From Nathaniel Hellerstein
You tracked the trajectory of a water molecule from a crashed comet to being printed on your magazine (14 November, p 30). As water is a molecule, not an element, when it is in liquid form, it can and does dissociate into OH- and H3O+ ions; then it swiftly reassociates, but with different atoms. What is the half-life of an individual water molecule, in pH 7 solution at room temperature?
San Francisco, California, US
