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
exactly alike, and no two systems are identical in form or function. As a result, our analysis
of such systems tends to deal with statistical averages or probabilities. This means that it is
difficult to understand biological systems without having a good basic understanding of
chemistry, physics, and math (including probability and statistics).
The vast majority of introductory biology students have studied inorganic chemistry in their
high school and first-year college chemistry courses. Many students compartmentalize their
knowledge, however. In some cases, the compartmentalization is so extreme that the students
feel uncomfortable dealing with chemical formulas and ideas outside of chemistry classes.
Therefore, it is generally useful to review some of the basic ideas in chemistry and, at the
same time, demonstrate how they can be applied to understanding biological systems.
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
they can be used to determine the reactivity of elements;
• various types of chemical bonds and how they affect the structure and energetics
of molecules; and
• the difference between a mole and a molar equivalent and how a knowledge of
these can be used in biological applications.
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:
• H2O molecules are cohesive; they form hydrogen bonds with each other.
• H 2O molecules are adhesive; they form hydrogen bonds with polar surfaces.
• Water is a liquid at normal physiological (or body) temperatures.
• Water has a high specific heat.
• Water has a high heat of vaporization.
• Water’s greatest density occurs at 4°C.
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
understand the relationship between molecules of a substance and moles of that
substance. Similarly, most students can recite what a mole is; however, the majority have
not thought about how that knowledge can be applied. Therefore, much of this first
activity is devoted to making it clear that a balanced equation indicates not only the
number of molecules required but also the number of moles required. It also explains why
moles can be substituted for molecules in such equations.
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%.
Questions 5 and 6: These questions are designed to help students understand how a
knowledge of balanced equations and molar equivalents can be useful in biology.
Questions 7 and 8: The answers go into a little more detail than does Campbell Biology,
9th edition. Students obviously shouldn’t be asked to know the specific electronegativity
of each of the elements. However, using concrete numbers may help students understand
how their electronegativity is related to the type of bonds formed between elements.
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
most commonly found
as CO 2, N2, and O2 in
the atmosphere and as
H20, PO4, and S
compounds on Earth.
b. In what chemical
form(s) do animals
need to obtain these
elements?
With the exception of
oxygen and water,
animals obtain the
majority of these
elements in the form
of organic compounds.
c. In what chemical
form(s) do plants need
to obtain these
elements?
Plants can obtain C as
CO 2 , N as ammonia or
nitrite or nitrates,
phosphorus as
phosphates, sulfur as
sulfides or sulfates,
and so on. In other
words, plants obtain
these elements as
inorganic compounds.
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
grams of that mole or compound. For example, the mass number of Na is 23;
therefore, a mole of Na has a mass of 23 g. The mass number of water is 18;
therefore, a mole of water has a mass of 18 g.
4. One atom of Na can combine with one atom of Cl (chlorine) to produce one
molecule of NaCl (table salt).
4 Activity 2.1
b. What information do you need to calculate or determine the following?
The atomic number of an
element
The atomic number is
equal to the number of
protons (or electrons).
The mass number of an
element
The mass number is equal
to the number of protons
plus the number of
neutrons.
The weight in daltons of
one atom of an element
You can estimate the
weight in daltons as 1
dalton per proton or
neutron.Therefore, the
weight in daltons of an
element is approximately
equal to the number of
protons plus the number
of neutrons.
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 10 23 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 10 23 molecules and weighs 2 g.
Atomic number Mass number Weight in daltons
11 23 23
a. What is the weight of 1 mole of pure
sodium (Na)?
23 g
b. How many molecules of Na are in
1 mole of Na?
6.02 1023
a. If Cl has 17 electrons, 17
protons, and 18 neutrons,
what is its mass number?
35
b. What is the mass number
of NaCl?
23 + 35 = 58
c. How many grams of
NaCl equal a mole of
NaCl?
58 g

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