Water - a polar molecule - tends to be slightly positive on the hydrogen side and slight negative on the oxygen side. (See the illustration in the tutorial.) The electrostatic bond between the positive hydrogen side of this molecule and other megative ions or polar molecules is called a hydrogen bond.

Molecules and ions with which water forms hydrogen bonds (such as sodium chloride) are hydrophylic. On the other hand, Ions and molecules that do not form hydrogen bonds with water are hydrophobic.

At room temperature, most compounds with low molecular weights take gaseous form. With water, however, hydrogen bonding helps to keep it a liquid at room temperature.

Kept relatively close together, the moluecules at room temperature are unable to dissipate sufficiently to form a gas. Temperatures of 212°F (or 100°C) are required to break the hydrogen bonds and convert liquid water into water vapor.

When ionic compounds such as sodium chloride are added to water, hydrogen bonding will tend to pull those ionic compounds apart. This makes water a natural solvent.

Once ionic compounds dissolve, their anions and cations circulate through the water allowing further reactions to occur. Thus, water also sponsors and facilitates chemical reactions.

Water takes more heat to raise its temperature than other common compounds, since much of that heat is required to first break the hydrogen bonds.

Water also retains heat, so its temperature falls slowly.

This means that larger systems of water (such as the ocean or a body) tend to maintain more or less constant temperatures, which in turn helps the earth (and us) to maintain relatively constant temperatures.

At 32°F (or 0°C) and below, water molecules form hydrogen bonds in a chrystalline lattice structure. This bonding spaces the molecules a bit farther apart than usual, causing water to expand when it freezes. This results in ice being less dense than liquid water, which is why ice floats.

Floating ice is critical to life on Earth. Bodies of water freeze from the top down, providing an insulating layer of ice that keeps the water below from freezing. For the organisms living below the surface, this property helps them to survive the cold weather.