The Water Molecule

The Water Molecule Not only is water the most common of substances on the earth’s surface, but it also uncommon in many of its properties. Water makes life possible. Its properties largely determine the characterisrics of the oceans, the atmosphere, and the land. Seawater is salt water and it is much more than just salt in water. Seawater contains dissolved gases, nutrient molecules, and organic substances as well as salt. We need to understand the ocean through the physical and chemical characteristics of seawater and to follow the processes that influence and regulate it. What are the properties that make water such a special, useful, and essential substance ???? its the molecular structure.

The water molecule is deceptively simple, made up of only 3 atoms: 2 hydrogen atoms and 1 oxygen atom (H2O). Water is a tiny V-shaped molecule with the molecular formula H2O. In the liquid state, in spite of 80% of the electrons being concerned with bonding, the three atoms do not stay together as the hydrogen atoms are constantly exchanging between water molecules due to protonation/deprotonation processes. Both acids and bases catalyze this exchange and even when at its slowest (at pH 7), the average residence time is only about a millisecond. As this brief period is, however, much longer than the timescales encountered during investigations into water's hydrogen bonding or hydration properties, water is usually treated as a permanent structure. Water molecules (H2O) are symmetric with two mirror planes of symmetry. Because a water molecule has a slight positive charge on one end and a slight negative charge on the other...the attraction of the opposite charges, (electro-static charges) creates what is called surface tension, the weak attraction is called a hydrogen bond.

The approximate shape and charge distribution of water

Note that the average electron density around the oxygen atom is about 10x that around the hydrogen atoms.

Van der Waals radii

The mean van der Waals diameter of water has been reported as identical with that of isoelectronic neon (2.82 Å). Molecular model values and intermediate peak radial distribution data indicates however that it is somewhat greater (~3.2Å). The molecule is clearly not spherical, however, with about a ±5% variation in van der Waals diameter dependent on the axis chosen; approximately tetrahedrally placed slight indentations being apparent opposite the (putative) electron pairs.

WATER MOLECULES AT THE SURFACE IN A GLASS OF WATER

CLOUD DROPS WITHIN A CLOUD

A water molecule inside the drop is attracted equally in all directions by neighboring molecules. A molecule at the surface is pulled inward, but not outward because there are no water molecules to pull the surface molecule outward. So....a molecule from the interior of the drop must do work against the "cohesive" forces (forces of attraction) between water molecules. To minimize the amount of energy expended in creating a surface a mass of water will assume the shape that givesminimum surface area. That shape is a sphere. We say it has minimum surface area to volume ratio. For a small amount of water, that is a very small drop, the electro-static forces (hydrogen bonds) are stronger than external forces, like air turbulence and gravity. As the amount of water increases and the drop grows it will be deformed by the external forces which eventually will overwhelm the electrostatic forces.

Water - The Universal Solvent??? Water has been called the “universal solvent”. It is not. Many substances will not dissolve in water, many oils, for example. However water is very effective at dissolving molecules with “exposed” electrical charges, like NaCl. Water seems to act mysteriously in dissolving materials. Look at the following graphs:

Solubility of NaCl and Sucrose In Water

grams/100ml of water

600 500 400

Sucrose

300 200 100

NaCl

0 0

20

40

60

80

100

Temperature C

Question: Why is there difference in solubility?

120

Solubility of NaCl and Sucrose In Water

grams/100ml of water

600 500 400

Sucrose

300 200 100

NaCl

0 0

20

40

60

80

100

120

Temperature C

The solubility of sucrose (a sugar) and NaCl both increase with increasing temperatures. The increase of solubility of sucrose is great, for NaCl the increase is slight. Hypothesis: The solubility of solids increases with increasing temperature, but all solids are not the same. Sucrose which does NOT dissolve into ions (charged atoms) shows a great increase, NaCl which does dissolve into charged ions (Na+, Cl-).

Solubility of Oxygen in Water Equilibrium, 1 atm (maximum at given temp) Concentration mg/L

16 14 12 10 8 6 4 2 0 0

10

20

30

40

Temp C

The solubility of both CO2 and O2 decrease with increasing temperature. Hypothesis: The solubility of gasses decreases with increasing temperature.

