H2O, also known scientifically as water, is a fundamental chemical compound composed of two hydrogen atoms covalently bonded to a single oxygen atom. It is essential for all known forms of life, serving as a universal solvent and a critical component in biochemical processes. Beyond its biological significance, H2O plays a vital role in Earth's climate and weather systems, participating in processes like evaporation, precipitation, and the water cycle. It exists naturally in three states—solid (ice), liquid (water), and gas (steam)—and covers approximately 71% of Earth's surface, primarily in oceans, lakes, and rivers.