The polar nature of water also causes water molecules to be attracted to one another. As is shown in Figure 2-10, the positively charged region of one water molecule is attracted to the negatively charged region of another water molecule. This attraction is called a hydrogen bond. A hydrogen bond is the force of attraction between a hydrogen molecule with a partial positive charge and another atom or molecule with a partial or full negative charge. Hydrogen bonds in water exert an attractive force strong enough so that water "clings" to itself and some other substances.
Hydrogen bonds form, break, and reform with great frequency. However, at any one time, a great number of water molecules are bonded together. The number of hydrogen bonds that exist depends on the state that water is in. If water is in its solid state all its water molecules are hydrogen bonded and do not break. As water liquifies, more hydrogen bonds are broken than are formed, until an equal number of bonds are formed and broken. Hydrogen bonding accounts for the unique properties of water, some of which we will examine further. These properties include cohesion and adhesion, the ability of water to absorb a relatively large amount of energy as heat, the ability of water to cool surfaces through evaporation, the density of ice, and the ability of water to dissolve many substances.
Cohesion and Adhesion
Water molecules stick to each other as a result of hydrogen bonding. An attractive force that holds molecules of a single substance together is known as cohesion. Cohesion due to hydrogen bonding between water molecules contributes to the upward movement of water from plant roots to their leaves.
Related to cohesion is the surface tension of water. The cohesive forces in water resulting from hydrogen bonds cause the molecules at the surface of water to be pulled downward into the liquid. As a result, water acts as if it has a thin "skin" on its surface. You can observe water's surface tension by slightly overfilling a drinking glass with water. The water will appear to bulge above the rim of the glass. Surface tension also enables small creatures such as spiders and water-striders to run on water without breaking the surface.
Adhesion is the attractive force between two particles of different substances, such as water molecules and glass molecules. A related property is capillarity (KAP-uh-LER-i-tee), which is the attraction between molecules that results in the rise of the surface of a liquid when in contact with a solid. Together, the forces of adhesion, cohesion, and capillarity help water rise through narrow tubes against the force of gravity. Figure 2-11 shows cohesion and adhesion in the water-conducting tubes in the stem of a flower.
Water has a high heat capacity, which means that water can absorb or release relatively large amounts of energy in the form of heat with only a slight change in temperature. This property of water is related to hydrogen bonding. Energy must be absorbed to break hydrogen bonds, and energy is released as heat when hydrogen bonds form. The energy that water initially absorbs breaks hydrogen bonds between molecules. Only after these hydrogen bonds are broken does the energy begin to increase the motion of the water molecules, which raises the temperature of the water. When the temperature of water drops, hydrogen bonds reform, which releases a large amount of energy in the form of heat.
Therefore, during a hot summer day, water can absorb a large quantity of energy from the sun and can cool the air without a large increase in the water's temperature. At night, the gradually cooling water warms the air. In this way, the Earth's oceans stabilize global temperatures enough to allow life to exist. Water's high heat capacity also allows organisms to keep cells at an even temperature despite temperature changes in the environment.
As a liquid evaporates, the surface of the liquid that remains behind cools down. A relatively large amount of energy is absorbed by water during evaporation, which significantly cools the surface of the remaining liquid. Evaporative cooling prevents organisms that live on land from overheating. For example, the evaporation of sweat from a person's skin releases body heat and prevents overheating on a hot day or during strenuous activity.
Unlike most solids, which are denser than their liquids, solid water is less dense than liquid water. This property is due to the shape of the water molecule and hydrogen bonding. The angle between the hydrogen atoms is quite wide. So, when water forms solid ice, the angles in the molecules cause ice crystals to have large amounts of open space, as shown in Figure 2-12. This open space lattice structure causes ice to have a low density.
Because ice floats on water, bodies of water such as ponds and lakes freeze from the top down and not the bottom up. Ice insulates the water below from the cold air, which allows fish and other aquatic creatures to survive under the icy surface.
Ice (solid water) is less dense than liquid water because of the structure of ice crystals. The water molecules in ice are bonded to each other in a way that creates large amounts of open space between the molecules, relative to liquid water.
Word Roots and Origins solvent from the Latin solvere, meaning "to loosen"
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