SUBSTANCE

0oc

20oc

40oc

60oc

80oc

100oC

CHANGE 0o to 100o

Solids – concentration in grams / 100ml NaCl

Sucrose

35.7

36.0

36.6

37.5

38.5

39.6

+10.6%

179.2

203.9

238.1

294

378

498

+178%

Liquids – concentration in mg / L (ppm) NH3 (ammonia)

75

55

37

22

12

6

-92.0%

O2

14.6

9.65

6.11

4.67

3.23

2.44

-83.3%

CO2

1.34

0.690

0.421

0.294

0.226

0.184

-86.3%

Salt in water….. Composition of seawater Seawater is a solution of salts of nearly constant composition (Rules of Constant Proportion), dissolved in variable amounts of water. There are >70 elements dissolved in seawater but only 6 make up >99% of all the dissolved salts; all occur as ions - electrically charged atoms or groups of atoms: Chloride (Cl): 55.04 wt% Sodium (Na): 30.61 wt% Sulphate (SO4): 7.68 wt% Magnesium (Mg): 3.69 wt% Calcium (Ca): 1.16 wt.% Potassium (K): 1.10 wt.% Oceanographers use salinity -- the amount (in grams) of total dissolved salts present in 1 kilogram of water -- to express the salt content of seawater. Normal seawater has a salinity of 35 grams/kilogram (or litre) of water -- also expressed as 35‰. Seawater from Wormly in southern England is used as the international standard for seawater composition. As well as major elements, there are many trace elements in seawater - e.g., manganese (Mn), lead (Pb), gold (Au), iron (Fe), iodine (I). Most occur in parts per million (ppm) or parts per billion (ppb) concentrations. They are important to some biochemical reactions - both from positive and negative (toxicity) viewpoints.

Processes controlling seawater composition Salts dissolved in seawater come from three main sources: • volcanic eruptions gases in the atmosphere ----- through precipitation. underwater eruptions ------ direct • chemical reactions between seawater and hot, newly formed volcanic rocks of spreading zones (mid-oceanic ridges)-----removal of magnesium and some sulphate from the seawater. ocean’s water circulates every 5-10 million years ------- near constant ratios of major constituents. • chemical weathering of rocks on the continents release soluble constituents like silica and elements like sodium, calcium, potassium and magnesium ------ remain in ocean, does not enter hydrological cycle.

Dissolved gases in seawater Small amounts of dissolved gases (nitrogen, oxygen, carbon dioxide, hydrogen, and trace gases) in seawater. Water at a given temperature and salinity is saturated with gas when the amount of gas entering the water equals the amount leaving during the same time. Surface seawater --- normally saturated with atmospheric gases (O2, N2). Solubility of gases determined by water’s temperature and salinity. Temperature

or

Salinity

Dissolved gases

Once water sinks below the ocean surface, dissolved gases can no longer exchange with the atmosphere except by movement of gas molecules through the water -- diffusion (slow process), or by mass water mixing. Nitrogen and rare inert gases (argon, helium, etc.) behave this way - their concentrations are conservative and only affected by physical processes. Some dissolved gases (O2, CO2). are non-conservative and actively participate in chemical and biological processes that change their concentrations.

Salinity

Salinity map showing areas of high salinity (36 o/oo) in green, medium salinity in blue (35 o/oo), and low salinity (34 o/oo) in purple. Salinity is rather stable but areas in the North Atlantic, South Atlantic, South Pacific, Indian Ocean, Arabian Sea, Red Sea, and Mediterranean Sea tend to be a little high (green). Areas near Antarctica, the Arctic Ocean, Southeast Asia, and the West Coast of North and Central America tend to be a little low (purple).

