AP Biology Properties of Water

Water and it’s life giving properties
 How many drops of water can you fit on a penny?
 How does water do to ink?
 What does a drop of water look like?
 Can you float a paperclip on water?

Observations
 What observations did you make about water?
 What do you already know?

The Chemical
Properties of Water

Your Objective
Be able to state at least 2
properties of water that help
support life
AND
Be able to explain how the
chemical nature of water gives
rise to that property

Water
and Life
 Life on Earth
began in
water and
evolved there
for 3 billion
years.
 Modern life still remains tied
to water
 Cells are composed of 70%-
95% water

•Water is found
as a liquid over
71% of the
earth
•The abundance
of water is a
major reason
Earth is
habitable

 Studied in isolation, the water molecule is
deceptively simple
 Its two hydrogen atoms are joined to one
oxygen atom by single covalent bonds
The structure of water
H
O
H

 But the electrons of the covalent bonds are not
shared equally between oxygen and hydrogen
This unequal sharing makes water a polar
molecule
Oxygen is more electronegative than
hydrogen, so it has a greater pull on the
electrons
() ()
() ()

 The polarity of
water results in
weak electrical
attractions
between
neighboring water
molecules
These
interactions
are called
hydrogen
bonds
(b)
()
Hydrogen bond
()
()
()
()
()
()
()

Quick Think
 Why is a molecule
of water said to
have polar covalent
bonds?
 What kind of bonds
hold individual
water molecules
together?

Water’s Life Supporting
Properties
 The polarity of water molecules and the
hydrogen bonding that results explain most
of water’s life-supporting properties
 Water’s cohesive nature
 Water’s ability to moderate temperature
 Floating ice
 Versatility of water as a solvent

 Water molecules
stick together as a
result of hydrogen
bonding
 This is called
cohesion
The Cohesion of Water
Cohesion is
vital for water
transport in
plants
Microscopic tubes

 Which of our mini-lab demos seems to
illustrate cohesion?

Surface tension is the measure of how difficult
it is to stretch or break the surface of a liquid
Hydrogen bonds
give water an
unusually high
surface tension
Cohesion
between water
molecules form a
skin-like surface Can support animals like “water
striders” in ponds

Surface Tension
 Water drops are round because all the
molecules on the edge are pulled to the
middle.

Adhesion
 Water will also
adhere to other polar
substances
 This is called
adhesion
 It is due to the polar
nature of the water
molecule
Caused by adhesion the water runs
along the glass and does not fall
straight.

Capillary Action
 Glass has polar
molecules.
 Glass can hydrogen
bond.
 Attracts the water
molecules.
 Some of the pull is up.

Meniscus
 Water curves up
along the side.
 This makes the
meniscus.

Quick Think
 How are adhesion and
cohesion similar and
different?
 Give an example of how
these properties help
support life.

8-15-14 Warm up
 Use the clay and toothpicks to make a
model of a water molecule
 Working with someone near you, use your
models to demonstrate cohesion.
HW:
1.) Read section 3-1
and 3-2 in your
textbook
2.) Watch
CrashCourse
Biology – Water on
YouTube
3.) water worksheet

Objective:
Explain and
diagram the
transpiration-
cohesion
mechanism
responsible
for water
transport in
plants

Water moderates temperature
 Because of hydrogen bonding, water has
a strong resistance to temperature
change

 Heat and temperature are related, but
different
 Heat is a measure of the amount of kinetic
energy in the atoms and molecules in
something
 Temperature measures the intensity of the
heat
 Whenever 2 objects meet, the cooler object
absorbs heat from the warmer object until
they are the same temperature

 Water has a high specific heat
 Specific heat = the amount of heat that must
be absorbed or lost to change the
temperature of 1g of the substance 1°C

 much of the absorbed heat is used to
break hydrogen bonds, not increase the
kinetic energy of the molecules

Earth’s giant water
supply causes
temperatures to
stay within limits
that permit life

How water moderates
temperature
Water also has:
High heat of fusion
• The temp at which liquid turns solid
High heat of vaporization
• The temp at which liquid turns to gas

Quick Think
Why does it take so
much energy to
change the
temperature of water?

