Water – Chapter 3

(–) (–)

O

HH

(+) (+)

What Make Water Unique?

• Polarity and hydrogen bonds!!

Polar vs. Nonpolar Molecules• Polar bond = simply a type of covalent bond in

which the electrons are shared but not equally. This happens when two different atoms come together, such a C and O. Each atom has a different ability to draw electrons to itself when it shares electrons which creates charged areas of the atoms

• Nonpolar bond = two of the same atoms come together, such as the diatomic molecule N2, or triatomics such as O3, etc. A nonpolar covalent bond could be viewed as having "pure" covalent character. There is perfectly equal sharing so no charge is created.

Water is Polar!!• Polarity: In some covalent

bonds, electrons are more attracted to one part of the molecule than the other. As a result, they will spend more time around one atom than another. This creates a slightly negative charge on that half of the molecule

Polarity of WaterIn a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. – Oxygen is larger so the electrons stay around the

oxygen and not the hydrogen.– This makes water charged. – Oxygen area negative. – The region near the two hydrogen

atoms has a partial positive charge.• A water molecule is a polar molecule

with opposite ends of the

molecule with opposite charges.

Unequal electron sharing creates polar molecules

• Atoms in a covalently bonded molecule continually compete for shared electrons– The attraction (pull) for shared electrons is

called electronegativity– More electronegative atoms pull harder– OXYGEN IS ONE OF THE MOST

ELECTRONEGATIVE ELEMENT THAT EXIST. F IS FIRST IN LINE!!!

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Unequal electron sharing creates polar molecules

• Water has atoms with different electronegativities– Oxygen attracts the shared electrons more strongly than

hydrogen– So, the shared electrons spend more time near oxygen – The result is a polar covalent bond

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Water Molecule

Why do you think this drop stays together?

Hydrogen bond

Hydrogen Bonds

• This polarity causes the H atoms to be positive charge which causes it to be attracted to a nearby negative charged molecule. This will form a hydrogen bond.

• HYDROGEN bonds: weak bond between positive H atoms in a molecule and a negative charge between another molecule.

• Water has a variety of unusual properties because of attractions between these polar molecules.– The slightly negative regions of one molecule

are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond.

– Each water molecule can form hydrogen bonds with up to four neighbors.

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Fig. 3.1

HYDROGEN BONDS1. Hold water molecules

together

2. Each water molecule can form a maximum of 4 hydrogen bonds

3. The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds.

4. They form, break, and reform with great frequency

Extraordinary Properties of water are a result of hydrogen bonds!!!!!

They hold water molecules together!!

Hydrogen Bonds

• Hydrogen

• Bonds

• Between

• H2O

• Molecules!!

Hydrogen Bonds Between DNA!!

Water and Its Affect on Organisms

Look at these pictures and describe the properties of water that affects these organisms!!

Water’s Importance to Organisms!!1.Water in cells2.Ice floats so life can live below3.Transport material throughout organism4.Dissolve ionic compound like sugars and

salts(materials in blood).5.Part of photosynthesis and energy

processes6.Surface tension organisms walk on it7.Regulates temperature around large

bodies of water8.Water absorbs heat caused by cell

processes. Regulates body temperatures.

WATER’S LIFE-SUPPORTING PROPERTIES

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Water and Its Properties

Water’s Properties

1. Cohesion & Adhesion

2. High Specific Heat

3. High Heat of Vaporization

4. Solid water (ice) is less dense than liquid

5. Solvent

6. Transparent

1. Cohesion• Cohesion: The attraction of molecules of

the same type. The hydrogen bonds between water molecules hold them together.

• Examples: Cytoplasm is held together, and blood, Drops of dew held together.

•

1. Cohesion• Water clings to polar

molecules through hydrogen bonding– Cohesion refers to

attraction to other water molecules.

• responsible for surface tension

– Surface tension - a measure of the force necessary to stretch or break the surface of a liquid

– Some animals can stand, walk, or run on water without breaking the surface.

Adhesion• Adhesion: attraction between particles

that are different.

Examples: Blood to vessels, water to xylem in plant. Clinging of one substance to another.

Organisms Depend on Cohesion• Adhesion is responsible for

the transport of the water through the xylem in plants against gravity.

• Capillary Action – combination of adhesion and cohesion that moves water up the xylem.

• Water molecules stick to walls of vessels and water sticks to each other.

