The phrase as fast as possible points up a major difficulty in dealing with spontaneous processes. Some of them occur quite rapidly, but others are so slow as to be imperceptible. A rapid spontaneous process occurs when 2 mol H2O is mixed with 2 mol “heavy water,” D2O, made from the isotope deuterium, $${}_{\text{1}}^{\text{2}}\text{H}$$, or D. The two species start to transfer protons and deuterons (D+ ions) as soon as they are stirred together, and we rapidly obtain a mixture consisting of 2 mol H—O—D and 1 mol each of H—O—H and D—O—D. Assuming that deuterium atoms behave the same chemically as ordinary hydrogen atoms, this is what the laws of probability would predict. There are four equally likely possibilities for a randomly selected water molecule: