The energy for life may come ultimately from the sun, but it is stored in carbon-hydrogen-oxygen compounds, and released by the combination of these molecules with oxygen.

Oxygen is almost unique in its chemical reactivity, and in the energy that it gives off when it combines with other atoms. This arises from its high affinity for electrons, or its electronegativity. Only fluorine is more reactive, but it is a thousand times rarer than oxygen on the crust of the Earth, and thus was bypassed as a reactant for the energy-releasing machinery.

Reactions in solution are so much more rapid than solid-state processes that we can hardly imagine life evolving on a planet without large amounts of a liquid that is a good solvent for other chemical substances.

One of the most common oxygen compounds on Earth is water. Because its molecules are polar, water is an excellent solvent for salts and molecules with polar bonds. It dissolves carbon dioxide with the release of acidic protons, and ammonia with the creation of basic hydroxide ions. These protons and hydroxide ions in turn catalyze, or speed up, reactions that otherwise would proceed extremely slowly.

Many other compounds are acids or bases in aqueous solution. Liquid ammonia has solvent properties similar to those of water, and has been suggested as a possible medium for extraterrestrial life on a cold planet. Ammonia has one disadvantage that affects its usefulness as a solvent medium in which life might evolve: Water ice floats on water, but ammonia ice sinks in liquid ammonia.