• Caused by Hydrogen bonds

•Water sticking to water - cohesion

•Water sticking to the wall is adhesion

•Plants have specialized structures to transport water: xylem

• water molecules are “dragged” from the roots to the top of the tree by capillary action and cohesion: hydrogen bonds help water molecules to each other

Capillary action water evaporates from leaves = transpiration

adhesion, cohesion and

capillary action

All thanks

to hydrogen

bonding!

water taken up by roots

Specific Heat is the amount of energy required to change the temperature of a substance.

Water has a high specific heat – it absorbs a lot of energy before it begins to heat up!Takes a lot of heat to break apart the bonds.

HOW WOULD THIS HELP ORGANISMS!!

Specific Heat

2. SPECIFIC HEATA. Moderates Earth’s climate – large bodies

of water absorb heat in the summer and release heat in the winter. This is one reason CA is mild.

B. Regulates body temperatures – absorbs energy released by the cell during cell processes.

A. Moderation of temperature

• Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for:

• Prevention of temperature fluctuations that are outside the range suitable for life.

• Coastal areas having a mild climate

• A stable marine environment

Moderates Temperatures on Earth

Celsius Scale at Sea Level

100oC Water boils

37oC Human body temperature

23oC Room temperature

0oC Water freezes

 

Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature.

B. REGULATES Body Temperatures

As an organism completes cells processes such as photosynthesis and respiration heat is released. Water absorbs this energy released by the cell during cell processes. Without water, organisms would overheat.

3. High Heat of Vaporization

**Because it also involves the breaking of hydrogen bonds, water resists vaporizing (evaporating).

Consequently, it takes a lot of heat to evaporate water. This high heat of vaporization is also utilized by organisms as a cooling process, e.g., sweat or panting.

3. EVAPORATIVE COOLING

• As a liquid evaporates, the surface of the liquid that remains behind cools - Evaporative cooling.

• Evaporative cooling moderates temperature in lakes and ponds and prevents terrestrial organisms from overheating.

• Evaporation of water from the leaves of plants or the skin of animals removes excess heat.

4. Ice Floats• When water freezes, hydrogen bonds lock

water molecules into a crystalline pattern with empty spaces. This makes ice less dense than water. So ice floats.

• Organisms can live below the ice!!!!

– When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to the maximum of four partners.

– As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state.

– Ice is about 10% less dense than water at 4oC.

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Fig. 3.5

Solid water (ice) is less dense than liquid

same mass but a larger volume

• Ice is less dense than water: the molecules are spread out to their maximum distance

Density = mass/volume

Density of WaterThe density of water:

1. Prevents water from freezing from the bottom up.

2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.

3. Makes transition between season less abrupt.

4. This creates an insulation for life below ice!!

5. Water is Transparent• The fact that water is clear allows light

to pass through it. NOT BECAUSE OF H BONDS!!– Aquatic plants can receive sunlight– Light can pass through the eyeball to

receptor cells in the back

6. Water Solubility

• Water Universal Solvent – Dissolves other polar molecules!!!

• Solubility: the ability to be dissolved. What would cause this?

• Charged ends of molecules attract each other making them dissolve.

6. Solvent for LifeHydrophilic

– Ionic compounds dissolve in water

– Polar molecules (generally) are water soluble

• Hydrophobic– Nonpolar

compounds

Why important to Organisms?• TRANSPORT - Water transports molecules

dissolved in it– Blood, a water-based solution, transports

molecules of nutrients and wastes organisms– Nutrients dissolved in water get transported

through plants– Unicellular organisms that live in water absorb

needed dissolved substances

Ionization of Water• Some water molecules will break apart in

a charged or polar compound!!• The charged parts of the water molecules

are pulled part• Ionization: breaking apart of water

molecules.• Water ionizes into H+ and OH-

• These ions will pull apart molecules which creates acids and bases.

• pH scale expresses hydrogen ion (H+) concentration in a solution.

pH

• When water breaks in H+ ions it will pull apart ionic bonds to form acids and bases!!!

• pH is a measure of hydrogen ion (H+) concentration in water solution

The more hydrogen ions present in a water solution, the higher the Molar concentration, and therefore the lower the pH.

Acids and Bases

• Some molecules form ions when they are dissolved in water.

• Caused because water is polar!!

• Example HCL – breaks into H+ and Cl-.

• .– The hydrogen atom leaves its electron behind

and is transferred as a single proton - a hydrogen ion (H+).

– The water molecule that lost a proton is now a hydroxide ion (OH-).

– .

Ionization of Water Molecules

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Unnumbered Fig. 3.47

• A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion:

– H2O <=> H+ + OH-

• This reaction is reversible.

