Here's why:

* Caustic soda is the common name for sodium hydroxide (NaOH).

* Inorganic compounds generally do not contain carbon-hydrogen bonds (C-H bonds) or are derived from living organisms.

* Organic compounds are based on a carbon skeleton and usually contain C-H bonds.

Sodium hydroxide (NaOH) does not contain carbon, so it is classified as an inorganic compound.