CAS stands for Chemical Abstracts Service, a division of the American Chemical Society that maintains the world’s largest registry of chemical substances. When chemists, regulators, or safety professionals refer to a “CAS number,” they mean a unique numerical identifier assigned to every known chemical substance in this registry, which now contains over 290 million entries. Think of it as a Social Security number for chemicals: one number, one substance, no confusion.
Why Chemistry Needs a Numbering System
Chemicals are surprisingly hard to name consistently. A single compound can have a common name, a trade name, and a systematic name that follows international naming rules. Aspirin, for instance, is also acetylsalicylic acid, and its systematic name is even longer. Some chemicals have dozens of synonyms across different languages and industries. Even “correct” systematic names can contain errors in alphabetical ordering, stereochemistry labels, or functional group priority, all of which change the meaning of the name entirely.
A CAS Registry Number cuts through this problem. Water is 7732-18-5. Caffeine is 58-08-2. Ethanol is 64-17-5. No matter what language you speak, what country you work in, or what you call the substance colloquially, the CAS number points to exactly one chemical identity. This eliminates the kind of ambiguity that could, in a lab or industrial setting, lead to dangerous mix-ups.
How CAS Numbers Are Structured
A CAS Registry Number contains up to 10 digits separated into three groups by hyphens. The first group (on the left) has two to seven digits, the second group always has two digits, and the final group is a single check digit. So a short one looks like 58-08-2 (caffeine), while a longer one might run seven digits in the first group for a more recently registered substance.
The numbers themselves don’t encode any chemical information. They’re assigned sequentially as new substances are registered, so a higher number generally means a more recently cataloged compound. The check digit at the end exists purely for error detection. It’s calculated from the other digits using a simple mathematical formula, which lets databases automatically catch typos. If you accidentally transpose two digits, the check digit won’t match and the system flags the error.
What Gets a CAS Number
The registry goes far beyond simple molecules. Organic and inorganic compounds, metals, alloys, polymers, coordination compounds, minerals, mixtures, and biological sequences like proteins and nucleic acids all receive CAS numbers. If a substance can be distinctly identified, it qualifies. The registry currently covers over 275 million unique chemicals and grows continuously as new compounds are synthesized or discovered.
Each registered substance gets more than just a number. The CAS record includes the chemical’s name, molecular formula, molecular weight, and a structure image. Many entries also list synonyms (systematic names, common names, and trade names), along with basic physical properties like boiling point, melting point, and density when that data is available.
Where CAS Numbers Show Up in Practice
If you’ve ever looked at a Safety Data Sheet (SDS), the document that accompanies hazardous chemicals in workplaces, you’ve seen CAS numbers in action. OSHA requires that Section 3 of every SDS list the chemical ingredients by name and CAS number. This lets safety officers, emergency responders, and workers identify exactly what they’re handling, regardless of the product’s brand name or the manufacturer’s preferred terminology.
Beyond workplace safety, CAS numbers are the standard identifiers used in regulatory frameworks around the world. Environmental agencies use them to track controlled substances. Pharmaceutical companies use them to register active ingredients. Customs authorities use them to classify chemical imports and exports. Academic researchers use them to search literature databases without worrying about name variations across journals. In short, any time precision matters when referring to a chemical, the CAS number is the reference point.
Looking Up a CAS Number
CAS operates a free public tool called CAS Common Chemistry that lets anyone search for substances. You can enter a CAS number directly, or search by chemical name, molecular formula, or several structural notation formats used in chemistry software (SMILES, InChI, and InChIKey). A basic search returns a list of matching substances with their names, CAS numbers, and structure images. Selecting a specific substance pulls up the full record: all known synonyms, identifiers, structural data, experimental properties, and any history of replaced or deleted CAS numbers for that entry.
This matters practically because if you encounter an unfamiliar CAS number on a product label or in a research paper, you can look it up in seconds and get a clear picture of what the substance is, what it’s commonly called, and what its basic physical properties are.
CAS Numbers vs. Chemical Names
Systematic chemical naming follows rules set by the International Union of Pure and Applied Chemistry (IUPAC), and CAS has its own slightly different naming conventions built up over decades of indexing. Both systems aim to describe a molecule’s structure through its name, but they occasionally produce different names for the same compound. For simple molecules this rarely causes confusion, but for complex organic compounds with multiple functional groups and stereocenters, the names can diverge enough to trip up even experienced chemists.
The CAS number sidesteps this entirely. It doesn’t try to describe the molecule. It just labels it. This is why databases, regulations, and safety documents lean on CAS numbers as the definitive cross-reference, even when they also include one or more chemical names. When you need to be absolutely certain two people are talking about the same substance, the CAS number is the simplest way to verify it.