• At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-.

• In pure water only one water molecule in every 554 million is dissociated.

– At equilibrium, the concentration of H+ or OH- is 10-7M (25°C) .

Acids

• Acids are substance that when placed in water are pulled apart into H+ ions and a negative ion.

• Increases the concentration of H+.– Have many H+ ions– Sour taste– HCl is hydrochloric acid or stomach acid

Bases

• Bases combine with H+ ions when dissolved in water, thus decreasing H+ concentration. Less H ions. Higher pH– Have many OH- (hydroxide) ions– Bitter taste– NaOH = sodium hydroxide or baking soda

Acids and Bases• An acid is a substance that

increases the hydrogen ion concentration in a solution.

• Any substance that reduces the hydrogen ion concentration in a solution is a base.– Some bases reduce H+ directly by

accepting hydrogen ions.

• Strong acids and bases

complete dissociate in water.• Weak acids and bases

dissociate only partially and reversibly.

pH

pH Scale• The pH scale in any aqueous solution :

– [ H+ ] [OH-] = 10-14

• Measures the degree of acidity (0 – 14)

• Most biologic fluids are in the pH range from 6 – 8

• Each pH unit represents a tenfold difference (scale is logarithmic)– A small change in pH actually indicates a

substantial change in H+ and OH- concentrations.

ProblemHow much greater is the [ H+ ] in a solution with pH 2 than in a solution with pH 6?

Answer:

pH of 2 = [ H+ ] of 1.0 x 10-2 = 1/100 M

pH of 6 = [ H+ ] of 1.0 x 10-6 = 1/1,000,000 M

10,000 times greater

Effects of pH changes on Organisms

1. Denatures enzymes: enzymes speed up every reaction in your body. If pH changes, then they are broken apart.

2. Stomach acid: must be a certain pH. When it changes upset stomach.

3. Lactic acid builds up during exercise4. Change of pH in marine ecosystems can be

detrimental.

Buffers

• Buffers– act as a reservoir for hydrogen ions,

donating or removing them from solution as necessary

– Offer protection from extreme pH levels– Produced naturally by organisms:

• Organisms can’t tolerate much pH change• Cells function best within a narrow pH range

Buffers• A substance that eliminates large sudden

changes in pH.• Buffers help organisms maintain the pH of

body fluids within the narrow range necessary for life. – Are combinations of H+ acceptors and

donors forms in a solution of weak acids or bases

– Work by accepting H+ from solutions when they are in excess and by donating H+ when they have been depleted.

Examples of Buffers

• Lactic Acid builds up in body during exercise. Raises H ion concentration or lowers pH. The job of the kidneys is to remove these H ions

• Carbonate is the main buffer in the blood and phosphate is the main buffer within cells. These elements bond with the H ions so they remove them!!

Acid Precipitation• Rain, snow or fog with more strongly acidic than pH

of 5.6• West Virginia has recorded 1.5• East Tennessee reported 4.2 in 2000• Occurs when sulfur oxides and nitrogen oxides

react with water in the atmosphere– Lowers pH of soil which affects mineral

solubility – decline of forests– Lower pH of lakes and ponds – In the

Western Adirondack Mountains, there are lakes with a pH <5 that have no fish.

pH Marine Ecosystems

• Very high ( greater than 9.5) or very low (less than 4.5) pH values are unsuitable for most aquatic organisms. Young fish and immature stages of aquatic insects are extremely sensitive to pH levels below 5 and may die at these low pH values. High pH levels (9-14) can harm fish by denaturing cellular membranes.

• Changes in pH can also affect aquatic life indirectly by altering other aspects of water chemistry. Low pH levels accelerate the release of metals from rocks or sediments in the stream. These metals can affect a fish’s metabolism and the fish’s ability to take water in through the gills, and can kill fish fry.

• At high pH (>9) most ammonium in water is converted to toxic ammonia (NH3), which can kill fish. Moreover, cyanobacterial toxins can also significantly influence fish populations.

• THE decline in freshwater fish populations in parts of southern Norway is associated with increasing acidity in rivers and lakes. The salmon has been eliminated from many rivers, and hundreds of lakes have lost their trout populations. The chief cause of increased acidity is acid precipitation which is the product of the emission, oxidation and long-distance transport of air pollutants, particularly sulphur dioxide.

• http://aqua-culture.blogspot.com/2007/01/effects-of-high-and-low-ph-levels-in.html

• Great reading