reacción
/reaɡˈθjon/ · B1 · noun · f
chemical reaction
🧪 re + acción = action back: what comes back when something acts on you
From Latin re- (back) and actio (action). Spanish nouns ending in -ción are feminine and match English -tion words: reacción = reaction, acción = action. The stress falls on the last syllable: re-ac-CIÓN.