case in inorganic chemistry, challenge the law of constant composition. As a result, distinguishing between a mixture and a compound can be challenging, as seen in substances like palladium hydride. Broader definitions of chemicals or chemical substances expand upon this, such as: ‘the term “chemical substance” refers to any organic or inorganic substance with a specific molecular identity, encompassing any combination thereof resulting from chemical reactions or naturally occurring.