Conditions for Life: Water We all know water. It has three different states: as a liquid we may drink or swim in, as ice on which we may skate in the winter, and as a vapour that takes the form of clouds in the air. Our own bodies contain a lot of water - about two thirds of our weight - and we could not live without it. The water molecule consists of an oxygen molecule (O) that is bound to two hydrogen atoms (H) and the chemical formula is accordingly: H2O. Many chemical substances - but far from all - dissolve in water. What are then the outstanding properties of this polar solvent which are so precious for life? The properties of water The first one is the range of temperatures in which water is maintained in a liquid state. Water evaporates at 100 °C and freezes at 0 °C at normal pressure conditions. This range can be further increased by addition of salts into the solution or at high pressure as at the bottom of the ocean. There has been speculation about the possibility of other solvents being used to sustain life, such as ammonia or methane, but in both cases the range in which the solvent is liquid is smaller and the temperatures are far below 0 °C, which would seriously affect the kinetics of the biological reactions. Another important property is the different density of the liquid and solid state, which allows ice to float on liquid water. Both properties, which are a consequence of the atomic structure of the water molecule, are fundamental for the existence of liquid water over an important range of temperatures. Polarity is also an important property of water, because it allows it to interact with non-polar molecules, like fatty acids, generating membranes, a basic cellular structure required for compartmentalisation. The utilization of non-polar solvents, like methane or ethane, would require the use of polar substances for the formation of membranes, with physico-chemical properties quite different from the unified version of our Earth reference systems. In relation to metabolism, water is interesting because it dissociates and produces protons (i.e. hydrogen nuclei, H+), which are very important for the control of acidity (pH) and for energy storage. Moreover, its redox potential (a chemical property) can be used to generate reducing power for the assimilation of carbon dioxide (CO2) during the oxygenic photosynthesis. But life also requires other elements to subsist.