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Water has strong intermolecular forces

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Normally, partial charges such as those found in a water molecule result in an intermolecular force known as a dipole-dipole force, in which the positive end of one molecule attracts the negative end of another molecule. The very high electronegativity of oxygen combined with the fact that a hydrogen atom has only one electron results in a charge difference significantly greater than you’d normally expect. This charge difference leads to stronger-than-expected intermolecular forces (stronger than the dipole-dipole forces), and these forces have a special name: hydrogen bonds.

The term hydrogen bond doesn’t refer to an actual bond to a hydrogen atom but to the overall interaction of a hydrogen atom bonded to either oxygen, nitrogen, or fluorine atoms with an oxygen, nitrogen, or fluorine on another molecule (intermolecular) or the same molecule (intramolecular). Hence the term intermolecular force. (Note that although hydrogen bonds occur when hydrogen bonds to fluorine, you don’t normally find such combinations in biological systems.)

Hydrogen bonds in oxygen- and nitrogen-containing molecules are very important in biochemistry because they influence reactions between such molecules and the structures of these biological molecules. The interaction between water and other molecules in which there may be an opportunity for hydrogen bonding explains such properties as solubility in water and reactions that occur with water as a solvent (more on that in a minute).

One environmentally important consequence of hydrogen bonding is that, upon freezing, water molecules are held in a solid form that’s less dense than the liquid form. The hydrogen bonds lock the water molecules into a crystalline lattice that contains large holes, which decreases the density of the ice. The less-dense ice — whether in the form of an ice cube or an iceberg — floats on liquid water. In nearly all other cases where a solid interacts with water, the reverse is true: The solid sinks in the liquid. So why is the buoyancy of ice important? Ask ice fishermen! The layer of ice that forms on the surface of cold bodies of water insulates the liquid from the cold air, protecting the organisms that live under the ice.

Biochemistry For Dummies

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