SDS Studio™Definition: A CAS number is the unique numeric identifier that the Chemical Abstracts Service assigns to a chemical substance, written in the form 64-17-5 (ethanol).
The CAS number appears in SDS section 3 next to the EC and index numbers. It is the most common key for looking up classifications, limits and regulatory lists.
Some substances have several CAS numbers or none (for example complex reaction products), so a CAS alone is not always enough to identify the right entry.
The last digit of a CAS number is a check digit calculated from the others, which helps catch typing errors.