Chapter 3: What’s so great about water?
Copyright  2010 Scott A. Bowling
Life as we know it requires water:

all organisms mostly liquid water

most metabolism requires aqueous (water) medium

many organisms live in liquid water or in an
environment dominated by water in its various
states (solid, liquid, or gas)
.
Chapter 3: What’s so great about water?
Copyright  2010 Scott A. Bowling
Some numbers:

cells are typically 70% or more water by mass

about 75% of the Earth’s surface is covered by
liquid water
.
Chapter 3: What’s so great about water?
Copyright  2010 Scott A. Bowling
Some numbers:

cells are typically 70% or more water by mass

about 75% of the Earth’s surface is covered by
liquid water
But then, just being common on the Earth doesn’t make
something essential for life. A large percentage of
the Earth’s crust is sand, but we don’t consider
sand a requirement for life.
.
Chapter 3: What’s so great about water?
Copyright  2010 Scott A. Bowling
Some numbers:

cells are typically 70% or more water by mass

about 75% of the Earth’s surface is covered by
liquid water
But then, just being common on Earth doesn’t make
something essential for life. A large percentage of
the Earth’s crust is sand, but we don’t consider
sand a requirement for life.
What is it about water that makes it so special?
.
Copyright  2010 Scott A. Bowling
Chapter 3: What’s so great about water?

polar nature of water molecules

What properties of water are important
for life?

Acids and Bases

Some useful definitions
.
Copyright  2010 Scott A. Bowling
Chapter 3: What’s so great about water?

polar nature of water molecules

What properties of water are important
for life?

Acids and Bases

Some useful definitions
.
Copyright  2010 Scott A. Bowling
•
Draw a water molecule (structural
formula)
•
Then draw in four more around it that
are connected to it by hydrogen bonds.
.
polar nature of water molecules
oxygen atoms are electron seeking
(electronegative), especially compared to hydrogen;
thus for an oxygen-hydrogen bond:
Copyright  2010 Scott A. Bowling


the oxygen atom has a partial (-) charge

the hydrogen atoms have a partial (+) charge
.
Copyright  2010 Scott A. Bowling
polar nature of water molecules
the polar character of water allows water molecules to form up to 4 hydrogen bonds
.
Copyright  2010 Scott A. Bowling
•
Draw a water molecule (structural
formula)
•
Then draw in four more around it that
are connected to it by hydrogen bonds.
.
Copyright  2010 Scott A. Bowling
Chapter 3: What’s so great about water?

polar nature of water molecules

What properties of water are important
for life?

Acids and Bases

Some useful definitions
.
Copyright  2010 Scott A. Bowling
•
List and describe at least four properties
of water that result from its polar
nature/hydrogen bonds.
•
Describe how water acts as a
temperature buffer (creates temperature
stability).
.
What properties of water are
important for life?
all of this come in some way from
water’s polar nature  hydrogen bonds
and similar interactions
Copyright  2010 Scott A. Bowling


water is the principal solvent in living things

water exhibits both cohesive and adhesive
forces

water helps maintain a stable temperature

ice (solid water) floats in liquid water
.
water is the principal solvent in living things
Copyright  2010 Scott A. Bowling

highly polar = excellent solvent for other polar
substances, and for ionic compounds
.
Copyright  2010 Scott A. Bowling
water is the principal solvent in living things

hydrophilic substances – interact readily
with water

hydrophobic substances – do not interact
readily with water

nonpolar substances

good components for membranes
.
Copyright  2010 Scott A. Bowling
water exhibits both cohesive and adhesive forces

cohesive forces: attraction of water molecules each other

give water a high surface tension
.
water exhibits both cohesive and adhesive forces
adhesive forces: water molecules to be attracted to
other kinds of molecules
Copyright  2010 Scott A. Bowling


how things are made wet
capillary action: water moving through narrow tubes
even against gravity


results from cohesion and adhesion

living organisms take advantage of this
.
water helps maintain a stable temperature
high specific heat of water  temp. stability
Copyright  2010 Scott A. Bowling


