Water is composed of hydrogen and oxygen atoms and its molecules form tetrahedral structures with hydrogen bonds. It has unique physical properties including high heat capacity and heat of vaporization, allowing it to regulate temperatures. Water exists in three phases - liquid, solid ice, and gas vapor - and its density peaks at 4°C, allowing ice to float. Water's polarity allows it to dissolve many substances and hydrate ions, serving as a universal solvent in the human body and worldwide.