The salinity of seawater is usually 35 parts per thousand (also written as o/oo) in most marine areas. This salinity measurement is a total of all the salts that are dissolved in the water. Although 35 parts per thousand is not very concentrated (the same as 3.5 parts per hundred, o/o, or percent) the water in the oceans tastes very salty. The interesting thing about this dissolved salt is that it is always made up of the same types of salts and they are always in the same proportion to each other (even if the salinity is different than average). The majority of the salt is the same as table salt (sodium chloride) but there are other salts as well. The table below shows these proportions:

Sodium chloride (NaCl)

Chemical Ion Contributing to Seawater Salinity

Concentration in o/oo (parts per thousand) in average seawater

Proportion of Total Salinity (no matter what the salinity)

Chloride

19.345

55.03

Sodium

10.752

30.59

Sulfate

2.701

7.68

Magnesium

1.295

3.68

Calcium

0.416

1.18

Potassium

0.390

1.11

Bicarbonate

0.145

0.41

Bromide

0.066

0.19

Borate

0.027

0.08

Strontium

0.013

0.04

Fluoride

0.001

0.003

Other

less than 0.001

less than 0.001

Variations occur in ocean salinity due to several factors. the relative amount of evaporation or precipitation in an area. Evaporation

Precipitation (rain)

Salinity

very large river emptying into the ocean. The runoff from most small streams and rivers is quickly mixed with ocean water by the currents and has little effect on salinity. But large rivers (like the Amazon River in South America) may make the ocean have little or no salt content for over a mile or more out to sea.

freezing and thawing of ice also affects salinity. thawing of large icebergs (made of frozen fresh water and lacking any salt) salinity …… freezing of seawater salinity temporarily.

60° N 20° N

S min = 33 ppt S max = 36 ppt

0° (Equator) 20° S

S min = 33 ppt

60° N

Equator zone: S (avg) = 35 ppt dilution of seawater (precipitation/rainfall)

evaporation

Latitudes bordering equator line dilution of seawater (precipitation/rainfall)

Evaporation >>>>> rainfall

evaporation

Icemelting >>>>>> evaporation

Salinity Surface zone Halocline

Deep zone

Many marine organisms are highly affected by changes in salinity. This is because of a process called osmosis which is the ability of water to move in and out of living cells, in response to a concentration of a dissolved material, until an equilibrium is reached. In general the dissolved material does not easily cross the cell membrane so the water flows by osmosis to form an equilibrium. Marine organisms respond to this as either being osmotic conformers (also called poikilosmotic) or osmotic regulators (or homeosmotic).

Marine algae (left) and marine feather duster worms (right) are osmotic conformers.

Osmotic conformers have no mechanism to control osmosis and their cells are the same salt content as the liquid environment in which they are found (in the ocean this would be 35 o/oo salt). If a marine osmotic conformer were put in fresh water (no salt), osmosis would cause water to enter its cells (to form an equilibrium), eventually causing the cells to pop (lysis). If a marine osmotic conformer were put in super salty water (greater than 35 o/oo salt) then osmosis would cause the water inside the cells to move out, eventually causing the cells to dehydrate (plasmolyze). These marine osmotic conformers include the marine plants and invertebrate animals which do not do well in areas without a normal salinity of 35 o/oo.

Arctic charr fish (left) and humpback whales (right) are osmotic regulators.

Osmotic regulators have a variety of mechanisms to control osmosis and the salt content of their cells varies. It does not matter what the salt content is of the water surrounding a marine osmotic regulator, their mechanisms will prevent any drastic changes to the living cells. Marine osmotic regulators include most of the fish, reptiles, birds and mammals. These are the organisms that are most likely to migrate long distances where they may encounter changes in salinity. An excellent example of this is the salmon fish. The fish is about 18 o/oo salt so in seawater it tends to dehydrate and constantly drinks the seawater. Special cells on the gills (called chloride cells) excrete the salt so the fish can replace its lost water. When a salmon migrates to fresh water its cells start to take on water so the salmon stops drinking and its kidneys start working to produce large amounts of urine to expell the water.

Temperature

Temperature

Seawater temperature map showing areas of warmer water in red and areas of cooler water is blue. White areas represent ice. Notice the upward finger of cold water in the South Pacific off of South America and the downward finger of cold water in the North Pacific off of the West Coast of the USA. The reasons for these become apparent when you learn about the major ocean currents

Earth's surface ocean currents are caused by the winds, continental land mass obstruction and the Coriolis effect.

