The chemical compound formed when oxygen and hydrogen combine is water, scientifically denoted as H2O. In this compound, two hydrogen atoms bond with one oxygen atom through covalent bonds. Water is essential for all known forms of life, serving as a solvent, medium for biochemical reactions, and regulator of temperature in organisms. It exists in three states—solid (ice), liquid (water), and gas (water vapor)—depending on temperature and pressure. Water's unique properties, like high specific heat and cohesiveness, are critical for supporting life, influencing weather patterns, and shaping Earth’s environments.