specific heat – energy to raise the temp.
of 1 gram of something 1ºC

h-bonds make water specific heat high


1 calorie / gram ºC
comparatively, takes more energy
gain/loss to change water temp.
.
water helps maintain a stable temperature
high specific heat of water  temp. stability
Copyright  2010 Scott A. Bowling


much of the ecosphere is water

most biological organisms >70% water

temperature stability is critical for most life
.
water helps maintain a stable temperature
high heat of vaporization of water helps cool
the ecosphere and biological organisms
Copyright  2010 Scott A. Bowling


heat of vaporization: energy to move 1 gram from
liquid to gas
.
water helps maintain a stable temperature
high heat of vaporization of water helps cool
the ecosphere and biological organisms
Copyright  2010 Scott A. Bowling


heat of vaporization: energy to move 1 gram from
liquid to gas

h-bonds make water specific heat high

540 calories / gram
.
water helps maintain a stable temperature
high heat of vaporization of water helps cool
the ecosphere and biological organisms
Copyright  2010 Scott A. Bowling


heat of vaporization: energy to move 1 gram from
liquid to gas

h-bonds make water specific heat high

540 calories / gram
organisms use this for cooling


examples: sweating; evaporative cooling of a leaf
.
Describe how water acts as a
temperature buffer (creates temperature
stability).
Copyright  2010 Scott A. Bowling
•
.
.
Copyright  2010 Scott A. Bowling
ice floats
Copyright  2010 Scott A. Bowling

liquid water becomes denser as it cools –
but only up to a point
.
ice floats
Copyright  2010 Scott A. Bowling

liquid water becomes denser as it cools –
but only up to a point
at 4ºC:

water begins to expand as it cools further
– that is, it gets less dense from then on –
due to hydrogen bonds becoming locked
in place
.
Copyright  2010 Scott A. Bowling
ice floats
at 0ºC:

ice freezes into a crystal
.
Copyright  2010 Scott A. Bowling
ice floats
at 0ºC:

ice freezes into a crystal

based on the hydrogen bonds
.
Copyright  2010 Scott A. Bowling
ice floats
at 0ºC:

ice freezes into a crystal

based on the hydrogen bonds

floating ice keeps lakes, etc., from freezing
solid and is important for temperature
cycling on the planet
.
List and describe at least four properties
of water that result from its polar
nature/hydrogen bonds.
Copyright  2010 Scott A. Bowling
•
.
Copyright  2010 Scott A. Bowling
Chapter 3: What’s so great about water?

polar nature of water molecules

What properties of water are important
for life?

Acids and Bases

Some useful definitions
.
Copyright  2010 Scott A. Bowling
•
Define acids and bases.
.
Acids and Bases
acids are proton donors
Copyright  2010 Scott A. Bowling

.
Acids and Bases
acids are proton donors
Copyright  2010 Scott A. Bowling


dissociates to yield hydrogen ions (H+) in solution
HA (an acid)
H+ + A- (an anion)
.
Acids and Bases
acids are proton donors
Copyright  2010 Scott A. Bowling


dissociates to yield hydrogen ions (H+) in solution
HA (an acid)

H+ + A- (an anion)
H+ = one proton (mostly)
.
Acids and Bases
acids are proton donors
Copyright  2010 Scott A. Bowling

dissociates to yield hydrogen ions (H+) in solution

HA (an acid)
H+ + A- (an anion)
H+ = one proton (mostly)


when the atom loses its electron to become a
hydrogen ion, all that remains is the nucleus
.
Acids and Bases
acids are proton donors
Copyright  2010 Scott A. Bowling

dissociates to yield hydrogen ions (H+) in solution

HA (an acid)
H+ + A- (an anion)
H+ = one proton (mostly)


when the atom loses its electron to become a
hydrogen ion, all that remains is the nucleus

thus, hydrogen ions are sometimes referred to
as protons
.
Acids and Bases
acids are proton donors
Copyright  2010 Scott A. Bowling

dissociates to yield hydrogen ions (H+) in solution

HA (an acid)
H+ + A- (an anion)
H+ = one proton (mostly)


when the atom loses its electron to become a
hydrogen ion, all that remains is the nucleus

