In its bulk liquid form, whether in a bathtub or an ocean, water is a relatively benign substance with little chemical activity. But down at the scale of tiny droplets, water can turn surprisingly reactive, Stanford researchers have discovered.Photo shows water microdroplet condensate formed on the surface of a glass container containing cold water (left) and an image of water microdroplets formed on a polished silicon surface (right). (Image credit: Jae Kyoo Lee and Hyun Soo Han)In microdroplets of water, just millionths of a meter wide, a portion of the H2O molecules present can convert into a close chemical cousin, hydrogen peroxide, H2O2, a harsh chemical commonly used as a disinfectant and hair bleaching agent.Stanford scientists first reported this unexpected behavior in forcibly sprayed microdroplets of water last year. Now in a new study, the research team has shown the same Jekyll-and-Hyde transformation happens when microdroplets simply condense from the air onto cold surfaces. The new results suggest that waters hydrogen peroxide transformation is a general phenomenon, occurring in fogs, mists, raindrops and wherever else microdroplets form naturally.The surprising discovery could lead to greener methods for disinfecting surfaces or promoting chemical reactions. Weve shown that the process of forming hydrogen peroxide in water droplets is a widespread and surprising phenomenon thats been happening right under our noses, said study senior author Richard Zare, the Marguerite Blake Wilbur Professor in Natural Science and a professor of chemistry in the Stanford School of Humanities and Sciences.The researchers also speculate that this newly recognized chemical ability of water could have played a key role in jumpstarting the chemistry for life on Earth billions of years ago, as well as produced our planets first atmospheric oxygen before life emerged. This spontaneous production of hydrogen peroxide may be a missing part of the story of how the building blocks of life were formed on early, Zare said.The co-lead authors of the new study, published in Proceedings of the National Academy of Sciences, are Stanford staff scientists Jae Kyoo Lee and Hyun Soo Han.Along with Zare and other Stanford colleagues, Lee and Han made the initial discovery of hydrogen peroxide production in water microdroplets last year. Some outside researchers who went over the studys results were skeptical, Zare said, that such a potentially common phenomenon could have gone undiscovered for so long. Debate also ensued over just how the hydrogen peroxide would ever actually form.The argument was that people have been studying water aerosols for years, and of course water is ubiquitous and has been intensively studied since the dawn of modern science, so if this hydrogen peroxide formation in microdroplets were real, surely someone would have seen it already, said Zare. That led us to want to explore the phenomenon further, to see in what other circumstances it might occur, as well as learn more about the fundamental chemistry going on.Zare and colleagues decided to investigate condensation, a scenario where microdroplets readily form naturally, without the aid of an external force such as a nebulizer instrument. Condensation occurs when water vapor (gas) in the air transitions into a liquid upon contacting a cooler surface; for instance, when the bathroom mirror fogs over after a shower.The Stanford team condensed water into multiple chilled materials, including silicon, glass, plastic and metal. The researchers then wiped a test strip that changes color in the presence of hydrogen peroxide over the condensed water. Sure enough, the strip turned blue. The low, yet detectable amounts of hydrogen peroxide (on the order of parts per million) that formed varied based on factors such as the temperature of the surface and the relative humidity in the test chamber. The researchers also noted that the hydrogen peroxide formed in microdroplets became diluted as the size of the water droplets grew, which might explain why this chemical transformation had been overlooked for so long.