1. 2
The Chemical Context of Life

2. Water: The Solvent of Life
A solution is a liquid that is a homogeneous mixture of substances
A solvent is the dissolving agent of a solution
The solute is the substance that is dissolved
An aqueous solution is one in which water is the solvent
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3. Water is a versatile solvent due to its polarity, which allows it to form hydrogen bonds easily
When an ionic compound is dissolved in water, each ion is surrounded by a sphere of water molecules called a hydration shell
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4. Na Cl Cl Na Figure 2.21 Figure 2.21 Table salt dissolving in water
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5. Water can also dissolve compounds made of nonionic polar molecules
Even large polar molecules such as proteins can dissolve in water if they have ionic and polar regions
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6.  - -  Figure 2.22 Figure 2.22 A water-soluble protein
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7. Hydrophilic and Hydrophobic Substances
A hydrophilic substance is one that has an affinity for water
A hydrophobic substance is one that does not have an affinity for water
Oil molecules are hydrophobic because they have relatively nonpolar covalent bonds
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8. Solute Concentration in Aqueous Solutions
Most chemical reactions in organisms involve solutes dissolved in water
Chemical reactions depend on the concentration of solutes, or the number of molecules in a volume of an aqueous solution
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9. Molecular mass is the sum of all masses of all atoms in a molecule
Numbers of molecules are usually measured in moles, where 1 mole (mol)  6.02  1023 molecules
Avogadro’s number and the unit dalton were defined such that 6.02  1023 daltons  1 g
Molarity (M) is the number of moles of solute per liter of solution
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10. Acids and Bases
Sometimes a hydrogen ion (H) is transferred from one water molecule to another, leaving behind a hydroxide ion (OH−)
The proton (H) binds to the other water molecule, forming a hydronium ion (H3O)
By convention, H is used to represent the hydronium ion
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11. H H O O O H H  O H H H H 2 H2O Hydronium ion (H3O) Hydroxide
Figure 2.UN03 H H O O O H H  O H H H H
2 H2O
Hydronium
ion (H3O)
Hydroxide
ion (OH)
Figure 2.UN03 In-text figure, water dissociation, p. 37
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12. H and OH− are very reactive
Though water dissociation is rare and reversible, it is important in the chemistry of life
H and OH− are very reactive
Solutes called acids and bases disrupt the balance between H and OH− in pure water
Acids increase the H concentration in water, while bases reduce the concentration of H
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13. This is a reversible reaction, as shown by the double arrows:
A strong acid like hydrochloric acid, HCl, dissociates completely into H and Cl− in water:
HCl → H + Cl−
Ammonia, NH3, acts as a relatively weak base when it attracts a hydrogen ion from the solution and forms ammonium, NH4
This is a reversible reaction, as shown by the double arrows:
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14. The hydroxide ions then combine with hydrogen ions to form water
Sodium hydroxide, NaOH, acts as a strong base indirectly by dissociating completely to form hydroxide ions:
NaOH → Na  OH−
The hydroxide ions then combine with hydrogen ions to form water
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15. Weak acids act reversibly and accept back hydrogen ions
Carbonic acid, H2CO3, acts as a weak acid:
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16. The pH Scale
In any aqueous solution at 25C, the product of H and OH− is constant and can be written as
[H+][OH] = 1014
The pH of a solution is defined as the negative logarithm of H concentration, written as
pH = log [H+]
For a neutral aqueous solution, [H] is 107 M, so
log [H]  (7)  7
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17. Acidic solutions have pH values less than 7
Basic solutions have pH values greater than 7
Most biological fluids have pH values in the range of 6 to 8
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18. 2 Gastric juice, lemon juice
Figure 2.23
pH Scale 1
Battery acid
2 Gastric juice, lemon juice H H H
