Biology 12
Note Outline: Water,
Acids, Bases and Buffers
1
A: Basic Chemistry of Water
1. The molecular formula for water is ___H2O________
2. The type of chemical bonds within the water molecule (i.e. intramolecular
bonds), which bind the two hydrogen’s to the oxygen, is __covalent.
3. Since there is an uneven sharing of electrons between the oxygen and the two
hydrogen’s, the oxygen has a partial ___negative______________ charge and
the hydrogen’s have a partial ____positive________ charge. Thus, the oxygen
end of the water molecule is slightly ___negative_____ while the hydrogen
end is slightly ___positive____________
4. Since the water molecule has opposite partial charges at either end, it is
described as a _____polar_______ molecule.
5. Since the water molecule is a ___polar_________ molecule, weak
__hydrogen__ bonds form between water molecules (i.e. intermolecular
bonding)
6. Due to the ____Hydrogen___ bonds between water molecules, water’s heat
capacity (i.e. the ability to store heat without changing temperature) is
___high_________
7. Due to the __hydrogen___ bonds between water molecules, water’s heat of
vaporization (i.e. the amount of heat required to change water from a
__liquid________ to a __gas__________) is _____high_____
8. Since water molecules are ___polar_____, they arrange themselves in a very
organized lattice structure when water freezes. Therefore, solid water (i.e. ice)
is __less________ dense than liquid water.
B: Biologically Significant Properties of Water
1. Since water consists of _polar______ molecules, it is able to effectively
dissolve other __polar____molecules. Since many of the molecules found in
organisms are __polar____, water is an ideal solvent in which these molecules
are dissolved and in which they are able to chemically interact.
2. Since the ___hydrogen_____ bonds between water molecules give water a
___high____ heat capacity, water can absorb a large amount of heat with a
minimal change in _temperature_______. This helps to maintain a relatively
stable body temperature and environmental temperature.
Biology 12
Note Outline: Water,
Acids, Bases and Buffers
2
3. Since the __hydrogen_____ bonds between molecules give water a
___high_____ heat of vaporization, water, in the form of ___sweat_______,
acts as an effective cooling mechanism.
4. Due to the ___hydrogen_____ bonds between water molecules, they tend to
‘stick’ together (i.e. they create a _cohesive__________ surface tension). This
allows water molecules to ‘pull’ each other along as they move through a
medium such as soil and over surfaces such as mucous membranes in animals
and root hairs in plants. This helps to ensure that those surfaces of organisms,
which must be kept moist in order to function properly, will have a continuous
layer of __fluid________ over them.
5. Since the ____polar____ water molecules arrange themselves in a regular
____lattice_____ structure when water freezes, ice is ___less____ dense then
liquid water. Thus, ice floats on water. The layer of ice that forms on the
surface protects the organisms living in the liquid water below.
C: pH, Acids, Bases and Buffers
1. pH:
- pH is a measure of the __hydrogen____ion concentration of a
solution.
- The pH scale runs from ___0_______ to ___14_____.
- The lower the pH number, the higher the ____hydrogen____ion
concentration.
- E.G. A solution in which the pH = 3 has a __higher_______ion
concentration than a solution with a pH = 12.
2. Acids:
- An acid is a substance that dissociates (i.e. splits up) in water and
__increases_____ the hydrogen ion (H+) concentration.
- Since hydrogen ions are simply protons, acids are sometimes
called __proton_____ donars.
- By increasing the ___hydrogen_______ ion concentration, acids
____decrease_______ the pH of a solution.
- Thus, acidic solutions have a ___high_____ hydrogen ion
concentration and a _____low____ pH number.
- Therefore, acidic solutions have a pH, which is __less _ than 7.
Biology 12
3. Bases:
Note Outline: Water,
Acids, Bases and Buffers
3
- When hydrogen ion concentration in a solution is high, the
hydroxide ion (OH-) is
low
. . therefore, in acidic
solutions the hydrogen ion concentrations is higher than the
hydroxide ion concentration.
A base is a substance that dissociated in water and increases the hydroxide
ion concentration.
- The hydroxide
ions from a base can combine with hydrogen ions
to form water.
- therefore, bases are sometimes called proton
acceptors
.
- since bases decrease
hydrogen ion concentration of a solution,
they increase
the pH of the solution.
- Thus, basic solutions have a
low
hydrogen ion concentration
and a
high pH number.
- therefore, basic solutions have a pH which is
higher
than 7.
- in basic solutions, the hydrogen ion concentrations is
less
than
the hydroxide ion concentration.
4. Neutral Solutions:
- in a neutral solution (ex. Pure water), the pH =
7
- in a neutral solution, the hydrogen ion concentration is
hydroxide ion concentration.
.
equal to the
5. Buffers: - buffers are compounds which are capable of stabilizing the pH
of a
solution by absorbing excess hydrogen
or
hydroxide
ions.
- By stabilizing the pH of solutions within an organism, buffers help to
maintain
pH
within the organism. (called homeostasis)
- an example of a buffer which helps to maintain the pH in the blood of
humans is
bicarbonate ion (HCO3--)
.
6. Simple pH calculations:
a. pH formula: pH = -log[H+]
where [H+] is the hydrogen ion
concentration
b. Examples:
i. if pH = 4, then [H+] = 1x10-4 and [OH--] = 1 x 10-10
ii. if pH = 12, then [H+] =
1x10-12
and [OH--] = 1x10-2
+
-9
iii. if pH = 9, then [H ] =
1x10
and [OH--] = 1x10-5
iv. if [H+] = 1 x 10 -8. Then pH = 8 and [OH--] = 1 x 10-12
v. if [H+] = 1 x 10 -2. Then pH =
2 and [OH--] =
1x10-12
vi. if [OH--] = 1 x 10 -3. Then pH = 11
and[H+] = 1x10-11
--14
vii. . if [OH ] = 1 x 10 . Then pH =
1 and[H+] = 1x100
**** this last one never really happens!****
4
Note Outline: Water,
Acids, Bases and Buffers
Biology 12
D. Questions:
1. Draw the molecular structure of two water molecules, one above the other. Indicate in
your diagram where the intramolecular bonds are and state their type and where the
intermolecular bonds are and state their type.
2. Complete the following table:
A
B
C
D
E
F
G
H
I
J
K
pH
8
7
13
0
1
4.2
12.6
10.6
7.4
14
7
[H+]
1 x 10 -8
1 x 10 -7
1 x 10 -13
0
1 x 10-1
1 x 10 -4.2
1 x 10-12.6
1 x 10 -10.6
1 x 10 -7.4
1 x 10-14
1 x 10 -7
[OH-]
1 x 10 -6
1 x 10 -7
1 x10-1
1 x 10 -14
1 x 10 -13
1 x 10 -9.8
1 x 10 -1.4
1 x 10-3.4
1 x 10 -6.6
1
10-7
3. Give as many reasons as you can to explain why the polarity of water is so important
to organisms.
Biology 12
Note Outline: Water,
Acids, Bases and Buffers
5
- cohesion of water sticks your lungs to your ribs (pleural membranes) and keeps your
lungs from collapsing.
-excellent solvent (with polar or charged molecules)
- increases efficiency of diffusion
- excellent coolant (sweat) (high heat of vapourization)
- temperature moderator (high heat of fusion, high specific heat)
ETC.!!!