Choose to view chapter section with a click on the section heading. The Water Planet

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►The Water Planet

►Water’s Unique Properties

►The Inorganic Chemistry of Water

►The Organic Chemistry of Water

►Chemical Factors That Affect Marine Life

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The Water Planet

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The Water Planet

Water covers about 71% of the Earth’s surface. Considering the depth and volume, the world’s ocean provides more than 99% of the biosphere – the habitable space on Earth.

The vast majority of water onEarth can’t be used directly fordrinking, irrigation, or industrybecause it’s salt water.

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The Water Planet

As the population increases, so does the need for water.Part of the solution to meeting this demand lies in

understanding what water is, where it goes,and how it cycles through nature.

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The Water Planet

The Hydrologic Cycle.

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The Water Planet

The ocean is vast.Considering depth and volume, the world’s ocean

provides more than 99% of the biosphere - the habitable space on Earth.

The entire ocean is a biosphere - organisms live everywherein the water column.

How does this compare toland animals?

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The Water Planet

Our ocean on planet Earth averages 3,730 meters (12,238 feet) deep and it contains more than 1.18 trillion cubic kilometers (285 million cubic miles) of water.

The deepest known pointin any ocean is theChallenger Deep inthe Mariana Trench.

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Water’s Unique Properties

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The Polar Molecule

Molecules are the simplest part of a substance. Compared to many important

molecules, water is a simplemolecule - it’s chemicalsymbol = H2O.

Consists of three atoms - twohydrogen and one oxygen.

The way it’s held togethergives it unique properties.

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The Polar Molecule Covalent bonding.

Covalent bonds result in theformation of molecules.

The hydrogen atoms bondto the oxygen atoms with acovalent bond.

A covalent bond is formed byatoms sharing electrons. Thismakes water a very stablemolecule. In water, the oxygenatom shares the electrons of two single-electron hydrogen atoms.

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The Polar Molecule

A molecule with positive and negative charged ends has polarity and is called a polar molecule.Water is a polar molecule.The two hydrogen atoms

have a net excess positivecharge.

The oxygen atom has anet excess negative charge.

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The Polar Molecule

The water molecule’s polarity allows it to bondwith adjacent water molecules.

The positively chargedhydrogen end of one watermolecule attracts thenegatively charged oxygenend of another water molecule.

This bond between watermolecules is called ahydrogen bond.

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The Polar Molecule

Hydrogen bonds. Individual hydrogen bonds are

weak compared to covalentbonds - only 6% as strong.

Hydrogen bonds can easilybreak and reform.

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The Effects of Hydrogen Bonds

Being a polar molecule, water has these characteristics:Liquid Water. The most important characteristics

of the hydrogen bonds is the ability to make water a liquid at room temperature.Without them, water would be a gas - water

vapor or steam at room temperature.Hydrogen bonds hold the molecules together,

requiring more energy (heat) to form steam.

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Being a polar molecule, water has these characteristics:Cohesion/Adhesion. Because hydrogen bonds

attract water molecules to each other, they tend to stick together. This is cohesion. Water also sticks to other materials due to its polar nature. This is adhesion.

Example of adhesion - a raindrop clinging to the surface of a leaf.

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Being a polar molecule, water has these characteristics:Viscosity. This is the tendency for a fluid (gas or

liquid) to resist flow. Most fluids change viscosity as temperatures

change.The colder water gets, the more viscous it

becomes. It takes more energy for organisms to move through it, and drifting organisms use less energy to keep from sinking.

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Being a polar molecule, water has these characteristics:Surface Tension. A skin-like surface formed due

to the polar nature of water. Surface tension is water’s resistance to objects attempting to penetrate its surface.

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Being a polar molecule, water has these characteristics: Ice Floats: as water cools enough to turn from a

liquid into solid ice, the hydrogen bonds spread the molecules into a crystal structure that takes up more space than liquid water, so it floats.

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If ice sank, the ocean would be entirely frozen – or at least substantially cooler – because water would not be able to retain as much heat.

The Earth’s climate would be colder – perhaps too cold for life.

So, the question is… “Howimportant is Hydrogen Bondsto your very existence?”

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The InorganicChemistry of Water

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Solutions and Mixtures in Water

A solution occurs when the molecules of one substance are evenly dispersed among the molecules of another substance.

