The Properties of Water
The main constituent of the oceans is, of course, water. The presence of large amounts of liquid water on Earth’s surface over much of its history has resulted from a fortunate combination of factors. Among them are water’s unusually high freezing and boiling points for a molecule of its size, and its relative chemical stability. Water also has other remarkable properties that contribute to the characteristics of oceans—from their ability to support life to effects on climate. Underlying these properties is water’s molecular structure.
The Water Molecule
A molecule of water (H2O) consists of two hydrogen (H) atoms bound to one atom of oxygen (O). Crucial to formation of the bonds between the oxygen and hydrogen atoms are four tiny negatively charged particles called electrons, which are shared between the atoms. In addition, six other electrons move around within different regions of the oxygen atom. This electron arrangement makes the H2O molecule chemically stable but gives it an unusual shape. It also produces a small imbalance in the distribution of electrical charge within the molecule. An important result of this is that neighboring water molecules are drawn to each other by forces called hydrogen bonds.
Water's Surface Tension
One special property of liquid water that can be directly attributed to the attractive forces between its molecules is its high surface tension. In any aggregation of water molecules, the surface molecules tend to be drawn together and inward toward the center of the aggregation, forming a surface “skin” that is resistant to disruption. Surface tension can be thought of as the force that has to be exerted or countered to break through this skin. Water’s high surface tension has various important effects. Perhaps the most crucial is that it is vital to certain processes within living organisms—for example, water transport in plants and blood transport in animals. Surface tension also allows small insects such as sea skaters to walk and feed on the ocean surface, and it even plays a part in the formation of ocean waves.
Water's Heat Capacity
A second property of liquid water that can be attributed to hydrogen bonding is its unusually high heat capacity, which exceeds that of nearly all other known liquids. When heat is added to water, most of the heat is used to break hydrogen bonds linking the molecules. Only a fraction of the energy increases the vibrations of the water molecules, which are detected as a rise in temperature. This means that areas of ocean can absorb and release huge amounts of heat energy with little change in temperature. It also means that movements of water—ocean currents—transfer enormous amounts of heat energy around the planet. This role of ocean currents is vital to Earth’s climate.
Three States of Water
The temperatures at which water changes between its three states—melting point (ice to liquid water) and boiling point (liquid water to water vapor)—are both high compared with substances having similarly sized molecules. For ice to melt and water to vaporize, high levels of energy are needed to break all the hydrogen bonds. Water is also unusual in that its solid form is slightly less dense than its liquid form, so ice floats in liquid water. The reason for this is that the molecules in ice are loosely packed, whereas those in liquid water move around in snugly packed groups. The fact that ice floats on liquid water is important because it allows the existence of large areas of polar sea ice. These affect heat flow between ocean and atmosphere and help stabilize ocean temperatures and Earth’s climate.