Role of Water
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Water is the base of life
Life on earth probably evolved in water
Living cells are 70-95% H2O
Water covers about ¾ of the earth
In nature water exist in all three physical states of matter –
solid, liquid and gas
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Water is the base of life
It can be quite correctly argued that life exists on Earth
because of the abundant liquid water.
Other planets have water, but they either have it as a gas
(Venus) or ice (Mars).
Recent studies of Mars reveal the presence sometime in
the past of running fluid, possibly water.
Water can exist in all three states of matter on Earth, while
only in one state on our two nearest neighboring planets.
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Polarity of water
Water is polar molecule. Its polar
bonds and assymetrical shape give
water molecules opposite charges on
opposite sides.
Four valence orbitals of O point to
corners of a tetrahedron.
Two corners are orbitals with
unshared pairs of electrons and weak
negative charge.
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Polarity of water
Two corners are occupied by H atoms which are in polar
covalent bonds with O.
Oxygen is so electronegative, that shared electrons spend
more time around the O causing a weak positive charge
near H’s.
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Polarity of water
Hydrogen bounding orders
water into a higher level of
structural organization.
The polar molecules of
water are held together by
hydrogen bonds.
Positively charged H of one
molecule is attracted to the
negatively charged O of
another water molecules.
Each water molecule can form a maximum of four
hydrogen bonds with neighboring water molecules.
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Polarity of water
Water has extraordinary properties that emerge as a
consequence of its polarity and hydrogen-bonding. Some of
these properties are that water:
•Has cohesive behavior
•Resists changes in temperature
•Has a high heat of vaporization and cools surfaces as it
evaporates
•Expands when it freezes
•Is a versatile solvent
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Polarity of
water
Water is polar molecule.
Other molecules, such as Ethane, are nonpolar, having
neither a positive nor a negative side.
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Hydrogen
bonds
Consequently, water has a great interconnectivity of
individual molecules, which is caused by the individually
weak hydrogen bonds that can be quite strong when taken
by the billions.
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Cohesion of water molecules
Collectively, the hydrogen bonds hold the substance
together – a phenomenon called cohesion.
Cohesion contributes to the transport of water against
gravity in plants. Adhesion of water to the walls of the
vessels helps counter the downward pull of gravity.
Water has a great surface tension. At interface between
water and air hydrogen is bounded to one another and to the
water below.
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Water moderates temperatures on Earth
Water absorbs heat from the warmer air and release the
stored heat to the cooler air.
Everything what moves has kinetic energy; the faster a
molecule moves, the grater its kinetic energy.
Heat is a measure of the total quantity of kinetic energy due
to molecular motion in a body of matter
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Water moderates temperatures on Earth
Temperature measures the intensity of heat due to the
average kinetic energy of the molecules.
When the two objects are brought together, molecules in the
cooler object speed up at the expense of the kinetic energy
of the warmer one.
A calorie is the amount of heat energy required for raising
the temperature of 1 g of water by 1C.
Conversely, one calorie is the amount of heat that one gram
of water releases when it cools down by one degree Celsius.
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Water’s high specific heat
Water has a high specific heat, which means that it resists
temperature changes when it absorbs or releases heat.
The specific heat of a substance is defined as the amount of
heat that must be absorbed or lost for 1g of that substance
to change its temperature by 1C.
The specific heat of water is 1 calorie per gram per degree
Celsius, abbreviated as 1 cal/g/ C.
It is unusually high when compared to the other substances
(0.6 for alcohol).
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Water’s high specific heat
•As a result of hydrogen bonding among water
molecules, it takes a relatively large heat loss or gain for
each 1C change in temperature.
•Hydrogen bonds must absorb heat to break, and they
release heat when they form.
•Much of the absorbed heat energy is used to disrupt
hydrogen bonds before water molecules can move
faster (increase temperature)
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Water’s high specific heat
A large body of water can act as a heat sink, absorbing
heat from sunlight during the day and summer (while
warming only few degrees) and releasing heat during the
night and winter as the water gradually cools.
As a result:
•Water, which covers three-fourths of the planet, keeps
temperature fluctuations within a range suitable for life
•Coastal areas have milder climates than inland.
•The marine environment has a relatively stable
temperature.
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Evaporative cooling
The transformation from the liquid to gas is called
vaporization.
•Molecules with enough kinetic energy to overcome the
mutual attraction of molecules in a liquid, can escape into
the air.
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Evaporative cooling
Heat of vaporization is the quantity of heat a liquid must
absorb for 1g of it to be converted from the liquid to the
gaseous state.
•For water molecules to evaporate, hydrogen bonds must
be broken which requires energy.
Water has high heat of vaporization – for each gram of water
580 cal are needed (ammonia and alcohol two times less).
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Evaporative cooling
With liquid evaporation the surface of the liquid that remains
behind cools down.
Evaporative cooling occurs because the “hottest” molecules
(with greater energy) are the most likely to leave as gas.