Water can act as a solvent. (often called the “universal solvent”)A solvent is the more abundant substance in a

solution; usually a liquid. A solute is the substance being dissolved and is less

abundant in the solution. Solutes are usually a solid or gas.

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A mixture is the combination of two or more substances that are not chemically bonded, and not in fixed proportions to each other.Two kinds of mixtures: Homogeneous and

Heterogeneous.

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Two kinds of mixtures:Homogeneous - has a uniform appearance

throughout. Examples: Milk, fog, sugar and water, smoke, and stirred up dust in the air. Colloid - a homogeneous solution with intermediate

particle size between a solution and suspension. Like milk.

Heterogeneous - is not uniform throughout and consists of visibly different substances. Examples: India ink, oil and water, sand and water. Suspension - a heterogeneous mixture of larger, visible

particles that will settle out on standing. Like sand and water.T

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Water can be part of both homogeneous and heterogeneous mixtures.Water’s polar nature makes it good solvent.Regarding salt, let’s examine why water is a good

solvent.

Salt crystals are held together by ionic bonding. Sodium loses an electron, making it positive. Chlorine gains and electron making it negative. The opposites attract forming salt.

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Water dissolves salt.Water’s charged ends pull apart - dissociate - the

salt (sodium chloride - NaCl) crystal.Sodium and chlorine become charged particles -

ions.

Salt - sodium chloride - NaCl in its crystal or solid state.

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Water dissolves salt.The positive sodium ion is attracted to the

negative oxygen side of the water molecule.The negative chlorine ion is attracted to the

positive hydrogenside of thewater molecule.

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Salts and Salinity

Salinity refers to sodium chloride and many other salts dissolved in seawater.

Salinity includes the total quantity of all dissolvedinorganic solids in seawater (ions).

Dissolved salts includes sodium chloride and everything else.

Scientist’s measure salinity in various ways – current method uses how well water conducts electricity.

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Salts and Salinity

Salinity is expressed in parts per thousand (‰). The ocean’s average salinity is 35, meaning 35‰ or

35g/kg. Variations as small as 0.01 are of importance to

oceanographers. The ocean’s salinity varies from near zero at river

mouths to more than 40‰ in confined, arid regions. The proportion of the different dissolved salts

never change, only the relative amount of water.

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The Colligative Properties of Seawater

Colligative properties are properties of a liquid that may be altered by the presence of a solute and are associated primarily with seawater.Pure water doesn’t have colligative properties.

Fresh water, with some solutes, can have colligative properties to some degree.

The strength of the colligative properties depends on the quantity of solute.

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The colligative properties of seawater include:Raised boiling point. Seawater boils at a higher

temperature than pure fresh water.Decreased freezing temperature. As salinity

increases, water resists freezing (why salt is put on a road during ice/snow storms).

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The colligative properties of seawater include:Ability to create osmotic pressure. Liquids flow

or diffuse from areas of high concentration to areas of low concentration until the concentration equalizes.

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Osmosis occurs when this happens through asemi-permeable membrane, such as a cell wall.Because it contains dissolved salts, water in seawaterexists in lower concentration than in fresh water.

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The colligative properties of seawater include:Electrically conductive. Ability to conduct an

electrical current. A solution that can do this is called an electrolyte.

Decreased heat capacity. Takes less heat to raise the temperature of seawater.

Slowed evaporation. Seawater evaporates more slowly than fresh due to the attraction between ions and water molecules.

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The Principle of Constant Proportions

In seawater no matter how much the salinity varies, the proportions of several key inorganic elements and compounds do not change. Only the amount of water and salinity changes. This constant relationship of proportions in

seawater is called the principle of constant proportions.

This principle does not apply to everything dissolved in seawater – only the dissolved salts.

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Dissolved Solids in Seawater

Next to hydrogen and oxygen, chloride and sodium are the most abundant chemicals in seawater.

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Determining Salinity, Temperature,and Depth

If you know how much you have of any one seawater chemical, you can figure out the salinity using the principle of constant proportions.

Chloride accounts for 55.04% of dissolved solids – determining a sample’s chlorinity is relatively easy.

