Dangerous Chemicals

http://www.dhmo.org/facts.html
Chapter 3
Water & the Fitness of
the Environment
Water supports all life!
Properties of Water
¾ of the Earth is covered in water
 Composition of cells = 70-95% water
 3 billion years before life on land
 All life on Earth requires water

Brainstorming

What are the properties of water that
make is so suitable for supporting life?

Polarity of water molecules results
in hydrogen bonding
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Water molecule=polar molecule
Water molecules form hydrogen bonds
These H-bonds are what gives water its unique
properties
LE 3-2
What are the 4 emergent
properties of water that contribute
to Earth’s fitness for life?
 I. Cohesion; adhesion; surface tension
 II. High specific heat
 III. Expansion upon freezing
 IV. Versatility as a solvent
Cohesion
Cohesion: Water molecules stick together
 Adhesion: Water molecules stick to other
substances

Water-conducting cells
LE 3-3
100 µm
Surface tensionhow hard it is to
break the surface
of a liquid
 Surface tension is
related to
cohesion

Moderation of Temperature
H2O absorbs heat from warmer air
 H2O releases heat from cooler air
 Water absorbs or releases a large amount
of heat with only a slight change in its
own temperature

Heat and Temperature


Heat- measure of the total amount of kinetic
energy due to molecular motion
Temperature measures intensity of heat due to
the average kinetic energy of molecules
Which has more heat?
An ocean (30°C) vs Blue whale (38°C)
A boiling kettle of water (100°C) vs an Iceberg (-20°C)
Water’s High Specific Heat

The specific heat of a substance is the
amount of heat that must be absorbed or
lost for 1 g of that substance to change its
temperature by 1ºC

Specific heat of water – 1 cal/g/°C

Water has an unusually high specific heat
◦ Heat is absorbed when H-bonds break
◦ Heat is released when H-bonds form
Brainstorm
Take a few minutes to write a list of
reasons for why the specific heat of water
is important for biological processes. Be
ready to explain your answer
Evaporative Cooling
Evaporation- liquid to gas
 Heat of vaporization- heat a liquid must
absorb for 1 g to be converted to gas
 Liquid evaporates  Remaining liquid
becomes cooler
 Evaporative cooling…

◦ Stabilizes temperature of bodies of water
◦ Helps organisms maintain body heat
Insulation of Bodies of Water by
Floating Ice

Frozen water is less dense than liquid
water (it floats)
◦ As the temperature of water decreases, hbonds are not broken as fast
 More hydrogen bonds  greater spacing
 The density of ice < liquid water

What would happen if ice sank?
Why is Water the Solvent of Life?
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Solution-liquid homogeneous mix of
substances
Solvent- dissolving agent of a solution
Solute- substance dissolved
Water is a versatile solvent due to its
polarity
An aqueous solution is one in which water
is the solvent
Water forms hydrogen bonds with
ionic solutes
Hydration shell 
Water can also dissolve:
Polar molecules
 Some proteins

(b) – (c) Lysozyme molecule in an aqueous environment.
(a) Lysozyme molecule
in a nonaqueous environment
.
What substances will NOT dissolve
in water?
Hydrophobic Substances
Hydrophobic substances are nonpolar
They do not dissolve in water (which is polar)
Hydrophilic substances are polar
They easily dissolve in water
“Like dissolves like”
Solute Concentration in Aqueous
Solutions
Chemical reactions depend on collisions of
molecules and therefore on the
concentration of solutes in an aqueous
solution.
 What is a mole?

Concentration
Molarity (M)- # of moles of solute per L of
solution
 Molecular mass - sum of all masses of all
atoms in a molecule
 Avogadro’s Number – the number of
atoms/molecules per mole.

◦ 6.02 x 10^23
How would you make 1 liter of a
0.5 M solution of NaCl?
How many moles of NaCl do you need?
 What is the molecular mass of NaCl?

◦ Na - ~23 g/mol
◦ Cl - ~35.5 g/mol

How many grams of NaCl do you need?
Extra Practice
How many molecules are in 60 grams of
NaCl?
 Determine the molarity of the solution:
100 g KCl dissolved into 0.75 L of
solution.
 How many grams of KCl are necessary to
get 0.5 L of a 0.10M solution?

Water acts as both an acid and a
base.
Water dissociates to form H+ (or H3O+)
and OH- ions.
 At equilibrium – 10^-7M
 Hydronium (H3O+) and hydroxide (OH-)
ions are extremely reactive.