Earth's surface winds are the result of the wind cells and Coriolis effect.

Temperature The temperature of seawater varies with the amount of sun that hits that area the length of time the angle of the sun's rays. The longer the time and the more direct the rays of the sun fall on the ocean, the greater the temperature of seawater. Tropical areas get more year-round sun and more direct sun (almost 90 degrees, straight down for most of the year at noon) have warmer surface waters than polar areas. surface ocean temperatures are warm in the tropics (up to 30 or more degrees C) and cooler at the poles (down to -2 degrees C). But, when we look below the surface we find that the oceans are also vertically stratified and marine scientists recognize a basic three layered ocean - the upper mixed layer, the main thermocline, and deep (bottom) water.

The three layered ocean with the upper mixed layer (yellow), main thermocline (green), and deep (bottom) water (blue).

The upper mixed layer is all one temperature but that temperature can vary from -2 degrees C, at the poles, to +30 degrees C, in the tropics. It all depends on the latitude and effects of the sun's heat and may be highly seasonal. The depth of this layer can be anything between the surface and 200 meters deep - usually the 200 meter depth is near the equator. The volume of this upper mixed layer is only about two percent of the volume of the ocean water. The main thermocline is an area of rapidly decreasing temperature with depth. This changes with latitude and may begin at 200 meters (the bottom of the mixed layer) in the tropics where it may end at close to 1,000 meters (or anywhere above that depending on the strength of the sun). It may also begin right at the surface of the ocean in high temperate areas and extend to a variety of depths. This layer shifts up and down with the seasons in the temperate areas. The main thermocline comprises only 18 percent of the volume of the ocean water.

The sunlit and wind-driven upper layer of Earth's oceans rests on relatively colder and denser waters. At times, there is a distinct temperature difference between the wind-stirred Surface Zone and the quieter Deep Zone below. Such a drop in temperature with depth is shown in red at left (<<<). Although temperature generally decreases with depth, there is a layer where temperatures drop abruptly called the Thermocline.

When heated, seawater volume expands and density decreases. So, Sun-warmed surface waters generally float on top of colder, denser waters below. This leads to layering of water -- or stratification -by temperature. However, stratification can be "undone" by other forces including wind and tides.

Temperature versus Depth

Deep (or bottom) water is always one cold temperature ranging between -2 to +5 degrees C. It is below the main thermocline (at the bottom of the thermocline there is no longer a decrease in water temperature with depth ... it is all one cold temperature). It is not affected by the seasons. This layer has most of the seawater and comprises close to 80 percent of all ocean water by volume. It is under the tropical areas, most temperate areas when there is a main thermocline, and is all the way to the surface in the polar areas (where there is no thermocline). Seawater temperature affects marine organisms by changing the reaction rates within their cell(s). Although each species has a specific range of temperature at which it can live, the warmer the water gets the faster the reactions and the cooler the water the slower the reactions. An organism's response to water temperature is considered to be cold blooded (or poikilothermic) or warm blooded (homeothermic) depending on their ability to control their internal body temperature. If any species is moved out of its temperature tolerance range it may die in a short time although temperatures on the cool side of the range are easier for organisms to tolerate than temperatures on the warm side because cell reactions just slow down in the cold but may speed up over six times the normal levels for each 10 degrees C of heat.

Marine algae (left) and marine green turtles (right), a reptile, are cold blooded.

Cold blooded (poikilothermic) marine organisms lack any temperature controls. These include marine plants, invertebrates, most fish and marine reptiles. These species each have their specific temperature tolerance range within which they must live (some are adapted to polar temperatures, some to tropical temperatures). Some have a narrow range (and are thus very restricted) and some have a wide range (and are thus less restricted).

The walrus (left) and dolphins (right) are warm blooded.

Warm blooded (homeothermic) marine organisms have some type of internal temperature controls to maintain a constant body temperature. These organisms include a few fish, all sea birds and mammals. This ability allows these warm blooded marine species to migrate over vast distances through various temperatures without problems and include some of the animals on Earth with the longest migrations.

