The Polarity of Water

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The polarity of water
Polarity means that the molecule has both a positively and negatively charged end. More important, the polarity of water is responsible for effectively dissolving other polar molecules, such as sugars and ionic compounds such as salt. Ionic compounds dissolve in water to form ions. This is important to remember because for most biological reactions to occur, the reactants must be dissolved in water. Because water is able to dissolve so many common substances, it is known as the universal solvent. Substances that cannot be dissolved by water (such as oils) are called fat soluble and are non-polar, nonionic compounds that are strongly covalently bonded. Insoluble substances make excellent containers of water, such as cell membranes and cell walls.

The polarity of water molecules results in hydrogen bonding. A hydrogen bond forms when the oxygen of one water molecule is electrically attracted to the hydrogen of a close molecule. Hydrogen bonding between water molecules is the basis for water’s unusual properties. Organisms depend on the cohesion of water molecules. Hydrogen bonding makes water molecules stick to each other, and this cohesion helps pull water upward in the microscopic vessels of plants. Hydrogen bonding is also responsible for water’s surface tension. Hydrogen bonding gives water a high specific heat. Heat is absorbed when hydrogen bonds break and is released when hydrogen bonds form, helping minimize temperature changes to within limits that permit life. Evaporative cooling is based on water’s high heat of vaporization. Water molecules must have a relatively high kinetic energy to break hydrogen bonds. The evaporative loss of these energetic water molecules cools a surface.

Oceans and lakes don’t freeze solid because ice floats. Ice is less dense than liquid water because its more organized hydrogen bonding causes expansion into a crystal formation. Floating ice allows life to exist under the frozen surfaces of lakes...
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