What Is a CAS Number? The ID Card Every Chemical Carries

A CAS number is a unique identifier given to a single chemical substance, written as three groups of digits separated by two hyphens, such as 7732-18-5. It is the same everywhere in the world, whatever language the label is in, and it is designed so that two people using it can be certain they are talking about the same substance. The digits themselves carry no information about the molecule.

You will meet one on safety data sheets, supplier datasheets, regulatory filings and database entries, usually next to a molecular formula and a molecular weight. This article explains where the number comes from, how it is built, why salts and mirror-image forms get their own, and what it can and cannot tell you. It is an explanation of an identifier, not a guide to any substance.

One substance, many names, one number

Chemistry has a naming problem. The same substance can be called by its systematic name, a shorter common name, an older trade name, a name in another language, and several abbreviations. Water alone is also dihydrogen oxide, oxidane and aqua. A database search on one of those names can miss records filed under another.

CAS describes its answer plainly: a CAS Registry Number is a unique and unambiguous identifier for a specific substance, which lets people communicate clearly and links together the available data and research about that substance 1. The organisation notes that the numbers have been the standard for identifying chemical substances since 1965, and that governments rely on them in regulatory applications because they are unique, easy to validate and recognised internationally 1.

Picture a passport number. Whatever name someone goes by, and however it is spelled in different alphabets, the number on the document points to exactly one person. A CAS number does the same job for a substance.

Who assigns it and how the number is built

CAS, short for Chemical Abstracts Service, is a division of the American Chemical Society. It maintains the CAS Registry, a database of disclosed chemical substances drawn from the scientific literature and other sources. At the time of writing, the CAS page describes it as holding over 290 million substances, and the figure keeps growing 1. A number is assigned when a substance is first recorded in the registry. The same number is then used for it from that point on.

The format is simple. A number can have up to ten digits, split by hyphens into three parts, and the right-most digit is a check digit used to verify the validity of the whole number 1. The first part has two to seven digits, the second part always has two, and the third is the single check digit.

The check digit is worth a moment because it shows how modest the system is. Take the digits before it, read them from right to left, multiply the first by 1, the second by 2, the third by 3, and so on, then add the results. The last digit of that total is the check digit. For water, 7732-18-5, the digits before the check digit read from the right are 8, 1, 2, 3, 7, 7. That gives 8 + 2 + 6 + 12 + 35 + 42, which is 105. The last digit is 5, matching the number. The arithmetic is there to catch a mistyped digit, nothing more.

Why the digits do not describe the molecule

Here is the surprising part. The number is not a code that you can decode. Numbers are handed out in the order that substances are registered, so a low number mostly means an early registration, and neighbouring numbers are usually unrelated chemicals. Nothing in 7732-18-5 tells you that it is two hydrogens and an oxygen.

That is deliberate, and it is a strength. A number that encoded structure would have to change whenever the way of describing structure changed. A meaningless number never needs to. Its only job is to be a stable key that connects every record about one substance, however it is named.

The flip side is that you cannot judge anything from the digits. A longer number does not mean a bigger molecule. A number with a familiar-looking first group does not mean a familiar family. If you want to know what a substance is, you look the number up, which means comparing it against a record, not reading it.

Salts, isomers and hydrates get their own numbers

Because the number identifies a specific substance, any change that makes a different substance gets a different number. Three kinds of change matter most.

• A salt. When a molecule that carries a charge is paired with an oppositely charged partner, the pair is a different substance from the molecule on its own, and it is registered separately.
• An isomer. Isomers are molecules with the same atoms joined in different arrangements, including mirror-image pairs. They behave differently, so they are recorded separately.
• A hydrate. This is a solid that holds a fixed amount of water within its structure. Dry and hydrated forms are different substances with different numbers.

This matters for peptides, which are often supplied as a salt. A peptide in its plain form and the same peptide as an acetate salt are, to the registry, two substances, and so two numbers. The companion article on salt forms explains why a peptide often needs a partner ion. For present purposes, the point is that a label quoting one number may be describing the plain peptide while the powder in the container is a salt, which has a different one.

Where you meet CAS numbers: labels, safety data sheets, databases

A safety data sheet is a standard document that describes a substance's hazards and handling, and it lists the CAS number near the top. That is its purpose there: to make sure a person reading the sheet and a person reading a regulation are looking at the same substance. Supplier datasheets and customs paperwork use it for the same reason.

Public databases use it as a cross-reference. PubChem, run by the US National Library of Medicine, lists a substance's alternative names and identifiers, and the CAS number usually sits among them. That is how one database can be matched to another 3.

Mixtures are the awkward case. A blend of two substances is usually not a single substance, so it normally has no single number of its own; each component has one. A product described by one CAS number is therefore, as a rule, claiming to be one substance.

What a CAS number cannot tell you about what is in the vial

This is the section that matters most. A CAS number is a statement about a name. It says that when this document says peptide X, it means the substance recorded under this number. It does not say that the container in front of you holds that substance.

A number can be printed on any label, by anyone, whatever is inside. The check digit confirms only that the digits are internally consistent, not that they belong to the right substance, and not that the substance is what the label claims. Checking identity, amount and purity takes testing of the material itself, which is the job of the documents and measurements discussed elsewhere in this series.

How to look one up

Looking a number up means finding the record it points to and comparing that record with the name, the formula and the molecular weight on the document you started from. A mismatch between them is a reason to pause and ask which is right.

CAS runs a free resource called CAS Common Chemistry, described as an open community resource with nearly 500,000 substances taken from the full registry, focused on common and frequently regulated chemicals 12. If a number or name is not found there, that is not proof of anything: the free collection is a small slice of the whole, and a newer or less common research compound may simply not be included. CAS also offers a paid lookup service for substances outside the free set 1.

General databases such as PubChem list the number among a compound's synonyms, so searching by number often works there too 3. The most useful habit is to compare. A document that states a name, a number, a formula and a weight should agree with itself, and with the record the number leads to.

Nothing here is guidance about using any substance. Material sold for laboratory research use only has not been assessed by a medicines regulator, and an identifier is not a statement about quality or suitability.