Lecture Notes-Seawater Chemistry

The chemistry of seawater is quite complex, yet, these complexities are what enable life and the physical properties of the oceans to exist. The chapter in the book contains a thick and useful dose of information. For the lecture, I will boil it down (pun intended!) and provide you with the key points.

Uniqueness of the Water Molecule

A molecule is a combination of two or more elements. Each element (as of last week, we know of 114 of them) refers to a unique atom composed of a set amount of PROTONS, NEUTRONS, and ELECTRONS. Familiar elements are hydrogen, oxygen, lead, and uranium. Water and salt are not elements, but are molecules-combinations of several elements.

Each water molecule is a combination of two hydrogen atoms and one oxygen atom, bound together by strong covalent bonds. We can therefore write the formula for water as H2O. Because of the way these atoms combine, water is a very polar molecule. That is, it acts like a magnet in which one side attracts atoms (or molecules) that are low in electrons and the other side attracts atoms (or molecules) that are high in electrons. This allows one water molecule to readily attract another water molecule (cohesion). The chemical force that is created is called a "hydrogen bond." Hydrogen bonds are very strong and allow water molecules to attach to other substances (adhesion).

This attraction allows water to rip apart weaker substances, thus, we say water is a powerful "solvent." Oil is one substance that is not readily dissolved in water. When any substance has been so thoroughly dissolved in a body of water that no more can dissolve, we say the water is saturated with respect to that substance. If more of the substance is added, it will "de-dissolve" or come out of solution as a solid. This is called "precipitation."

Why the Oceans are Salty

First of all, salt is a general term for a variety of compounds which are made up of a positive ion (needs electrons) and a negative ion (would like to give up electrons), not just common table salt. Water dissolves a lot of salts and the amount of those substances in seawater is called its salinity. Most of the "salt" in the oceans is actually table salt, or sodium chloride. In this salt, sodium, or Na, is the positive ion and chlorine, or Cl, is the negative ion. We can write table salt as NaCl. Before we get into the variety of salts, some quick math. You already have dealt with percentages. A percent or % is ratio of the material per 100 units. For instance, if you had a box of 100 peanuts and you ate 14 of them, you ate 14% of the box. Now, if you had 1000 peanuts and you ate 14 of them, you ate 1.4%. Another way to give a percent is not as a ration of 100, but as a ration of 1000, or "parts per thousand." This is shown as the ‰ amount. So, 14 peanuts out of 1000 peanuts is 14‰. The amount of dissolved salts in the ocean is usually given as percentage of 1000. The average salinity of the oceans is 35‰ by weight. The most common salt is NaCl. Other common salts in the ocean are made up of the ions Sulfate, Magnesium, Calcium, Potassium, and Bicarbonate. In addition, there are lots of ions that occur in very small amounts (a few parts per million or billion parts of water). These ions are "trace elements" and are useful in looking at how the oceans change over time and distance.

Having dissolved salts in water affects its special abilities. Specifically, four changes occur:

1. The heat capacity of water decreases with more salts added.

2. The freezing point of water becomes lower with more salts.

3. Seawater evaporates more slowly than water without salts.

4. The osmotic pressure rises with increasing salinity.

These four properties are called water's "Colligative Properties."

The salts in the ocean come from deep within the crust or mantle and are also eroded from the continents. Right now, the oceans are in a steady state. That is, the percentage of the dissolved salts is pretty much constant-add some, and the ocean taketh away. This consistency is called the "principal of constant proportions" or "Forchhammer's principle." The beauty of Forchhammer's Principle is that once you know the relative amounts of the ions, you can take a sample of ocean water anywhere, measure the amount of one ion, and calculate the amounts of all of the others. Modern technology allows us to measure the salinity of the oceans by measuring the amount of dissolved chlorine, and then multiply by 1.80655 to get the salinity. The amount of chlorine is calculated by measuring the pH of the water (see below). Average chlorinity is 19.2‰.

Dissolved Gases

In addition to salts, the oceans contain a lot of dissolved gases. The gases come in from the atmosphere as the top layer of the ocean gets mixed, or from respiration from plants and animals in the ocean, or it comes in from the deepsea vents. Nitrogen makes up 48% of the gas in seawater, Oxygen as a gas is 36%, and Carbon Dioxide as a gas is 15%. Compare this to the ratios in the atmosphere of Nitrogen -78%, Oxygen-21%, and Carbon Dioxide as 0.04%. Note that animals and plants must use the gas form of oxygen or carbon dioxide for respiration. So, even though oxygen is one of the two elements in the water molecule, it is OFF LIMITS to animals no matter how hard they suck. Likewise, carbon dioxide, or CO2 is very common in the oceans in other forms, but only as CO2 gas can it be used by plants. This is a good lead-in to the final topic, acidity.

Acidity of the Oceans

As you may recall, the pH scale is a logarithmic measure of the acidity of a substance. The scale ranges from 0 (most acidic) to 14 (least acidic or most basic). A pH of 7 is considered neutral. Pure H2O has a pH of 7. If the pH of pure water is raised by one value to pH 8, then the acidity went up 10 times. If it was raised two values to 9, then the acidity went up 100 times. This is what is meant by a "logarithmic scale." Because of the dissolved salts in the ocean, typical seawater is slightly alkaline (or basic), with a pH of 8. However, this is an average and the pH changes constantly, albeit minutely. The major change is brought on by plant growth. Also, the pH changes with temperature and pressure, and by depth. Near the surface, the water is warmer and plants take away CO2 gas. This causes a chain of reactions which remove free hydrogen atoms from the seawater, thus lowering acidity or raising the pH. Deeper in the water column, more CO2 is present, so the pH is lower (more acidic).

Key Points to Know:

1. Water is made up of two hydrogen atoms and one oxygen atom, H2O.

2. Water is a polar molecule, bound by strong covalent bonds. Water molecules are attracted to each other by weak hydrogen bonds.

3. Water is a powerful solvent.

4. If no more of a substance can be dissolved, than the water is saturated.

5. A salt is a compound made up of a positive and a negative ion.

6. Salinity is a measure of the total dissolved salts, or amount of ions.

7. The average salinity of the oceans is 35‰ by weight.

8. Common ions in the ocean are Chlorine, Sodium, Sulfate, Magnesium, Calcium, Potassium, and Bicarbonate.

9. Ions that occur in small amounts are called "trace elements".

10. Water's "Colligative Properties" are:

A. The heat capacity of water decreases with more salts added.
B. The freezing point of water becomes lower with more salts.
C. Seawater evaporates more slowly than water without salts.
D. The osmotic pressure rises with increasing salinity.

11. The salts in the ocean come from deep within the crust or mantle and are also eroded from the continents.

12. Right now, the oceans are in a steady state

13. The consistency of the ratios of ions in seawater is called the "principal of constant proportions" or "Forchhammer's principle."

14. Dissolved gases in the ocean are primarily Nitrogen (48%), Oxygen (36%), and Carbon Dioxide (15%).

15. In the atmosphere, the ratios are: Nitrogen -78%, Oxygen-21%, and Carbon Dioxide-0.04%.

16. The gases come in from the atmosphere through mixing at the surface, or from respiration from plants and animals in the ocean, or it comes in from the deep sea vents.

17. The pH scale is a logarithmic measure of the acidity of a substance, with 0 being most acidic and 14 most basic.

18. Pure H2O has a pH of 7, or is neutral.

19. Seawater has an average pH of 8.

20. The pH changes with temperature and pressure, and by depth.

21. Near the surface, ocean water is slightly more alkaline, with depth, it becomes more acidic.