Thermal and Chemical Properties of Water

A water molecule consists of two hydrogen atoms and one oxygen atom bonded chemically at an angle of 105 degrees, as illustrated below.


The electronic distribution in a water molecule is such that the total charge is zero, but there is a small excess negative charge on the oxygen and a small positive excess on the hydrogens. This property is called polarization, and allows water to participate in hydrogen bonding, where there is a weak attraction of a hydrogen on one molecule and an oxygen on another (see above figure). Water exhibits stronger hydrogen bonding than any other substance. This causes its freezing and boiling points to be higher than they would be otherwise, and is responsible for its large heat capacity.

Heat Capacity and Latent Heat

Water has a number of remarkable properties that are closely tied to the fact that it can exist in three distinct phases (solid ice, liquid water, and gaseous vapor) and to its thermal properties (that is, how water behaves when heat is added or subtracted). For substances having solid, liquid, and gas phases, there are three crucial thermal properties.

Thermal Properties
The most general thermal property is the heat capacity, which is the amount of heat (that is, energy) that must be added to a substance to raise its temperature by one degree celsius (this is often expressed in terms of the specific heat, which is the amount of heat required to raise the temperature of 1 gram of the substance by 1 degree). A second important thermal property is the latent heat of fusion, which is the amount of heat that must be added to convert one gram of solid material to a liquid at constant temperature. A third important thermal property is the latent heat of vaporization, which is the amount of heat that must be added to one gram of liquid to convert it to a gas at constant temperature.

Water Is Special
Water is special because (1) its heat capacity is larger than for any other substance except liquid ammonia, (2) its latent heat of vaporization is higher than that of any other substance, and (3) its latent heat of fusion is higher than for any other substance except ammonia. These properties ensure that water is liquid under a broad range of temperatures on Earth, and ensure that the oceans play central roles in the climate of the Earth.

Heat Reservoirs
The most direct influence of the oceans on the climate is that the oceans serve as an enormous reservoir of heat (because of water's large heat capacity) that moderates temperature fluctuations in the atmosphere. This is why, for example, the daily and seasonal temperature variations near oceans tend to be much smaller than the corresponding variations for locations not near large bodies of water. Here is a link to a map giving the current sea surface temperatures for the Earth.