The chemistry teacher used phenolphthalein for acid-base experiments.
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Both characters share the 酉 (yǒu, 'wine vessel') radical that runs through alcohol/chemistry words. The pinyin 'fēntài' is a sound loan from English 'phen-tha(lein)' — 酚 = phen (phenol), 酞 = tha (phthalein).
