Water: Substance and Solvent
Water, present in the geological environment as solid, liquid, vapor and supercritical fluid, is one of the most important components of Earth's fluid envelope. Without water and its special properties, Earth would be a vastly different planet in many ways. Water plays critical roles in atmospheric processes, including the absorption of radiant energy and the redistribution of energy through weather mechanisms, thus fundamentally influencing climate. In the lithosphere, water serves as an essential solvent and transport medium for many of the elements, a modifier of rock physical properties, and a lubricant for tectonic processes. The biosphere also depends on water for its solvent properties and, in photosynthesis, as an electron-donating nutrient.