Water is a polar molecule that is essential for life. Its polarity allows it to dissolve many other polar substances and act as a solvent for biological molecules and reactions in living cells and organisms. Water has unique physical properties, such as a high heat capacity and ability to form hydrogen bonds, that make it well-suited to regulate temperature and support life processes on Earth. The polarity and hydrogen bonding of water molecules also allow water to have high surface tension and influence many other physical phenomena.

2. Water
•Water is the most important
inorganic molecule
•All living organisms need water to
survive
•65% of human body weight is
water

3. Water and it’s importance
• Most important compound in living
organisms.
• Life processes rely on free movement of
molecules and ions.
– This occurs when substances are dissolved in
water.
• Water has unique properties that make it
vital to life.
– Many of water’s biological functions stem from
it’s chemical structure!

5. Polarity of Water
• Atoms of water molecules are bonded together by
covalent bonds.
• Sharing of electrons in the covalent bonds is not
equal
– oxygen attracts electrons more strongly than hydrogen
• This gives water an assymetrical distribution of
charge.
– assymetrical = uneven
– Uneven  opposite ends or poles are created
– POLAR covalent bonds exist within water molecules

6. This causes an unequal
sharing of electrons

8. Hydrogen bonds form
between water molecules
Oxygen is slightly
negative
(more electronegative)
Hydrogen is slightly positive
(less electronegative)
Neighboring
molecules held by
a hydrogen bonds
H-bonds are not true
bonds, but an
attractive force.

10. Water is a versatile solvent
• When water and another solution are mixed,
you have an aqueous solution.
• Solute = what is being dissolved
• Solvent = what is doing the dissolving
• Polarity of water allows a variety of substances
to dissolve easily
– ionic compounds
– polar molecules
– Molecules with charged parts attract to water.
• Substances that repel water include non-polar
and non-ionic substances
• Salt dissolving
(animation)

13. Water as a solvent: Examples
• Polarity of water attracts other polar molecules
– ex: sugar
• These molecules are considered to be “hydrophilic”
or
– water loving
• Molecules that are non-polar are repelled
– Ex: oil
– Non-polar molecules are considered
“hydrophobic” or
– water fearing
• oil and water
animation
• More about dissolving

14. Importance to Living Things
• The chemistry of living things involves the
study of solutions ~~AQUEOUS solutions
• Water is vital to life because the chemical
reactions (metabolism) of living things must
take place in aqueous solutions.
• This occurs when substances are dissolved in
water.
• Life processes rely on free movement of
molecules and ions. (ex: electrical signals in
nervous systems)

15. Examples of aqueous solutions:
• Blood plasma
– Liquid portion only
– Ions, larger molecules, gases all dissolved in water
• Cytosol
– The gelatin-like aqueous solution inside cells.
• Interstitial fluid
– Intercellular fluid of multicellular organisms
~11 L in average adult human body
• Aquarium water….

16. Temperature Stabilizer
• Water can absorb great amount of heat before it’s
temperature changes considerably
– Due to hydrogen bonds
– Energy added to water disrupts hydrogen bonds
– Hydrogen bonds are constantly breaking and reforming
• Cells release heat as a result of metabolism
– Water helps to minimize temperature changes in cells
• This also allows large bodies of water and large organisms to
regulate temperature
– If bonds stay broken, water molecules at the surface escape into the
atmosphere
• Evaporation
• This takes away some energy and cools surface left behind
– This is why sweating cools you down!

17. High Specific Heat
Substance C (J/g o
C)
Air 1.01
Aluminum 0.902
Copper 0.385
Gold 0.129
Iron 0.450
Mercury 0.140
NaCl 0.864
Ice 2.03
Water 4.18
• Water (liquid) has a
high specific heat
compared to other
substances.
• It requires more heat
to increase it’s
temperature
• 0-100 degrees Celsius
range!

19. Specific Heat Definiton:
• The quantity of heat required to raise the
temperature of a substance by one degree
Celsius is called the specific heat capacity
of the substance.

20. Phase Change Graph

21. Freezing and Expansion
• Water expands below 4o
C.
• Molecules are spaced further apart
• It is less dense, therefore ice floats in
water.
• Ice at surface of lakes
and oceans provides a
thermal insulator for life
below the surface.

23. Water is Cohesive
• Cohesion- ability of similar molecules to stick together
• Cohesion = stick TOGETHER
– Like molecules
– Like sticks to like
– WATER sticks to WATER
• Water attracts other water molecules
– Hydrogen bonds = attractions (water-water)
– Results in surface tension

24. Surface Tension
• Cohesion allows water to form an interface
with a surface.
• Surface tension is the measure of how
difficult it is to break this interface.
– Due to H-bonding between water molecules
• Ex: materials are able to rest on water (float),
as long as the surface tension is not broken.

25. Adhesion
• Sticking together of one substance to
another, different substance.
• Water is very adhesive; sticks to
OTHER substances like glue.
• Capillary action is adhesion and
cohesion in action.
– Capillary action is the process of pulling water
up against gravity in plant vessels

28. Imbibition
• Process of soaking into a hydrophilic
substance.
• Water soaking into seeds, paper towels,
or sponge.
• Important for plants (seed germination
usually requires imbibition)

29. pHpH
• pH refers to the dissociation of water
molecules.
• H2O H+
+ OH-
• In pure water, concentrations of H+ and OH-
are equal. (pH = neutral)
• Organisms are sensitive to changes in pH

30. AcidsAcids and BasesBases
• When acids dissolve in water they donate H+
to
the solution
• An acid is a substance that increases the H+
concentration
• A substance that reduces the H+
concentration
is a base
– By accepting H+
– Or by dissociating to form hydroxide ions that
combine with H+
to form water

31. pH
• In any solution, the product of H+
and OH-
concentrations is constant at 10 -14
[H+
][OH-
] = 10 -14
Neutral solution (pH 7) = [H+] = 10-7
[OH-] = 10-7
[10-7
] [10-7
] = 10-14
– If [H+] increased to 10-5
M, then [OH-] would be 10-9
M
[10-5
] [10-9
]= 10-14

32. PH Scale: pH scale is used to determine concentration
of these ions in a solution
Higher
concentration
of Hydrogen ions
H+
Higher
concentration of
Hydroxide ions OH-

34. Each step
is ten fold
Ex: How many
more H+
at pH
of 1 than pH of
3?
10
x
10
= 100

35. pH Changes
• H+
and OH-
are very reactive
• pH changes greatly affect a cell’s proteins
and other molecules by altering their
structure (shape).
• If the structure of a molecule changes, the
result is a change in function.
In our pH lab, what changes were observed?

36. Buffers
• The control of pH is very important for
living systems
• Most chemicals of life can only operate
properly within a narrow pH range
• Buffers are substances that minimize
changes in pH

37. Buffers
• As shown on the pH scales above, the pH of
your internal body systems varies greatly.
– Stomach fluid = acidic
– Urine = acidic
– Intestinal fluid = alkaline
– Blood = alkaline
• Complex buffering systems maintain the
proper pH values of your body’s many fluids