Water is composed of two atoms of hydrogen and one atom of oxygen. Because of this composition, water has unique properties that has made it essential to life on the planet.
The molecules in water form hydrogen bonds. Oxygen atoms in water have a slightly negative charge whereas hydrogen atoms are slightly positive. When the negative oxygen meets up with the positive hydrogen, they form a bond.
Water's boiling point is high because more heat is needed to break the hydrogen bonds between water molecules. Water is most dense as a liquid, unlike other elements and compounds. Ice can float on top of water. The layer of ice that floats on top of lakes in the winter actually keeps the water underneath from freezing also, enabling organisms living underneath to survive.