A CAS Registry Number identifies one substance. It carries no information about the substance itself — not its formula, not its class, not its hazards. That is precisely why it is useful: it survives every change of name, spelling, language and trade designation.
What the number looks like
A CAS number has three parts separated by hyphens: two to seven digits, then two digits, then a single check digit. Benzene is 71-43-2. Methanol is 67-56-1. Water is 7732-18-5.
The parts have no meaning. They are not a date, not a class, not a position in any table. Numbers are assigned in the order substances enter the registry, so a low number means only that the substance was registered early.
The check digit, and why it matters
The last digit is not arbitrary. It is computed from the others, which means a mistyped or invented CAS number can usually be detected without consulting any database.
The rule: take the digits to the left of the final hyphen, read them from right to left, multiply the first by 1, the second by 2, the third by 3 and so on, add the products, and take the remainder on division by 10. That remainder is the check digit.
For benzene, 71-43-2 — the digits are 7, 1, 4, 3. Reading from the right: 3×1 + 4×2 + 1×3 + 7×4 = 3 + 8 + 3 + 28 = 42. The remainder of 42 divided by 10 is 2, which is the check digit. The number is internally consistent.
This is worth knowing because a plausible-looking number that fails the check cannot be a real CAS number. It is one of the few identity errors in chemical documentation that can be caught mechanically, and it is caught before anything is published on this site.
What a CAS number does not tell you
It does not tell you the form. Many substances exist as several hydrates, salts or crystal forms, and these frequently carry different CAS numbers with different regulatory status. Working from the wrong one is a common and consequential mistake.
It does not tell you the concentration. A CAS number identifies a substance, not a preparation. Sulphuric acid has one number whether it is dilute or concentrated, while its classification changes with concentration.
It does not tell you the regulatory status. That comes from the inventories and from harmonised classification, not from the identifier.
And it is not the same thing as a CLP Annex VI index number, although the two are often confused. An index number such as 601-020-00-8 has four groups and identifies an entry in the harmonised classification list. A CAS number has three groups and identifies a substance. They coexist on the same document and mean different things.
Other identifiers you will meet beside it
- EC number — the European Community inventory number, in the form 200-753-7. Assigned by EINECS, ELINCS or the NLP list.
- InChI and InChIKey — structure-derived identifiers. Unlike a CAS number, these are computed from the structure itself, so two people with the same structure arrive at the same InChIKey without consulting a registry.
- SMILES — a compact line notation for a structure, readable by software and by a trained chemist.
- PubChem CID, ChemSpider ID, ChEMBL ID, UNII, KEGG, DrugBank, MeSH, HMDB — database-specific keys, each useful for reaching a particular body of data.
No single identifier is sufficient on its own. A well-characterised substance record resolves several of them and shows which ones were found, so a reader can judge how firmly the identity is pinned down rather than assuming it.
Searching by CAS number
Because a CAS number is unambiguous where a chemical name is not, it is the fastest way into a documentation catalogue. On MolGod every substance page is reachable by its number: search the catalogue by CAS number or by chemical name, and the substance page shows the identity resolved across sixteen systems before any purchase is involved.
Note: CAS Registry Number is a registered trademark of the American Chemical Society. MolGod.org is not affiliated with, endorsed by or acting on behalf of CAS or the American Chemical Society.