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Figure 3-01
Water is Central to Life
• Water has several properties that life is dependent upon including:
• Cohesion• High heat of vaporization• High specific heat• Less dense as a solid than a liquid • A “universal solvent”• All of these properties are due to the polar
nature of water molecules and hydrogen bonding
LE 3-3
Water-conducting cells
100 µm
Figure 3-04
LE 3-6
Na+
Na+
Cl–Cl–
+
+
+
+
+
+
++
–
–
–
–
–
–
–
–
–
–
–
Dissociation
• Many molecules will dissociate when the are placed in water
• Ions are separated and “released” into the water environment
• Table salts dissolves in water (dissociates into sodium and chlorine ions)
Acids
• When hydrogen chloride is placed in water it dissociates into Hydrogen ions (H+) and chlorine ions (Cl-)
• HCl is an acid
• Acids are substance that release H+ ions (protons) when they dissociate in water
• The more acidic a solution, the higher concentration of H+ ions (protons) it has.
Bases
• When sodium hydroxide is placed in water it dissociates into sodium ions (Na+) and hydroxide ions (OH-)
• NaOH is a base
• Bases are substance that release OH- ions when they dissociate in water
• The more acidic a solution, the higher concentration of OH- ions it has.
Water Dissociates
• Water molecules are not completely stable
• They spontaneously dissociate (at a very slow rate) into H+ and OH- ions.
• At any given moment only 1/554,000,000 water molecules in pure water dissociates
LE 3-UN53
Hydroniumion (H3O+)
Hydroxideion (OH–)
Spontaneous Dissociation of Water
• Is water and acid? Is it a base?
• Water is amphoteric
• At equilibrium, the [H+] concentration is 1X10-7 M.
• At equilibrium, the [OH-] concentration is 1X10-7 M.
pH Scale
• pH is the –log [H+]
• The pH scale measure the acidity (or basicity) of an aqueous solution
• The pH scale range from 0-14
• The lower the pH, the higher the [H+]
• The higher the pH the lower the [H+]
• A pH of 7 is considered to be neutral
Ph Scale
LE 3-8pH Scale
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14Oven cleaner
Household bleach
Household ammonia
Milk of magnesia
Seawater
Pure waterHuman blood
UrineRainwater
Black coffee
Tomato juice
Vinegar, beer, wine,cola
Digestive (stomach)juice, lemon juice
Battery acid
Neutral[H+] = [OH–]
Inc
rea
sin
gly
Ac
idic
[H+]
> [
OH
–]
Inc
rea
sin
gly
Ba
sic
[H+]
< [
OH
– ]
The importance of pH to life
• Living things typically exist is an aqueous environment
• The pH of the environment influences the charge of molecules present in that environment
• The charge of molecules influences their shape (structure)
• The shape of molecules determines their function
Case study-pH and Blood
• The normal pH of blood is about 7.4
• What are the consequences of a change in that pH?
• Effect on hemoglobin?
• Why is hemoglobin affected by a change in pH?
• What homeostatic mechanisms does the blood have to keep its pH constant?
Buffering Systems