thus, hydrogen ions are sometimes referred to
as protons

therefore, any substance that yields a proton is
an acid, or an acid is a proton donor
.
Acids and Bases
bases are proton acceptors
Copyright  2010 Scott A. Bowling


a base is a substance that can accept a proton
.
Acids and Bases
bases are proton acceptors
Copyright  2010 Scott A. Bowling


a base is a substance that can accept a proton

bases either dissociate in water to produce
hydroxide ions and a cation, or split water to form
a cation and hydroxide ion:
Na+ + OH-
NaOH
or
B (a base) + HOH
BH+ + OH-
.
Acids and Bases
Copyright  2010 Scott A. Bowling

water tends to slightly dissociate into hydrogen and hydroxide ions (H+
and OH-)
HOH
H+ + OH-
.
Acids and Bases
water tends to slightly dissociate into hydrogen and hydroxide ions (H+
and OH-)
Copyright  2010 Scott A. Bowling

HOH
H+ + OH-
in pure water, the concentrations of these ions are equal:

[H+] = [OH-] = 10-7 M
(note that the designation M stands for molar, the moles of a substance per liter of solution)
the product of these remains constant: [H+] x [OH-] = 10-14


acidic solutions have an elevated [H+], and thus reduced [OH-]

basic solutions have an elevated [OH-], and thus reduced [H+]
.
Copyright  2010 Scott A. Bowling
•
Define acids and bases.
.
What does pH stand for, and how does
the pH scale work?
Copyright  2010 Scott A. Bowling
•
.
Acids and Bases
pH scale shorthand notation for
proton concentration of a solution
Copyright  2010 Scott A. Bowling


the pH is -log[H+]

pure water ( [H+]= 10-7 M ) has
pH = 7

pH < 7 is acidic; pH > 7 is basic

pH of most living cells is usually
~ 7.2 to 7.4
.
What does pH stand for, and how does
the pH scale work?
Copyright  2010 Scott A. Bowling
•
.
Copyright  2010 Scott A. Bowling
•
How do pH buffers work?
.
Acids and Bases
buffers minimize pH changes
Copyright  2010 Scott A. Bowling


weak acids and weak bases serve as buffers

living things use buffers to prevent dramatic changes in pH,
which can kill them
.
Acids and Bases
buffers minimize pH changes
Copyright  2010 Scott A. Bowling


weak acids and weak bases serve as buffers

living things use buffers to prevent dramatic changes in pH,
which can kill them
.
02.20 Buffer Formation
Slide number: 2
+
H2O
Water
CO2
Carbon dioxide
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
02.20 Buffer Formation
Slide number: 3
+
H2O
Water
CO2
Carbon dioxide
H2CO3
Carbonic acid
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
02.20 Buffer Formation
Slide number: 4
–
+
+
H2O
Water
CO2
Carbon dioxide
H2CO3
Carbonic acid
HCO3–
Bicarbonate
ion
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
+
H+
Hydrogen
ion
Fig. 2.19
Copyright  2010 Scott A. Bowling
•
How do pH buffers work?
.
Copyright  2010 Scott A. Bowling
Chapter 3: What’s so great about water?

polar nature of water molecules

What properties of water are important
for life?

Acids and Bases

Some useful definitions
.
Copyright  2010 Scott A. Bowling
Some useful definitions

solvent – a liquid into which a
substance dissolves

solute – the dissolved substance

solution = solvent + solute

salts – form from acids and bases

water is formed

the cation of the base and the anion of the
acid form the salt
HCl + NaOH
NaCl + HOH
.
Some useful definitions
electrolytes are salts, acids, or bases that form ions
in water and thus can conduct an electrical current
when dissolved in water (pure water is a poor
conductor of electricity, but put in a salt and it
becomes an excellent conductor)
nonelectrolytes are substances like sugar that
dissolve in water but do not become ionic
mixtures - a mixture of 2 or more elements and/or
compounds; they can be broken down into elements
and compounds by simple physical means. There
are two types:
Copyright  2010 Scott A. Bowling





heterogeneous mixtures - mixtures that are not of uniform
composition throughout - a living organism is a good
example
homogeneous mixtures - mixtures that are completely
uniform throughout - a salt water solution is a good example
.