Water is both an acid and a base.
Bronsted Acid – A substance that donates
a proton in an aqueous solution.
 Bronsted Base – Proton acceptor.
 Amphoteric – A substance that can act as
either an acid or a base.
 Amphiprotic – A substance that can either
donate or accept a proton.

Concentrations of acids and bases.
Concentrations of H+ and OH- are equal
in pure water
 Product is constant: [H+][OH-] = 10^-14
 Dissolving acids in aqueous solutions
increases the concentration of H+ ions.
 Bases  [OH-] > [H+]

◦ NaHO  Na+ and OH◦ NH3  NH4+ and OH-
Practice

What is [H+] if [OH-] = 10^-8M?
◦ [H+][OH-] = 10^-14
◦ 10^-14/[OH-] = [H+]
◦ 10^-14/10^-8 = [H+] = 10^-6M
pH is a measure of acidity
pH + pOH = 14
 pH = -log [H+]

◦ Acid- pH < 7
◦ Base- pH > 7

pOH = -log [OH-]
◦ Acid – pOH > 7
◦ Base – pOH < 7
pH practice

What is the pH of a 0.10M solution of
NaOH?
◦ NaOH  Na+ + OH-; determine the ratio of
moles.
◦ pOH = -log [OH-]
◦ pH = 14 - pOH
pH practice

What is the concentration of a solution of
HCl if the pH = 3?
What makes an acid strong?
Strong acids/bases dissociate completely
 Weak acids/bases dissociate until
equilibrium is reached
 Examples:

◦ Strong base – NaOH
◦ Weak base – NH3
pH Scale
0
Increasingly Acidic
[H+] > [OH–]
1
Neutral
[H+] = [OH–]
Battery acid
2 Digestive (stomach)
juice, lemon juice
3 Vinegar, beer, wine,
cola
4 Tomato juice
5 Black coffee
Rainwater
6 Urine
7 Pure water
Human blood
8
Seawater
Increasingly Basic
[H+] < [OH–]
9
10
Milk of magnesia
11
Household ammonia
12
Household bleach
13
Oven cleaner
14
LE 3-8
Effect of pH on living cells
Most biological liquids – 6 ≤ pH ≤ 8
 Internal pH of most cells ~7

Brainstorming: What would happen to a cell
if the internal pH was decreased by 1?
Why?
Buffers
The internal pH of most living cells must
remain close to pH 7
 Buffers help to minimize changes in
concentrations of H+ & OH- in a solution
 Buffer: a solution composed of a weak
acid and its conjugate base (or a weak
base and its conjugate acid)

The Threat of Acid Precipitation
Acid precipitation (ppt)- rain, snow, or fog
w/ a pH < 5.6; caused by mixing
pollutants w/ water in the air
 Acid precipitation can damage life in lakes
and streams
 Effects of acid precipitation on soil
chemistry are contributing to the decline
of some forests

How do buffers work?
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NH3(g)+H2O(l)→NH4+(aq)+OH−(aq)
NH3 = weak base
NH4+ = conjugate acid
When an acid is added to a buffer
solution, the buffer is ready to accept
protons (H+).
When a base is added, the buffer is ready
to donate protons.
Think about it…
Why can’t buffers be made with strong
acids/bases?
Le Chatelier’s Priciple

For a system that is at equilibrium,
changes in (concentration, temperature,
pressure, volume) will cause the reaction
to shift either forward or toward the
reverse reaction in order to counteract the
change.
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
More
acidic
Acid
rain
Normal
rain
More
basic
LE 3-9
Exam practice
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Buffers are substances that help resist
shifts in pH by
A) releasing H+ in acidic solutions.
B) donating H+ to a solution when they
have been depleted.
C) releasing OH- in basic solutions.
D) accepting H+ when the are in excess.
E) both B and D
Exam practice
Buffers are substances that minimize
changes in the concentrations of H+ and
OH- ions in a solution
 The answer is E

Exam practice

Which of the following is responsible for the
cohesive property of water?
◦ A. H-bonds b/w the oxygen atoms of 2 adjacent
water molecules
◦ B. covalent bonds b/w Hydrogen atoms of 2
adjacent water molecules
◦ C. H-bonds b/w the oxygen atom of 1 water
molecule and the hydrogen atom of another
◦ D. covalent bonds b/w O atom of 1 water molecule
& H atom of another
◦ E. H-bonds b/w water molecules and other types of
molecules
Exam practice
The cohesive property of water is due to
the hydrogen bonding between water
molecules, specifically the hydrogen
bonds b/w the oxygen atom of one water
molecule and the hydrogen atom of
another water molecule
 The answer is C

Your pH investigation lab outcome should look like this