When Vitamin C Turns to Rust: Chemistry, Consequences, and How to Keep Your Serum Fresh

Few skincare ingredients hold as exalted a place as vitamin C. The résumé of ascorbic acid is almost unbelievable: it boosts collagen synthesis, inhibits the enzyme responsible for excess melanin production, neutralizes pollution-induced free radicals, and even enhances the efficacy of sunscreen when applied underneath. Yet the same chemistry that makes vitamin C a biochemical powerhouse also makes it frustratingly unstable. Leave a dropper bottle uncapped under bathroom lights, and a once crystal-clear serum can transform i