V.+Hydrogen+bonding

Hydrogen Bonds
Polar molecules, such as water molecules, have a weak, partial negative charge at one region of the molecule (the oxygen atom in water) and a partial positive charge elsewhere (the hydrogen atoms in water). Thus when water molecules are close together, their positive and negative regions are attracted to the oppositely-charged regions of nearby molecules. The force of attraction, shown here as a dotted line, is called a **hydrogen bond**. Each water molecule is hydrogen bonded to four others. The hydrogen bonds that form between water molecules account for some of the essential — and unique — properties of water. Two outcomes of this: The hydrogen bond has only 5% or so of the strength of a [|covalent bond]. However, when many hydrogen bonds can form between two molecules (or parts of the same molecule), the resulting union can be sufficiently strong as to be quite stable.
 * The attraction created by hydrogen bonds keeps water liquid over a wider range of temperature than is found for any other molecule its size.
 * The energy required to break multiple hydrogen bonds causes water to have a high heat of vaporization; that is, a large amount of energy is needed to convert liquid water, where the molecules are attracted through their hydrogen bonds, to water vapor, where they are not.
 * The evaporation of sweat, used by many mammals to cool themselves, achieves this by the large amount of heat needed to break the hydrogen bonds between water molecules.
 * Moderating temperature shifts in the ecosystem (which is why the climate is more moderate near large bodies of water like the ocean)