Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state (water vapor or steam). Water also exists in a liquid crystal state near hydrophilic surfaces.Under nomenclature used to name chemical compounds, Dihydrogen monoxide is the scientific name for water, though it is almost never used.
Why is water so important for life?
Every known form of life on earth, from the largest mammals to the smallest microbes, relies on water. Why? Because water is an extraordinarily versatile molecule - it's the perfect liquid medium in which to dissolve nutrients for ingestion or wastes for excretion, to transport important chemicals or even be used as one. Water has two particular physical properties that are unique among natural molecules: it remains liquid over an extremely broad range of temperatures, and it decreases in density when converted to solid phase (frozen). While this may seem a relatively minor point, its consequences (that ice floats) are critical to the evolution of life. If ice were more dense than water and the earth cooled slightly, ice formed on the oceans would sink and push the already cold water from the bottom to the surface, where it too would freeze and sink, repeating the cycle until all water on the planet was frozen. Not all scientists believe that the presence of water is "concrete" evidence of life, but liquid water certainly improves the likelihood of life taking hold and finding a hospitable environment. This should not be confused with ice, however, which we know is present in many planets and moons in the solar system. Remember that ice may not be only frozen water, but perhaps vapor from other gases - in either case not as conducive to life.