Seawater is the most abundant resource on Earth! Seawater has evolved to what it is over the billions of years that oceans have existed on Earth. This chapter examines the physical and chemical properties of water and seawater. 97.2% in the world ocean
2.15% frozen in glaciers and ice caps
0.62% in groundwater and soil moisture
0.02% in streams and lakes
0.001% as water vapor in the atmosphere
Seawater is composed of: Ions in seawater • Cl - 55% • Na - 30.4% • SO4 - 7.6% • Mg - 3.9% • Ca - 1.2% • K - 1.1%• all other dissolved components - <1 %  Figure 7.2. Components of seawater
Water is a polar substance. Each molecule of water has a negative charge associated with its oxygen, and a positive charge with it hydrogen (Figure 7.3). This polar character is responsible for its properties cohesion and adhesion. • Cohesion: water has high surface tension because water molecules stick together. • Adhesion: water "sticks" to things. • High capillary action: The cohesion and adhesion properties of water allow water to move upward against gravity in small confined spaces. The smaller the tube, the higher the water will rise. Capillary action helps plants to move water upward from their roots to their leaves. • Water is a powerful solvent: A solvent is a substance that dissolves a solute (a chemically different liquid, solid or gas), resulting in a solution. The polar character of the water molecule allows it to form weak bonds with other polar molecules. Substance held together with ionic bonds will readily dissolve in water. However, the solubility of chemical compounds in water is highly variable. The solubility of a chemical compound in water is defined as the maximum amount of the chemical that will dissolve in pure water at a specified temperature. Seawater is a solution.
Organic compound containing only carbon and hydrogen (hydrocarbon) are nonpolar and will dissolve in nonpolar solvents (like oil). However, many organic compounds have “functional groups with very electronegative elements” (i.e. oxygen), making the whole molecule polar, allowing them to dissolve in water (ex: sugar and starch can dissolve in water). Soap compounds (called surfactants) have molecules that are both; they have portions that behave as polar and non-polar ends. One end will stick to hydrocarbons and other non-polar substances whereas the other will stick to water and other polar molecules. This allows polar organic compounds to disperse in water.
pH is a measure of the acidity or alkalinity of a solution expressed on a logarithmic scale on which 7 is neutral, lower values are more acid, and higher values more alkaline. pH is an important measurement in seawater. Neutral water is a pH of 7.
A natural buffering system with seawater’s interaction with carbon dioxide. Seawater is generally always within a range of pH of 7.5 to 8.5. The interactions of dissolved components keep ocean water in a stable range. Organisms living in or near seawater have a limited tolerance for variations in pH and other factors. For instance, calcite (as in shell material) is stable within this range, but will dissolve if exposed to acidic conditions. Carbonate buffering keeps pH stable by precipitation (increase pH) or dissolution (decrease pH) of calcium carbonate - CaCO3.
The dynamic interactions of water molecules. Individual H2O molecules are V-shaped, consisting of two hydrogen atoms (depicted in white) attached to the sides of a single oxygen atom (depicted in red). Neighboring H2O molecules interact transiently by way of hydrogen bonds (depicted as blue and white ovals). Strong linkages—called covalent bonds—hold together the hydrogen (white) and oxygen (red) atoms of individual H2O molecules. Covalent bonds occur when two atoms—in this case oxygen and hydrogen—share electrons with each other. Because oxygen and hydrogen attract the shared electrons unequally, each end of the V-shaped H2O molecule adopts a slightly different charge. The area around the oxygen is somewhat negative compared to the opposite, hydrogen-containing end of the molecule, which is slightly positive. Opposites attract, so this lopsided charge difference allows bonds to form between the hydrogen and oxygen atoms of adjacent H2O molecules. Each H2O can bind to a maximum of four neighbors through these so-called hydrogen bonds. Although short-lived and much weaker than the covalent variety, hydrogen bonds contribute significantly to water chemistry because they are extremely abundant in H2O. Credit: Nicolle Rager Fuller, National Science Foundation |
The _____ of water allows individual water molecules to stick together.
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