Synonyms for caustic

adj burning, corrosive

Antonyms for caustic

Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.

acerbic

adjective. bitter, sharp, or sour

alkaline

adjective. being basic, not acid (chemically)
Roget's 21st Century Thesaurus, Third Edition Copyright © 2013 by the Philip Lief Group.