• Water is a polar molecule with an
asymmetrical shape.

Discuss oxygen’s electronegativity

A water molecule can form up to 4 H-bonds with neighboring water molecules [Fig 3.1]
Why? How?

• cohesive behavior
• resists temperature change
• high heat of vaporization and cooling property (evaporation)
• expands when freezes
• is a versatile solvent

I. Cohesion: phenomenon of a substance being held together by weak H-bonds
• Adhesion is the clinging of one substance to another
How are these important to trees?

Surface tension: a measure of how difficult it is to stretch or break the surface of a liquid
- At the water/air interface, H-bonds with water below as if a film is coating the
surface.

• Kinetic energy – energy of motion
• Heat – total KE due to molecular motion
• Calorie – amount of heat to raise 1g of water by 1

C
• Temperature – measure of heat intensity due to the average KE.

Water has a high specific heat that helps resist temperature change

Specific heat is the amt of heat that must be
absorbed or lost for 1 g of substance to change its temp by 1

C
• The high specific heat of water is due to Hbonds !!

Heat must be absorbed to break H-bonds

Heat must be released to form H-bonds

As temp of water drops, more H-bonds form, causing heat to be released which helps resist temp change
• This unique ability to act as a “heat sink” keeps temperatures on Earth from fluctuating too much

¾ of the planet is water!!!

Absorbs during days and summer

Releases during nights and winter

Real world example: coastal areas have milder
climates than inland;

Evaporative Cooling
• Vaporization – the transformation from liquid to
gas caused by breaking of H-bonds for water
• Heat of vaporization – amt of heat needed to convert 1g of liquid to gas
• 
Water has a high heat of vaporization!!

• Evaporative cooling – water molecules on the surface with the highest KE are most likely to escape into a gasous state

Remaining water molecules have lower KE
• How does this aid in cooling?

Expands when it freezes
• H-bonding important again!
• Water is less dense as a solid than a liquid.

Water is densest at 4

C

4

C

0

C expansion occurs (becomes less dense)
• Crystalline lattice forms. Each water molecule forming the maximum 4 H-bonds no longer break and reform
(the molecules don’t have enough KE to break the bonds).

Why is expansion of water important to life?

Prevents bottom of lakes from freezing

Ice on surface serves to insulate

Makes transitions between seasons less abrupt

• Water is the Solvent of Life
• Solution – homogenous mixture of 2 or more substances
• Solvent – dissolving agent (H
2
O)
• Solute – substance dissolved in solution (NaCl)
• Aqueous solution – solution in which H
2
O is the solvent
• Polarity contributes to the versatility of water as a solvent

Hydrophobic vs. Hydrophilic
Define these?
Most biochemical reactions have water as the
solvent!!!

Two factors that are important for aqueous solutions:solute concentration + pH
Review of solute concentration:
Molecular weight: weight of all of all atoms in a molecule
 measured in Daltons; H
2
O = 18 Daltons
Mole: amount of substance that has a mass in grams equal to its molecular weight in
Daltons.

Determine a mole of sucrose (C
12
H
22
O
11
):
C = 12 dal
H = 01 dal
O = 16 dal
12

12 = 144
01

22 = 022
16

11 = 176
M.W. = 144 + 22 + 176 = 342 dal
Mole = 342 g

Molarity (M) = # of moles of solute per L of solution

1 M sucrose solution = 342 g sucrose + 1 L of H
2
O
Advantage of the mole:
 can weigh using grams instead of Daltons
 mole of one substance has the same # of
molecules as a mole of another substance (6.02

10 23 )

Review of pH:
Water can dissociate, forming a hydrogen ion
(H + ) and a hydroxide ion (OH ). pH – negative log
10
[H + ] (expressed in moles/L)
 used to measure H + concentration (acidity) pH scale ranges from 0 –14

7 is neutral; less than 7 is acidic; more than 7 is basic

Most biological fluids are between 6 – 8 pH
 exception is stomach (~1.5) pH unit represents 10 fold difference, so slight change in pH represents a large change in actual [H + ]

Acid: a substance that increases [H + ] of a solution
Base: a substance that reduces [H + ] of a solution
Buffers: substances that prevent sudden changes in pH.

Weak acids or bases that accept H + when there are in excess or donate H + when [H + ] of solution is depleted

Bicarbonate Buffer: H
2
CO
3

HCO
3
+ H +
If pH

, which way will the rxn proceed? To the left
If pH

, which way will rxn proceed? To the right
If [OH-]

, which way will rxn proceed? To the right
Why? To keep pH in biological range

Rain or snow with pH less than 5.6.

Lowest recorded value = 1.5
Major source: Fossil fuels by industry and cars
Sudden accumulation during spring melt problem at a time when many organisms are
laying eggs!!

Early meltwater often has pH as low as 3

Lakes w/ pH < 5 have no fish