In an anhydrous manner; without water or in a water-free state.
From anhydrous + -ly (adverb suffix). This creates an adverbial form allowing chemistry to describe how processes occur in completely water-free conditions.
When chemists say a reaction happens 'anhydrously,' they're emphasizing that even a single water molecule can wreck the whole reaction—they're working in conditions drier than the Sahara.
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