Density Temperature, salinity and pressure affect the density of seawater. Large water masses of different densities are important in the layering of the ocean water (more dense water sinks). As temperature increases water becomes less dense. As salinity increases water becomes more dense. As pressure increases water becomes more dense. A cold, highly saline, deep mass of water is very dense whereas a warm, less saline, surface water mass is less dense. When large water masses with different densities meet the denser water mass slips under the less dense mass. These responses to density are the reason for some of the deep ocean circulation models.

Dissolved Gases The concentration of dissolved oxygen and carbon dioxide are very important for marine life forms. Although both oxygen and carbon dioxide are a gas when outside the water, they dissolve to a certain extent in liquid seawater. Dissolved oxygen is what animals with gills use for respiration (their gills extract the dissolved oxygen from the water flowing over the gill filaments). Dissolved carbon dioxide is what marine plants use for photosynthesis. The amount of dissolved gases varies according to the types of life forms in the water. Most living species need oxygen to keep their cells alive (both plants and animals) and are constantly using it up. Replenishment of dissolved oxygen comes from the photosynthetic activity of plants (during daylight hours only) and from surface diffusion (to a lesser extent). If there are a large number of plants in a marine water mass then the oxygen levels can be quite high during the day. If there are few plants but a large number of animals in a marine water mass then the oxygen levels can be quite low. Oxygen is measured in parts per million (also called ppm) and levels can range from zero to over 20 ppm in temperate waters. It only reaches 20 when there are a lot of plants in the water, it is very sunny with lots of nutrients, and the wind is whipping up the surface into a froth. In any water mass there is a maximum amount of dissolved gas that can be found (after which the gas no longer dissolves but bubbles to the surface). This maximum amount increases with a decrease in temperature (thus cold water masses can hold more dissolved gases ... but they can also have none if it has been used up). So, just because a water mass is cold it does not mean it has a lot of dissolved gases. This concept is a little tricky but just remember that the amount of dissolved gases in seawater depends more on the types of life forms (plants and animals) that are present and their relative proportions.

Dissolved Nutrients Fertilizers, like nitrogen (N), phosphorous (P), and potassium (K), are important for plant growth and are called 'nutrients.' The level of dissolved nutrients increases from animal feces and decomposition (bacteria, fungi). Surface water often may be lacking in nutrients because feces and dead matter tend to settle to the bottom of the ocean. Most decomposition is thus at the bottom of the ocean. In the oceans most surface water is separated from bottom water by a thermocline (seasonal in temperature and marginal polar regions, constant in tropics) which means that once surface nutrients get used up (by the plants there) they become a limiting factor for the growth of new plants. Plants must be at the surface for the light. Nutrients are returned to surface waters by a special type of current called 'upwelling' and it is in these areas of upwelling that we find the highest productivity of marine life. Silica and iron may also be considered important marine nutrients as their lack can limit the amount of productivity in an area. Silica is needed by diatoms (one of the main phytoplanktonic organisms that forms the base of many marine food chains. Iron is just recently being discovered to be a limiting factor for phytoplankton.

pH pH is a measure of the acidity or alkalinity of a substance and is one of the stable measurements in seawater. Ocean water has an excellent buffering system with the interaction of carbon dioxide and water so that it is generally always at a pH of 7.5 to 8.5. Neutral water is a pH of 7 while acidic substances are less than 7 (down to 1, which is highly acidic) and alkaline substances are more than 7 (up to 14, which is highly alkaline). Anything either highly acid or alkaline would kill marine life but the oceans are very stable with regard to pH. If seawater was out of normal range (7.5-8.5) then something would be horribly wrong.

In summary, the salinity and pH of seawater are relatively stable measurements whereas temperature, dissolved oxygen, and nutrients may vary. The next lesson will introduce you to how all the water masses of the oceans move around and mix, or do not mix in general patterns. It is always important to remember that every local situation can be quite different than the 'average' so it is always important to take actual measurements.