Dipping deeper into acid. (Chemistry).

Experiments have now revealed just how a molecule of acid dissolves in water, strengthening scientists' fundamental understanding of this common chemical process. The results give other researchers a new tool for studying complicated chemical reactions, such as those that occur during the depletion of Earth's protective ozone layer.

To examine molecules of acid and water as they interact, A. Welford Castleman Jr. of Pennsylvania State Universityin State College and his coworkers introduced molecules of a simple acid, hydrogen bromide, into a vacuum chamber and then injected water vapor. Using a type of spectroscopy that employs fast pulses of laser light, the researchers watched the chemical process unfold.

The way an acid breaks down into ions in water has never been revealed with such step-by-step precision. In particular, Castleman and his coworkers found that it takes five water molecules to dissolve one hydrogen bromide molecule. Castleman and his colleagues report their results in the Oct. 5 Science.

Hydrogen bromide is representative of other simple acids, and it also plays a role in atmospheric chemistry, says Castleman. Humanmade chemicals, including some pesticides, can release hydrogen bromide into the upper atmosphere, where it participates in complicated reactions that contribute to ozone destruction.--J. G.