反応
はんのう · hannou · JLPT N1 · noun
chemical reaction
🏓 反 = back/against + 応 = respond: a response that bounces back
反 means back or against, and 応 means respond. A reaction is a response that comes back at the stimulus, like a return shot in ping-pong.
反 — hand 又 under cliff 厂
Originally a pictograph of a hand (又) turning something over against a cliff or slope (厂), depicting the action of flipping or reversing. From this came the core meaning 'to turn over,' which extended to 'opposite,' 'contrary,' and 'rebel against.' The character is purely semantic, with no phonetic component.