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3

Physical Properties of Seawater

3.1. MOLECULAR PROPERTIES OF WATER

Many of the unique characteristics of the ocean can be ascribed to the nature of water itself. Consisting of two positively charged hydrogen ions and a single negatively charged oxygen ion, water is arranged as a polar mole- cule having positive and negative sides. This molecular polarity leads to water’s high dielec- tric constant (ability to withstand or balance an electric field). Water is able to dissolve many substances because the polar water molecules align to shield each ion, resisting the recombina- tion of the ions. The ocean’s salty character is due to the abundance of dissolved ions.

The polar nature of the water molecule causes it to form polymer-like chains of up to eight molecules. Approximately 90% of the water molecules are found in these chains. Energy is required to produce these chains, which is related to water’s heat capacity. Water has the highest heat capacity of all liquids except ammonia. This high heat capacity is the primary reason the ocean is so important in the world climate system. Unlike the land and atmosphere, the ocean stores large amounts of heat energy it receives from the sun. This heat is carried by ocean currents, exporting or importing heat to various regions. Approxi- mately 90% of the anthropogenic heating associated with global climate change is stored

Descriptive Physical Oceanography

in the oceans, because water is such an effective heat reservoir (see Section S15.6 located on the textbook Web site http://booksite.academic press.com/DPO/; “S” denotes supplemental material).

As seawater is heated, molecular activity increases and thermal expansion occurs, reducing the density. In freshwater, as tempera- ture increases from the freezing point up to about 4 C, the added heat energy forms molecular chains whose alignment causes the water to shrink, increasing the density. As temperature increases above this point, the chains break down and thermal expansion takes over; this explains why fresh water has a density maximum at about 4 C rather than at its freezing point. In seawater, these molecular effects are combined with the influence of salt, which inhibits the formation of the chains. For the normal range of salinity in the ocean, the maximum density occurs at the freezing point, which is depressed to well below 0 C (Figure 3.1).

Water has a very high heat of evaporation (or heat of vaporization) and a very high heat of fusion. The heat of vaporization is the amount of energy required to change water from a liquid to a gas; the heat of fusion is the amount of energy required to change water from a solid to a liquid. These quantities are relevant for our climate as water changes state from a liquid in the ocean to water vapor in the atmosphere and to ice at polar latitudes. The heat energy

! 2011. Lynne Talley, George Pickard, William Emery and James Swift. 29 Published by Elsevier Ltd. All rights reserved.

CHAPTER

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