Post on 21-Dec-2015
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Chapter 3 – Water
• Three-quarters of the Earth’s surface is submerged in water
• The abundance of water is the main reason the Earth is habitable
Figure 3.1
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Chapter 3 – Water
• Overview: The Molecule That Supports All of Life
– Water is the biological medium here on Earth
– All living organisms require water more than any other substance
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Polarity of Water Molecules
• Water is unique because of its structure and the interaction of its molecules
• Water is a polar molecule because oxygen is more electronegative than hydrogen
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
• The polarity of water molecules
– Allows them to form hydrogen bonds with each other
– Contributes to the various properties water exhibits
Hydrogenbonds
+
+
H
H+
+
–
–
– –
Figure 3.2
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Polarity of Water
• The hydrogen bonds between water molecules give water 4 unique properties:
1) Water’s cohesive behavior
2) Water’s ability to moderate temperature
3) Water’s expansion on freezing
4) Water’s versatility as a solvent
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Cohesion
Cohesion
– Is the bonding of a high percentage of the molecules to neighboring molecules
Water molecules pull each other along
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Cohesion
• Cohesion
– Helps pull water up through the microscopic vessels of plants
Water conducting cells
100 µmFigure 3.3
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Cohesion
• Adhesion is related to cohesion
• Adhesion – the clinging of one substance to another
Water sticks to the walls of cells, which helps it resist gravity
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Cohesion
• Surface tension
– Is a measure of how hard it is to break the surface of a liquid
Figure 3.4
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Moderation of Temperature
• Water moderates air temperature
– By absorbing heat from air that is warmer and releasing the stored heat to air that is cooler
This is one reason why Cape Cod has milder winters than Worcester County
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Moderation of Temperature
• Water has high specific heat
– That is, it takes a lot of energy to raise the temperature of water
Why the ocean is still cold here in the summer
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Moderation of Heat
Fast moving molecules can leave a liquid as gas
• Evaporative cooling – when the “hottest” or fastest moving molecules leave a liquid, thus cooling the remaining liquid
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Moderation of Heat
•Water’s evaporative cooling property helps stabilize the temperature of lakes and ponds and allows for the mechanism of temperature regulation in living things too
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Expansion of Water
• Water is less dense as a solid than as a liquid
– This is very rare!
• Water expands as it solidifies (freezes)
• As a result, ice floats
This plays an important role in Life
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Expansion of Water
• The hydrogen bonds in ice
– Are more “ordered” than in liquid water, making ice less dense
– The molecules are further apart in ice than liquid water
Liquid water
Hydrogen bonds constantly break and re-form
Ice
Hydrogen bonds are stable
Hydrogen bond
Figure 3.5
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Expansion of Water
Since ice floats in water
– Life can exist under the frozen surfaces of lakes and polar seas
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At what temperature is water the densest?
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
The Solvent of Life
Solution – a liquid that is completely homogenous mixture of two or more substances
Solvent – the dissolving agent of a solution
Solute – the substance dissolved
Aqueous solution – solution in which water is the solvent
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
The Solvent of Life
• Water is a versatile solvent due to its polarity
• If an ionic compound (NaCl) is placed in water, the ions exposed at the surface are attracted to the polarity of the water molecules
Result:
Water molecules separate individual ions and surround them, effectively “breaking up” the salt
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
The Solvent of Life
• The different regions of the polar water molecule can interact with ionic compounds called solutes and dissolve them
Negative
oxygen regions
of polar water molecules
are attracted to sodium
cations (Na+).
+
+
+
+Cl –
–
–
–
–
Na+Positive hydrogen regions
of water molecules cling to chloride anions
(Cl–).
++
+
+
–
–
–
–
–
–Na+
Cl–
Figure 3.6
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
The Solvent of Life
• Compounds don’t have to be ionic to be affected
• Water can also interact with polar molecules such as proteins or sugars
This oxygen is
attracted to a slight
positive charge on
the lysozyme
molecule.This oxygen is attracted to a slight
negative charge on the lysozyme
molecule.(a) Lysozyme molecule
in a nonaqueous
environment
(b) Lysozyme molecule (purple)
in an aqueous environment
such as tears or saliva
(c) Ionic and polar regions on the protein’s
Surface attract water molecules.
+
–
Figure 3.7
These compounds are still water soluble
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
The Solvent of Life
• A hydrophilic substance
– Has an affinity for water
• Anything ionic
• Polar molecule
• Substance doesn’t have to dissolve in water
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The Solvent of Life
• A hydrophobic substance
– Does not have an affinity for water
– Non-ionic and non-polar substances
– Seem to repel water
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Acids, Bases and pH
Polar water molecules don’t just break up ionic and polar compounds
• Water can also disassociate itself
H
Hydroniumion (H3O+)
H
Hydroxideion (OH–)
H
H
H
H
H
H
+ –
+
Figure on p. 53 of water dissociating
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
• Water can dissociate into
– Hydrogen ions – single protons with a charge of 1+
– Hydroxide ions – charged with a 1-
• This reaction is reversible and will reach a state of dynamic equilibrium
– When the water dissociates at the same rate that it is being re-formed
H
Hydroniumion (H3O+)
H
Hydroxideion (OH–)
H
H
H
H
H
H
+ –
+
Figure on p. 53 of water dissociating
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
In pure water there is 1 molecule disassociated for every 554 million molecules of water.
Despite its rarity, disassociated water molecules are extremely important to life
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
Certain solutes can alter the concentration of these ions
• An acid
– Is any substance that increases the hydrogen ion concentration of a solution
• A base
– Is any substance that reduces the hydrogen ion concentration of a solution
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
• Acids and bases that dissolve completely in water are considered strong
• Weak acids and bases bind reversibly to H+ or OH- and will eventually reach a state of equilibrium
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
• The pH of a solution
– Is determined by the relative concentration of hydrogen ions
– Is low in an acid
– Is high in a base
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
• pH scale ranges from 0 – 14
– Less than 7 is acidic
• The lower the number the stronger the acid
– 7 is neutral
– Greater than 7 is basic (alkaline)
• The higher the number the stronger the base
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
• The pH scale and pH values of various aqueous solutions
Incr
easi
ngly
Aci
dic
[H+]
> [
OH
–]
Incr
easi
ngly
Bas
ic[H
+]
< [
OH
–]
Neutral[H+] = [OH–]
Oven cleaner
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
pH Scale
Battery acid
Digestive (stomach) juice, lemon juiceVinegar, beer, wine,colaTomato juice
Black coffee RainwaterUrine
Pure waterHuman blood
Seawater
Milk of magnesia
Household ammonia
Household bleach
Figure 3.8
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
• The pH scale is a logarithmic scale
• Therefore, a solution with a pH of 3 isn’t twice as acidic as one with a pH of 6, it is 1000 times more acidic!
If a small change in the number of H+ has such a big change on the pH level, how can
concentrations be maintained?
Copyright © 2006 Cynthia Garrard publishing under Canyon Design
Acids, Bases and pH
• Buffers
– Are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution
– Consist of an acid-base pair that reversibly combines with hydrogen ions
– Works by accepting H+ from the solution when they are in excess and donating H+ to the solution when they have been depleted