The formula for determining salinity is based on the chloride compounds:

salinity ‰ = 1.80655 x chlorinity ‰ Sample of seawater is tested at 19.2‰ chlorinity:

salinity ‰ = 1.80655 x 19.2‰ salinity ‰ = 34.68‰

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Most commonly, salinity is determined with a salinometer.It determines the electrical conductivity of the water.

An important tool to measure the properties of seawater are the Argo floats - an array of 3,000 free-drifting floats.

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Salinometer Argo Float

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Another tool is the conductivity, temperature, and depth (CTD) sensor. The CTD profiles temperature and salinity with depth.

Another less accurate way to determine salinity is with a refractometer.

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A source of sea salts appears to be minerals and chemicals eroding and dissolving into fresh water flowing into the ocean.

Waves and surf contribute by eroding coastal rock. Hydrothermal vents change

seawater by adding some materials while removing others.

Scientists think these processes all counterbalance so the average salinity of seawater remains constant.

The ocean is said to be in chemical equilibrium.

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Why the Seas Are Salty

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Salinity, Temperature, and Water Density

Most of the ocean surface has average salinity, about 35‰. Waves, tides, and currents mix waters to make them more uniform.

Precipitation and evaporation have opposite effects on salinity. Rainfall decreases salinity by adding fresh water. Evaporation increases salinity by removing fresh water. Freshwater input from rivers lowers salinity. Abundant river input and low evaporation results in salinities

well below average.

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Salinity and temperature also vary with depth.Density differences causes water to separate into

layers.High-density water lies beneath low-density water.

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Water’s density is the result of its temperature and salinity characteristics: Low temperature and high salinity are features of high-

density water. Relatively warm, low-density surface waters are separated

from cool, high-density deep waters by the thermocline, the zone in which temperature changes rapidly with depth.

Salinity differences overlap temperature differences and the transition from low-salinity surface waters to high-salinity deep waters is known as the halocline.

The thermocline and halocline together make the pycnocline, the zone in which density increases with increasing depth.

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Global Salinity

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Acidity and Alkalinity

pH measures acidity or alkalinity. Seawater is affected by solutes. The relative

concentration of positively charged hydrogen ions and negatively charged hydroxyl ions determines the water’s acidity or alkalinity. It can be written like this:

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Acidic solutions have a lot of hydrogen ions (H+), it is considered an acid with a pH value of 0 to lessthan 7.

Solutions that have a lot of hydroxyl ions (OH-) are considered alkaline. They are also called basic solutions. The pH is higher than 7, with anything over 9 considered a concentrated alkaline solution.

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pH can be measured chemically or electronically. Pure water has a pH of 7 - neutral pH. Seawater pH ranges from 7.8 to 8.3 - mildly alkaline. Ocean’s pH remains relatively stable due to buffering.

A buffer is a substance that reduces the tendency of a solution to become too acidic or alkaline.

Seawater is buffered primarily by carbon dioxide.

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Seawater is fairly stable, but pH changes withdepth because the amount of carbon dioxidetends to vary with depth.

Shallow depths have less carbon dioxidewith a pH around 8.5.

Shallow depths have the greatest density of photosynthetic organisms which use the carbon dioxide, making the water slightly less acidic.

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Middle depths have more carbon dioxide andthe water is slightly more acidic with a lower pH.More carbon dioxide present from the respiration

of marine animals and other organisms, which makes water somewhat more acidic with a lower pH.

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Deep water is more acidic with no photosynthesisto remove the carbon dioxide.At this depth there is less organic activity, which

results in a decrease in respiration and carbon dioxide. Mid-level seawater tends to be more alkaline.

At 3,000 meters (9,843 feet) and deeper, the water becomes more acidic again.This is because the decay of sinking organic

material produces carbon dioxide, and there are no photosynthetic organisms to remove it.

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The variation of pH and total dissolved inorganic carbon with depth.

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The Organic Chemistryof Water

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Biogeochemical Cycles

Organic chemistry deals mainly with chemical compounds consisting primarily of carbon and hydrogen.

Inorganic compounds such as dissolved sea salts account for the majority of dissolved solids in seawater.

Dissolved organic elements also interact with organisms on a significant scale.These elements are crucial to life and differ from

the sea salts in several ways.

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Proportions of organic elements in seawater differ from the proportions of sea salts because:The principle of constant proportions does not

apply to these elements.These nonconservative constituents have

concentrations and proportions that vary independently of salinity due to biological and geological activity.

