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7.1: Properties of Seawater

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    10250
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    Properties of Seawater

    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.

    Distribution of Water on Earth

    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

    Water Cycle
    Figure 7.1. The water cycle.

    Components of Seawater

    Seawater is composed of:
    Water
    Dissolved matter: solids and gas (as ions)
    Suspended matter (dust and organic residues)

    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 %

    Seawater components
    Figure 7.2. Components of seawater

    Properties of Water

    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.

    Water molecule showing polar properties Polar attraction between water molecules Water adhesion and cohesion illustrated by water droplets of pine needles Capllary action of water in tubes of different sizes
    Figure 7.3. Water is a polar molecule. Figure 7.4. Water's polarity allows weak attraction between water molecules. Figure 7.5. Water droplets illustrate water's high cohesive and adhesive properties. Figure 7.6. Capillary action works because water adheres to material. The smaller the tube the higher the water will rise.

    Why don't oil and water mix?

    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.

    Oil on water oil spill containment
    Figure 7.7. Oil is a non-polar substance. Water is polar. Figure 7.8. Ship cleaning up after an oil spill.

    Properties of Water

    pH (acidity and alkalinity)

    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.

    Bicarbonate buffering of seawater

    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.

    Salt dissolves in water pH of household substances compared with water and seawater
    Figure 7.9. Salts can dissolve in water and then precipitate out again as water evaporates and concentrates salts in solution. Figure 7.10. pH of common household substances compared with pure water (pH=7) and normal seawater (pH=8.2).