1. CHAPTER 2 The Chemical Basis of Life
Modules 2.9 – 2.17

2. 2.9 Water is a polar molecule
THE PROPERTIES OF WATER
2.9 Water is a polar molecule
Atoms in a covalently bonded molecule may share electrons equally, creating a nonpolar molecule
If electrons are shared unequally, a polar molecule is created

3. In a water molecule, oxygen exerts a stronger pull on the shared electrons than hydrogen
This makes the oxygen end of the molecule slightly negatively charged
The hydrogen end of the molecule is slightly positively charged
Water is therefore a polar molecule
(–)
(–) O H H
(+)
(+)
Figure 2.9

4. 2.10 Overview: Water’s polarity leads to hydrogen bonding and other unusual properties
The charged regions on water molecules are attracted to the oppositely charged regions on nearby molecules
This attraction forms weak bonds called hydrogen bonds
Hydrogen bond
Figure 2.10A

5. Like no other common substance, water exists in nature in all three physical states:
as a solid
as a liquid
as a gas
Figure 2.10B

6. 2.11 Hydrogen bonds make liquid water cohesive
Due to hydrogen bonding, water molecules can move from a plant’s roots to its leaves
Insects can walk on water due to surface tension created by cohesive water molecules
Figure 2.11

7. 2.12 Water’s hydrogen bonds moderate temperature
It takes a lot of energy to disrupt hydrogen bonds
Therefore water is able to absorb a great deal of heat energy without a large increase in temperature
As water cools, a slight drop in temperature releases a large amount of heat

8. This leads to evaporative cooling
A water molecule takes a large amount of energy with it when it evaporates
This leads to evaporative cooling
Figure 2.12

9. 2.13 Ice is less dense than liquid water
Molecules in ice are farther apart than those in liquid water
Hydrogen bond
ICE
Hydrogen bonds are stable
LIQUID WATER
Hydrogen bonds constantly break and re-form
Figure 2.13

10. Ice is therefore less dense than liquid water, which causes it to float
If ice sank, it would seldom have a chance to thaw
Ponds, lakes, and oceans would eventually freeze solid

11. 2.14 Water is a versatile solvent
Solutes whose charges or polarity allow them to stick to water molecules dissolve in water
They form aqueous solutions
Na+ – –
Na+ + +
Cl–
Cl– – – + + –
Ions in solution
Salt crystal
Figure 2.14

12. 2.15 The chemistry of life is sensitive to acidic and basic conditions
A compound that releases H+ ions in solution is an acid, and one that accepts H+ ions in solution is a base
Acidity is measured on the pH scale:
0-7 is acidic
8-14 is basic
Pure water and solutions that are neither basic nor acidic are neutral, with a pH of 7

13. (Higher concentration of H+) (Lower concentration of H+)
pH scale
The pH scale H+
OH–
Lemon juice; gastric juice
(Higher concentration of H+)
Increasingly ACIDIC
Grapefruit juice
Acidic solution
Tomato juice
Urine
NEUTRAL
[H+] = [OH–]
PURE WATER
Human blood
Seawater
Neutral solution
(Lower concentration of H+)
Increasingly BASIC
Milk of magnesia
Household ammonia
Household bleach
Oven cleaner
Basic solution
Figure 2.15

14. Cells are kept close to pH 7 by buffers
Buffers are substances that resist pH change
They accept H+ ions when they are in excess and donate H+ ions when they are depleted
Buffers are not foolproof

15. 2.16 Connection: Acid precipitation threatens the environment
Some ecosystems are threatened by acid precipitation
Acid precipitation is formed when air pollutants from burning fossil fuels combine with water vapor in the air to form sulfuric and nitric acids
Figure 2.16A

16. These acids can kill fish, damage buildings, and injure trees
Regulations, new technology, and energy conservation may help us reduce acid precipitation
Figure 2.16B

17. 2.17 Chemical reactions rearrange matter
REARRANGEMENTS OF ATOMS
2.17 Chemical reactions rearrange matter
In a chemical reaction:
reactants interact
atoms rearrange
products result
Figure 2.17A
2 H2 + O2 
2 H2O

18. Living cells carry out thousands of chemical reactions that rearrange matter in significant ways
Beta-carotene
Vitamin A
(2 molecules)
Figure 2.17B