High concentration of water in the air inhibits evaporation.
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Evaporative cooling
Water’s high heat of vaporization:
•Moderates the Earth’s climate.
•Solar heat absorbed by tropical seas dissipates
when surface water evaporates (evaporative cooling)
•As moist tropical air moves poleward, water vapor
releases heat as it condenses into rain
•Stabilizes temperature in aquatic ecosystems
(evaporative cooling).
•Helps organisms from overheating by evaporative
cooling.
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The structure of ice.
Each molecule is hydrogen-bonded to four neighbors in a
three-dimensional crystal with open channels.
Because the hydrogen bonds make the crystal spacious,
ice contains fewer molecules than an equal volume of liquid
water.
In other words, ice is less dense than liquid water.
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The structure of ice.
•Water is densest at 4C.
•Water contracts as it cools to 4C
•As water cools from 4C to freezing (0C), it expands and
becomes less dense than liquid water (ice floats)
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The structure of ice.
•When water begins to freeze, the molecules do not have
enough kinetic energy to break hydrogen bonds.
•As the crystalline lattice forms, each water molecules forms a
maximum of four hydrogen bonds, which keeps water
molecules further apart than they would be in the liquid state.
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The structure of ice.
Expansion of water contributes to the fitness of the
environment for life:
•Prevents deep bodies of water from freezing solid from
the bottom up.
•Since ice is less dense, it forms on the surface first. As
water freezes it releases heat to the water below and
insulates it.
•Makes the transitions between seasons less abrupt. As
water freezes, hydrogen bonds form releasing heat. As
ice melts, hydrogen bonds break absorbing heat.
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Universal
solvent
Water has been referred to as the universal solvent.
Living things are composed of atoms and molecules within
aqueous solutions (solutions that have materials dissolved in
water).
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Universal
solvent
Solutions are uniform mixtures of the molecules of two or
more substances.
The solvent is usually the substance present in the greatest
amount (and is usually also a liquid).
The substances of lesser amounts are the solutes.
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Hydrophilic, hydrophobic
Any substance (ionic or polar) that has an affinity for water is
hydrophilic, even if the substance does not dissolve (cotton).
Non-ionic and non-polar substances that repel water are
termed hydrophobic.
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A water-soluble protein.
Even a molecule as large as a
protein can dissolve in water if
it has enough ionic and polar
regions on its surface.
The mass of purple here
represents a single such
protein molecule, which water
molecules are surrounding.
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How to prepare solutions
Everything is done in moles.
The advantage of measuring a quantity of chemicals in
moles is that a mole of one substance has exactly the same
number of molecules as a mole of any other substance.
A mole (M) is equal to molecular weight – the sum of the
weights of all the atoms in a molecule.
The number of the molecules in a mole is called Avogadro’s
number, 6.02x1023.
To get a litre (L) of 1 M of sucrose in water we need to
weight 342 g of sucrose and add water up to 1L.
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Water dissociation
Water tends to disassociate into H+ and OH- ions.
In this disassociation, the oxygen retains the electrons and
only one of the hydrogens, becoming a negatively charged
ion known as hydroxide.
Pure water has the same number (or concentration) of H+ as
OH- ions.
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Water dissociation
Acidic solutions have more H+ ions than OH- ions.
HCl  H+ + ClBasic solutions have the opposite.
NaOH  Na+ + OHAn acid causes an increase in the numbers of H+ ions and a
base causes an increase in the numbers of OH- ions.
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pH of some common items
The pH scale is a logarithmic scale representing
the concentration of H+ ions in a solution.
As the H+
concentration
increases the
OHconcentration
decreases and
vice versa.
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If we have a solution with one in every ten molecules being
H+, we refer to the concentration of H+ ions as 1/10.
Remember from algebra that we can write a fraction as a
negative exponent, thus 1/10 becomes 10-1.
Conversely 1/100 becomes 10-2, 1/1000 becomes 10-3, etc.
Logarithms are exponents to which a number (usually 10)
has been raised. For example log 10 (pronounced "the log of
10") = 1 (since 10 may be written as 101).
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The log 1/10 (or 10-1) = -1.
pH, a measure of the concentration of H+ ions, is the
negative log of the H+ ion concentration.
If the pH of water is 7, then the concentration of H+ ions is
10-7, or 1/10,000,000.
In the case of strong acids,
such as hydrochloric acid (HCl),
an acid secreted by the lining of your stomach,
[H+] (the concentration of H+ ions, written in a chemical
shorthand) is 10-1; therefore the pH is 1.
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Buffers
Biological fluids resist changes to their own pH when acids
or bases are introduced because of the presence of buffers.
Buffers are substances that minimize changes in the
concentrations of H+ and OH- in a solution.
The change of pH to less than 7 and more than 7.8 is lethal.
The mode of action of a buffer:
-donate the hydrogen ions when they have been depleted;
-accept the hydrogen ions from the solution when they are in
excess.
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Reading
Ch. 3 (46-57)
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