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All life depends on material from the nonliving part of the Earth.The continuous flow of elements and compounds

between organisms (biological form) and the Earth (geological form) is the biogeochemical cycle.

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Organisms require specific elements and compounds to stay alive.Aside from gases used in respiration or

photosynthesis, those substances required for life are called nutrients. The primary nutrient elements related to seawater

chemistry are carbon, nitrogen, phosphorus, silicon, iron, and a few other trace metals.

Not all elements and compounds cycle at the same rate. The biogeochemical cycle of the various nutrients affects

the nature of organisms and where they live in the sea.

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Carbon

Carbon is the fundamental element of life. Carbon compounds form the basis for chemical

energy and for building tissues. The seas have plenty of carbon in several forms. It

comes from:Carbon dioxide in the air.Natural mineral sources - such as carbonate

rocks.Organisms - excretion and decomposition.

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Carbon

Carbon cycle.The movement ofcarbon betweenthe biosphereand the nonlivingworld is described

by the carbon cycle.

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Nitrogen

Nitrogen is another element crucial to life on Earth. Organisms require nitrogen for organic

compounds such as protein, chlorophyll, and nucleic acids.

Nitrogen makes up about 78% of the air and 48% of the gases dissolved in seawater.

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Nitrogen

Gaseous nitrogen must be converted to a chemically usable form before it can be used by living organisms.

Bacteria “fixes” nitrogen from the air into usable compounds - nitrate, nitrite and ammonium.

In these forms, autotrophs take up the nitrogen and incorporate it into their systems as protein.

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Nitrogen

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The nitrogen cycle.Bacteria and humanscarry out many of theimportant steps of thenitrogen cycle.The cycle has fourstages: Assimilation,Decomposition,Nitrification,Denitrification.

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Phosphorus and Silicon

Phosphorus is another element important to life because it is used in the ADP/ATP cycle, by which cells convert chemical energy into the energy required for life.Phosphorus combined with calcium carbonate is a

primary component of bones and teeth.

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The phosphorus cycle.Dissolved phosphorous iscarried to the sea byrunoff and leaching fromland. Phosphorus is usedby plants, then recycledthrough animals until it isreleased from wasteand decay.

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Silicon is used similarly by some organismsin the marine environment (including diatomsand radiolarians) for their shells and skeletons.Silicon exists in these organisms

as silicon dioxide, called silica.

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Iron and Trace Metals

Iron and other trace metals fit into the definition of a micronutrient.Micronutrients are substances essential to

organisms in very small amounts.These are essential to organisms for

constructing specialized proteins, including hemoglobin and enzymes. Plants need iron to produce chlorophyll.

Other trace metals used in enzymes include manganese, copper, and zinc.

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Chemical Factors thatAffect Marine Life

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Diffusion and Osmosis

Diffusion is the tendency for a liquid, gas, or solute to flow from an area of high concentration to an area of low concentration.

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Osmosis is diffusion through a semipermeablecell membrane.

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This has important implications with respect tomarine animals. Hypertonic - having a higher salt concentration,

and the water will diffuse into the cells. What happens when you put a marine fish into fresh water.

Isotonic - when water concentration inside the cellis the same as the surrounding water outside thecell. There is no osmotic pressure in either direction. Marine fish cells are isotonic.

Hypotonic - having a lower salt concentration than the surrounding water. It is what happens when you put a freshwater fish into seawater.

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Hypertonic

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Active Transport, Osmoregulators,and Osmoconformers

Osmosis through a semipermeable cell membrane is called passive transport.Passive transport moves materials in and out of a

cell by normal diffusion.

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Active Transport, Osmoregulators,and Osmoconformers

The process of cells moving materials from low to high concentration is called active transport.Active transport takes energy because

it goes against the flow of diffusion.

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Active Transport, Osmoregulators, and Osmoconformers

Marine fish that have a regulation process that allows them to useactive transport to adjust water concentrationwithin their cells are osmoregulators.

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Active Transport, Osmoregulators, and Osmoconformers

Marine organisms that have their internal salinity rise and fall along with the water salinity are osmoconformers.

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Documents

Choose to view chapter section with a click on the section heading. The Water Planet