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Examining the Physical Properties of Water

The medium in which biological systems operate is water, and physical properties of water influence the biological systems. Therefore, it is important to understand some properties of water that you learned in general chemistry.

Water is a polar molecule

Because it's polar, water has a tendency to "wet" substances. It's also a bent molecule, not linear. The hydrogen atoms have a partially positive charge; the oxygen atom has a partially negative charge. This charge distribution is due to the electronegativity difference between hydrogen and oxygen atoms (the attraction that an atom has for a bonding pair of electrons).

Normally, such partial charges 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 single electron of a hydrogen atom results in a charge difference significantly greater than you'd normally expect. This leads to stronger-than-expected intermolecular forces, called 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 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.

Water has strong intermolecular forces

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.

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.

Water has a high specific heat

Specific heat is the amount of heat required to change the temperature of a gram of water 1° Celsius. A high specific heat means it isn't easy to change the temperature of water. Water also has a high heat of vaporization. Humans can rid their bodies of a great deal of heat when their sweat evaporates from their skin, making sweat a very effective cooling method.

As a result of water's high specific heat and heat of vaporization, lakes and oceans can absorb and release a large amount of heat without a dramatic change in temperature. This give and take helps moderate the earth's temperature and makes it easier for an organism to control its body temperature. Warm-blooded animals can maintain a constant temperature, and cold-blooded animals can absorb enough heat during the day to last them through the night.

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