 When water molecules get cold, they move
apart, forming ice
The Biological Significance of Ice
Floating
A chunk of ice has fewer molecules than an equal volume
of liquid water
IceLiquid
water

 The density of ice is lower than liquid water
 This is why ice floats
Figure 2.15
Hydrogen bond
Liquid water
Hydrogen bonds
constantly break and re-form
Ice
Stable hydrogen bonds

Dipole Structure
 Ice floats in water because all ice
molecules are held in hexagons
 Center is open
space, making
ice 8% less
dense than
water.

 Since ice floats, ponds, lakes, and even the
oceans do not freeze solid
Marine life could not survive if bodies of water froze solid

 Floating ice insulates water below,
preventing freezing: critical for ocean
animals

Maximum density: 3.98oC
 Below this temp, form hexagonal polymers and
decrease density
 Above this, molecules are energetic, water behaves
like other liquids - expanding when warm and
contracting when cool

Quick Think
 Of all the properties we’ve learned so far,
which ones could you competently write
about in your essay?

 A solution is a liquid consisting of two or
more substances evenly mixed
Water as the Solvent of Life
The dissolving agent is called the solvent
The dissolved substance is called the solute
Ion in solution
Salt crystal

 Water is a good solvent because it is polar
 Ionic (salts) and polar (sugars) compounds
dissolve readily in water

Solvent Properties
 Water dissolves salts by surrounding the atoms in the
salt molecule and neutralizing the ionic bond holding the
molecule together

 Acid
Acids, Bases,
and pH
A chemical compound that donates H+ ions to solutions
Base
A compound that accepts H+ ions and removes them from
solution or a compound that dissociates in water to form
hydroxide ions

Basic
solution
Neutral
solution
Acidic
solution
Oven cleaner
Household bleach
Household ammonia
Milk of magnesia
Seawater
Human blood
Pure water
Urine
Tomato juice
Grapefruit juice
Lemon juice;
gastric juice
pH scale
To
describe
the acidity
of a
solution,
we use the
pH scale

pH scale (log scale)
Each number on the scale is 10x difference
from the number next to it
• pH 1 is 10x more acidic than pH 2, 100x more
acidic than pH 3, 1000x more acidic that pH 4,
and so on

pH - Percent Hydronium
A measure of the percent of hydronium ions
in the solution
The greater the percent hydronium ions, the
more acidic the solution is
H2CO3 -------------> H+ + HCO3 -
CARBONIC ACID HYDRONIUM BICARBONATE
ION ION

 Buffers are substances that resist pH change
They accept H+ ions when they are in excess
They donate H+ ions when they are depleted
Buffering is not
foolproof
Example: acid
precipitation
normal rain water
(pH of 5-6)
pH of acid rain is
between 3-4

Buffers
 Dissolved CO2 in water acts as a buffer, a
substance that prevents large shifts in pH.
Buffers help keep pool
and spa water clean

Buffer Systems
CO2 + H2O  H2CO3  H+ + HCO3
- H+ + CO3
-2
H2CO3 is carbonic acid,
H+ is the hydronium ion
HCO3
- is the bicarbonate ion
CO3
-2 is the carbonate ion

Buffer Systems
CO2 + H2O  H2CO3  H+ + HCO3
-H+ + CO3
-2
 Adding CO2 shifts the reaction to the right and
produces more H+ ions making the water more
acid.

Buffer Systems
CO2 + H2O  H2CO3  H+ + HCO3
- H+ + CO3
-2
 Removing CO2 shifts the reaction to the left,
combining H+ ions with carbonate and
bicarbonate ions reducing the acidity.

AP Biology Properties of Water

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AP Biology Properties of Water