Notes to Instructors
Chapter 2 The Chemical Context of Life
Chapter 3 Water and the Fitness of the Environment
What is the focus of these activities?
Living organisms function in the real world, so they are subject to all the laws of chemistry
and physics. In addition, biological organisms and systems are variable. No two organisms are
What are the particular activities designed to do?
Activity 2.1 A Quick Review of Elements and Compounds
The questions in this activity are designed to help students review and understand:
• atomic/molecular number, mass number, and atomic/molecular weight and how
Activity 3.1 A Quick Review of the Properties of Water
The questions in this activity are designed to help students review and understand the
properties of water and how they support life. Students are asked to review these key
properties:
•H
2O molecules are cohesive; they form hydrogen bonds with each other.
Notes to Instructors 1
In addition, students review pH and how it is related to both the ionization constant of
pure water and the concentration of H+ions in a solution.
What misconceptions or difficulties can these activities reveal?
Activity 2.1
Question 1: Many students don’t understand that nutrients for plants are inorganic and
most nutrients for animals (heterotrophs) are organic.
Questions 2 and 3: Most students know how to balance a chemical equation. Fewer
Question 4: Some students have difficulty understanding that a solution’s concentration
or molarity does not change if you aliquot or subdivide the solution into smaller volumes.
To test this, ask your students: “There is 10% sugar in this solution. If I pour half of it
into one beaker and the other half into another beaker, what percent sugar will I have in
each beaker?” More than half of the students will automatically answer 5%.
Activity 3.1
Most students have no difficulty stating the properties of water and the definition of pH.
On the other hand, not all of them have a good understanding of how these properties are
related to biological and other phenomena. Therefore, some questions ask students to
relate pH values to actual concentrations of H+ions in solution and to relate the properties
of water to common experiences they have had in class or in life.
2 Notes to Instructors
Answers
Activity 2.1 A Quick Review of Elements and Compounds
1. Table 2.1 (page 32) lists the chemical elements that occur naturally in the human
body. Similar percentages of these elements are found in most living organisms.
Activity 2.1 3
a. In what abiotic
(nonlife) chemical
forms are these
elements often found
in nature?
These elements are
b. In what chemical
form(s) do animals
need to obtain these
elements?
With the exception of
oxygen and water,
c. In what chemical
form(s) do plants need
to obtain these
elements?
Plants can obtain C as
CO2, N as ammonia or
2. A chemical element cannot be broken down to other forms by chemical reactions.
Each element has a specific number of protons, neutrons, and electrons.
a. What is the name of the following element, and how many protons, neutrons, and
electrons does it have?
Na
11
23
Name
Sodium
Number of protons
11
Number of electrons
11
Number of neutrons
12
c. How would you determine how many grams are in a mole of any chemical
element or compound?
A mole of any chemical element or compound is equal to the mass number in
4. One atom of Na can combine with one atom of Cl (chlorine) to produce one
molecule of NaCl (table salt).
4Activity 2.1
b. What information do you need to calculate or determine the following?
The atomic number of an
element
The atomic number is
The mass number of an
element
The mass number is equal
The weight in daltons of
one atom of an element
You can estimate the
c. What are the atomic number, mass number, and weight in daltons of the element
shown in part a?
3. One mole of an element or compound contains 6.02 1023 atoms or molecules of
the element or compound. One mole of an element or compound has a mass equal
to its mass number (or molecular weight) in grams. For example, 1 mole of hydrogen
gas (H2) contains 6.02 1023 molecules and weighs 2 g.
Atomic number Mass number Weight in daltons
a. What is the weight of 1 mole of pure
sodium (Na)?
b. How many molecules of Na are in
1 mole of Na?
a. If Cl has 17 electrons, 17
protons, and 18 neutrons,
b. What is the mass number
of NaCl?
c. How many grams of
NaCl equal a mole of
d. If you wanted to combine equal numbers of Na and Cl atoms in a flask, how
much Cl would you have to add if you added 23 g of Na? (Include an explanation
of the reasoning behind your answer.)
23 g of Na is equal to 1 mole of Na. A mole contains 6.02 1023 molecules
e. To make a one-molar (1 M) solution of NaCl, you need to add 1 mol of NaCl to
distilled water to make a final volume of 1 L (1,000 ml). A 1 Msolution is said to
have a molarity of 1. If you added 2 moles of NaCl to 1 L of distilled water,
you would make a 2 Msolution and its molarity would equal 2. You make up
a 1 Msolution of NaCl.
Activity 2.1 5
5. The summary formula for photosynthesis is
6 CO2+ 6 H2O →C6H12O6+ 6 O2
How many molecules of NaCl are in the
1 MNaCl solution?
How many molecules of NaCl are there
per ml of the solution?
f. Next, you divide this 1 Msolution of NaCl into four separate flasks, putting
250 mL into each flask.
How many grams
of NaCl are in each
flask?
How many
molecules of NaCl
are in each flask?
How many
molecules of NaCl
are there per ml of
distilled water?
What is the
molarity of NaCl in
each of the four
flasks?
a. How many molecules of carbon dioxide
and water would a plant have to use to
produce three molecules of glucose
(C6H12O6)?
