Science, Tech, Math › Science How CAS Numbers Are Assigned to Chemicals Share Flipboard Email Print Chris Ryan / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated August 12, 2019 Every chemical is assigned a CAS Number. Have you ever wondered what a CAS Number is and how they are assigned? Check out this very simple explanation that will give you everything you need to know about what a CAS Number is, plus how CAS Numbers are assigned. The Chemical Abstract Service or CAS The Chemical Abstract Service is a division of the American Chemical Society, and it maintains a database of chemical compounds and sequences. The CAS database currently contains over 55 million different organic and inorganic chemical compounds. Each CAS entry is identified by their CAS Registry Number, or CAS Number for short. CAS Numbers CAS Numbers are up to 10 digits long using the format xxxxxxx-yy-z. They are assigned to a compound as the CAS registers a new compound. The number has no significance to the chemistry, structure, or chemical nature of the molecule. The CAS Number of a compound is a useful way to identify a chemical over its name. For example, the compound CAS 64-17-5 refers to ethanol. Ethanol is also known as ethyl alcohol, ethyl hydrate, absolute alcohol, grain alcohol or hydroxyethane. The CAS Number is the same for all these names. The CAS Number can also be used to distinguish between stereoisomers of a compound. Glucose is a sugar molecule that has two forms: D-glucose and L-glucose. D-glucose is also called dextrose and has CAS Number 50-99-7. L-glucose is the mirror image of D-glucose and has a CAS Number of 921-60-8.