Watch out when a situation becomes volatile — it is likely to change for the worse suddenly. If you and your best friend have a volatile relationship, you frequently fight and make up.
Volatile from Latin volatilis, "fleeting, transitory," always gives the sense of sudden, radical change. Think of it as the opposite of stable. A person who is volatile loses his or her temper suddenly and violently. A volatile political situation could erupt into civil war. When the stock market is volatile, it fluctuates greatly. And in scientific language, a volatile oil evaporates quickly.