OH- H 3
Vinegar, wine,
cola
OH- H
Increasingly Acidic
[H] [OH_] H H H
Acidic
solution 4
Tomato juice
Beer 5
Black coffee
Rainwater 6
Urine
Saliva
OH-
OH-
Neutral
[H] [OH_]
OH- 7
Pure water
Human blood, tears H H
OH-
OH- H H H 8
Seawater
Inside of small intestine
Neutral
solution 9
Figure 2.23 The pH scale and pH values of some aqueous solutions 10
Increasingly Basic
[H] [OH_]
Milk of magnesia
OH-
OH- 11
OH- H
OH-
Household ammonia
OH-
OH- H
OH- 12
Basic
solution
Household
bleach 13
Oven cleaner 14
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19. H+ OH OH OH OH H+ H+ H+ OH OH OH H+ OH H+ H+ OH OH H+ H+ OH
Figure H+
OH
OH
OH
OH H+ H+ H+
OH
OH
OH H+
OH H+ H+
OH
OH H+ H+
OH
OH
OH H+
OH H+ H+ H+ H+ H+
Figure The pH scale and pH values of some aqueous solutions (part 1: ions)
Basic
solution
Neutral
solution
Acidic
solution
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20. Buffers The internal pH of most living cells must remain close to pH 7
Buffers are substances that minimize changes in concentrations of H and OH− in a solution
Most buffer solutions contain a weak acid and its corresponding base, which combine reversibly with H
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21. Carbonic acid is a buffer that contributes to pH stability in human blood:
Response to a rise in PH
H+ donor Response to H+ acceptor Hydrogen (acid) a drop in pH (base) ion
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22. Acidification: A Threat to Our Oceans
Human activities such as burning fossil fuels threaten water quality
CO2 is a product of fossil fuel combustion
About 25% of human-generated CO2 is absorbed by the oceans
CO2 dissolved in seawater forms carbonic acid; this causes ocean acidification
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23. As seawater acidifies, hydrogen ions combine with carbonate ions to form bicarbonate ions (HCO3–)
It is predicted that carbonate ion concentrations will decline by 40% by the year 2100
This is a concern because organisms that build coral reefs or shells require carbonate ions
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24. CO2 CO2  H2O H2CO3  H2CO3 H  HCO3  H  CO32 HCO3 
Figure 2.24
CO2
CO2  H2O H2CO3 
H2CO H  HCO3 
H  CO32 HCO3 
Figure 2.24 Atmospheric CO2 from human activities and its fate in the ocean
CO32  Ca2 CaCO3 
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25. Figure 2.UN01 In-text figure, Van der Waals interactions, p. 30
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26. [mmol CaCO3/(m2  day)] Calcification rate
Figure 2.UN04-1 20
[mmol CaCO3/(m2  day)]
Calcification rate 10
Figure 2.UN04-1 Scientific skills: interpreting a scatter plot with a regression line (part 1)
220
240
260
280
[CO32] (mol/kg of seawater)
Data from C. Langdon et al., Effect of calcium carbonate saturation state on
the calcification rate of an experimental coral reef, Global Biogeochemical Cycles
14:639–654 (2000).
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27. Figure 2.UN04-2
Figure 2.UN04-2 Scientific skills: interpreting a scatter plot with a regression line (part 2)
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28. Nucleus Protons ( charge) determine element Electrons ( charge)
Figure 2.UN05
Nucleus
Protons ( charge)
determine element
Electrons ( charge)
form negative cloud
and determine
chemical behavior
Neutrons (no charge)
determine isotope
Figure 2.UN05 Summary of key concepts: atom components
Atom
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29.  + H O  + H  +  + Figure 2.UN06
Figure 2.UN06 Summary of key concepts: hydrogen bonds  +
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30. Ice: stable hydro- Liquid water: gen bonds transient hydrogen bonds
Figure 2.UN07
Figure 2.UN07 Summary of key concepts: ice and liquid water
Ice: stable hydro-
gen bonds
Liquid water:
transient hydrogen
bonds
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31. Acidic [H]  [OH] Acids donate H in aqueous solutions. Neutral
Figure 2.UN08
Acidic
[H]  [OH]
Acids donate H in
aqueous solutions.
Neutral
[H]  [OH] 7
Figure 2.UN08 Summary of key concepts: pH scale
Bases donate OH
or accept H in
aqueous solutions.
Basic
[H]  [OH] 14
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32. Figure 2.UN09
Figure 2.UN09 Test your understanding, question 11 (female luna moth, Actias luna)
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33. Figure 2.UN10 Test your understanding, question 14 (how cats drink)
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