For each molecule of glucose produced,
b. How many moles of carbon dioxide and
water would a plant have to use to
produce 2 mole of glucose?
Because a mole of anything contains the
c. Refer to the summary formula for photosynthesis. If you know the number of
molecules or moles of any of the reactants used (or products produced), how
would you calculate the number of molecules or moles of all of the other
reactants needed and products produced?
If the formula is balanced and if it is a true representation of the overall reactions
6. A biologist places a plant in a closed chamber. A sensor in the chamber maintains
the carbon dioxide level at the normal atmospheric concentration of 0.03%. Another
sensor allows the biologist to measure the amount of oxygen produced by the plant
over time. If the plant produces 0.001 mole of oxygen in an hour, how much carbon
dioxide had to be added to the chamber during that hour to maintain the atmospheric
concentration of 0.03%?
For every mole of oxygen produced, 1 mole of carbon dioxide had to be consumed.
7. O2and NH3are both small covalent molecules found in cells. NH3is extremely
soluble in the aqueous environment of the cell, while O2is relatively insoluble. What
is the basis for this difference in solubility between the two molecules? In reaching
your answer, draw the structures of the molecules as valence shell diagrams (as in
Figure 2.12, page 38). Given these diagrams, consider the types of interactions each
molecule could have with water.
6Activity 2.1
H
H
HN OO
Ammonia (NH3)
Oxygen (O2)
8. Refer to pages 38–41 of Campbell Biology, 9th edition, which describe these types
of chemical bonds: nonpolar and polar covalent bonds, ionic bonds, hydrogen bonds,
and van der Waals interactions.
The molecule diagrammed here can also be represented by the formula CH3COOH.
Activity 2.1 7
H
H
CC
O
O
H
H
Explain how you could determine which of the bonds between elements in this
molecule are polar or nonpolar covalent bonds, ionic bonds, hydrogen bonds, and
van der Waals interactions.
The best way to determine the bond types is to determine each atom’s
electronegativity, or its attraction for electrons. As a general rule, the more filled the
outer electron shell of an atom is, the higher is its electronegativity. In addition, the
Activity 3.1 A Quick Review of the Properties of Water
1. Compounds that have the capacity to form hydrogen bonds with water are said to
be hydrophilic (water loving). Those without this capacity are hydrophobic (water
fearing).
8Activity 3.1
Is the molecule on the left hydrophilic or
hydrophobic? Explain your answer.
2. In addition to being polar, water molecules can dissociate into hydronium ions
(H3O+, often described simply as H+) and hydroxide ions (OH). The concentration
of each of these ions in pure water is 107. Another way to say this is that the
concentration of hydronium ions, or H+ions, is one out of every 10 million
molecules. Similarly, the concentration of OHions is one in 10 million molecules.
a. The H+ion concentration of a solution can be represented as its pH value. The
pH of a solution is defined as the negative log10 of the hydrogen ion
concentration.What is the pH of pure water?
b. Refer to the diagram of the molecule of acetic acid in question 1. The COOH
group can ionize to release a H+ion into solution. If you add acetic acid to water
and raise the concentration of H+ions to 104, what is the pH of this solution?
3. Life as we know it could not exist without water. All the chemical reactions of life
occur in aqueous solution. Water molecules are polar and are capable of forming
hydrogen bonds with other polar or charged molecules. As a result, water has the
following properties:
A. H2O molecules are cohesive; they form hydrogen bonds with each other.
B. H2O molecules are adhesive; they form hydrogen bonds with polar surfaces.
H
H
O
OH
Explain how these properties of water are related to the phenomena described in
parts a–h below. More than one property may be used to explain a given phenomenon.
a. During the winter, air temperatures in the northern United States can remain
below 0°C for months; however, the fish and other animals living in the lakes
survive.
Water’s greatest density occurs at 4°C. In a lake, the 4°C water sinks below the
b. Many substances—for example, salt (NaCl) and sucrose—dissolve quickly in water.
Water is very polar. The attraction of the polar water molecules for the Na+and
c. When you pour water into a 25-mL graduated cylinder, a meniscus forms at the
top of the water column.
Water is attracted to the polar molecules that make up the glass (or plastic)
apparently “pulled up” the inside edge of the cylinder.
d. Sweating and the evaporation of sweat from the body surface help reduce a
human’s body temperature.
Water has a high specific heat. The specific heat of water is 1 cal/g/°C. In other
e. A bottle contains a liquid mixture of equal parts water and mineral oil. You shake
the bottle vigorously and then set it on the table. Although the law of entropy
favors maximum randomness, this mixture separates into layers of oil over water.
The oil molecules are nonpolar and hydrophobic. The water molecules are polar
Activity 3.1 9
f. Water drops that fall on a surface tend to form rounded drops or beads.
Water molecules are cohesive and form hydrogen bonds with each other. As a
g. Water drops that fall on your car tend to bead or round up more after you polish
(or wax) the car than before you polished it.
The wax (or polish) is hydrophobic and therefore less polar than the surface was
h. If you touch the edge of a paper towel to a drop of colored water, the water will
move up into (or be absorbed by) the towel.
The polar water molecules adhere to the cellulose in the paper towel and cohere
10 Activity 3.1