Seawater Chemistry

The Water Molecule zWater is a compound and compounds are made up of two or more elements. zAn element is a substance of identical particles, called atoms. zAtoms can not be broken into simpler substances by chemical means.

The Water Molecule zAtoms have a nucleus which is made up of protons and neutrons. zProtons are positively charged particles and have a mass of one atomic weight unit. zNeutrons have no electrical charge, but they also have a mass of one atomic weight unit.

The Water Molecule zIn addition to the protons and neutrons in the nucleus atoms also have electrons orbiting around the nucleus. zThe electrons are negatively charged, but their mass is negligible, not even one atomic weight unit. In most discussions of atoms the mass of electrons is not even considered.

The Water Molecule zAtoms are electrically neutral, therefore there are as many electrons orbiting the nucleus as there are protons in the nucleus of the atom.

The Water Molecule zA water molecule is made from two atoms of hydrogen and one atom of oxygen. zA molecule is a group of atoms held together by chemical bonds. zChemical bonds are formed when electrons are shared or transferred between atoms.

The Water Molecule zA water molecule forms when electrons are shared between two hydrogen atoms and one oxygen atom. zThe bonds formed by shared pairs of electrons are known as covalent bonds. zIn water the angle formed by the two hydrogen atoms and the central oxygen atom is about 105-degrees.

The Water Molecule

zHydrogen bonds greatly influence the properties of water. zThe allow individual water molecules to stick together. We call this cohesion. zCohesion gives water a high surface tension, which gives water a “skin” capable of supporting a razor blade or a tiny walking insect.

The Water Molecule zAdhesion is the tendency of water to stick to other materials, this allows water to adhere to solids and make them wet. zCohesion and adhesion cause capillary action, the tendency for water to spread through a towel when one end is dipped in water.

The Water Molecule zWater is a powerful solvent. It will eventually dissolve almost any substance. zWater’s dissolving power results from the polar nature of the water molecule. zWhen water dissolves sodium chloride (NaCl) the result is that Na+ ions and Cl- ions are surrounded by water molecules.

The Water Molecule zAn ion is an atom that has an unbalanced electrical charge because it has either gained or loss one or more electrons. zSodium chloride is not covalently bounded. There is no sharing of electrons between sodium and chlorine.

The Water Molecule zSalts and other dissolved molecules can move through still water by a process called diffusion. zDiffusion is the movement of molecules from an area of higher concentration of those molecules to an area of lower concentration of those molecules.

Seawater zThe ocean’s salinity varies from about 3.3% to 3.7% by weight. zThe average salinity is usually given as 3.5% or 35 parts per thousand ( 0/00 ). zMost of the dissolved salts in the ocean have been separated into ions. The most abundant of these are sodium and chloride.

Seawater zAs the salinity increases, the freezing point of water becomes lower; the salts act as a sort of antifreeze. Therefore sea ice forms at a lower temperature than freshwater lakes. zDissolved salts tend to attract water molecules. Therefore seawater evaporates more slowly than freshwater.

Seawater z3.5% of seawater consists of dissolved substances. zIf we were to boil away 100 kilograms of seawater we would be left with a residue weighing 3.5 kilograms. zOceanographers prefer to use parts per thousand notation rather than percent (parts per hundred).

Seawater

zResidence Time zThe constancy of the ocean seems to suggest that ions are being added to the ocean at the same rate they are removed. zIons are removed by precipitating out and becoming part of the sediments on the ocean bottom

Seawater zOr by being extracted from seawater by animals and plants and used to form skeletal materials as well as being used in metabolic processes. zThe residence time is equal to the amount of an element in the ocean divided by the rate at which it is added (or removed).

Seawater

zDissolved Gases zMost gases in the air readily dissolve in seawater ant the ocean’s surface. zPlants and animals living in the ocean need these gases. zNo marine animal has the ability to break down water molecules to obtain oxygen directly.

Seawater zAcid-Base Balance zWater can separate to form hydrogen (H+) ions and hydroxyl (OH-) ions. zIn pure water, these two ions are present in equal amounts. zAn imbalance in the ions produces either an acidic or basic solution.

Seawater zAn acid is a substance that releases a hydrogen ion in solution. zA base is a substance that combines with a hydrogen ion in solution. zAcidity or alkalinity of a solution is measured in terms of the pH scale.

Seawater zThe pH scale measures the concentration of hydrogen ions in a solution. zAn excess of hydrogen (H+) ions makes the solution acid. zAn excess of hydroxide (OH-) ions makes the solution alkaline. zNeutral is in the middle of the scale at 7.

Seawater zSeawater is slightly alkaline with an average pH of about 8. zAt first this sounds odd because there are large amounts of carbon dioxide dissolved in the ocean. Dissolved carbon dioxide combines with water to from carbonic acid.

Seawater zSome carbonic acid brakes down to produce hydrogen ions, bicarbonate ions, and carbonate ions. zThe net effect of the bicarbonate and carbonate ions is to buffer seawater.

zIf you want to see how much you remember a quiz follows. zThe answers follow each question or set of questions, so write your answers on a blank sheet of paper.

Quiz z1. When an atom gains an additional electron it becomes: ya. An atomb. An ion yc. A moleculed. A quark

Answer z1. Ion

Quiz z2.The oxygen and hydrogen atoms in the water molecule are held together by: ya. Covalent bonds. Ionic bonds yc. Ester bondsd. Hydrogen bonds z3.When hydrogen bonds form between adjacent water molecules they form a “skin” which allows the water to support a razor blade. This is called: ya. adhesionb. capillary action yc. cohesiond. tension

Answers z3 a. Covalent bonds z4. C. cohesion

Quiz z5.Water is: ya. A good solvent yb. A poor solvent yc. A moderately good solvent yd. A very poor solvent z6.Pure water is: ya. A good conductor of electricity yb. A poor conductor of electricity

Answers z5. a. good solvent z6. b. A poor conductor

Quiz z7.The average salinity of the open ocean is: ya. 10 ppt.b. 25 ppt. yc. 35 ppt.d. 55 ppt. z8.True or false. Ice forms at a lower temperature in fresh water than in salt water.

Answers z7.C. 35 ppt. z8.False

Quiz z9.The most abundant ion in seawater is: xa. Magnesiumb. Chloride xc. Sodiumd. Sulfate z10.The average pH of seawater is: xa. 6.2b. 7 xc. 7.5d. 8.2

Answers z9. b. Chloride z10. D. 8.2

zThe End