the.com/bath bomb

means A molded lump of baking soda, citric acid, and scent that fizzes apart when dropped in water, releasing color, oil, and bubbles into your bath.

from A modern invention, not an old word — the fizzing trick comes from mixing a carbonate (baking soda) with an acid (usually citric acid), the same reaction that makes volcano science-fair projects foam. Dry, they're inert; wet, the acid and base react and release carbon dioxide, which is the fizz you see. The cosmetics company Lush is widely credited with popularizing the bath bomb as a retail product in the 1980s-90s and trademarking the name, though similar fizzing bath fizzies existed before. 'Bomb' here is just vivid marketing for 'thing that bursts dramatically' — no explosive chemistry involved, just gas escaping fast.