Answers to Open-Ended Questions
Hoefnagels Essentials 2/e
Chapter 2
Answers to Mastering Concepts Questions
2.1
1. Which four chemical elements do organisms require in large amounts?
Carbon, oxygen, hydrogen, and nitrogen are the chemical elements that organisms require
in large amounts.
2. Where in an atom are protons, neutrons, and electrons located?
An atom’s protons and neutrons are in its nucleus. A cloud of electrons surrounds the
nucleus.
3. What does an element’s atomic number indicate?
An element’s atomic number indicates the number of protons in its nucleus.
4. What is the relationship between an atom’s mass number and an element’s atomic
weight?
An atom’s mass number is the sum of protons and neutrons in its nucleus. An element’s
atomic weight is the average mass of all of the isotopes of that element.
5. How are the isotopes of an element different from one another?
Isotopes of the same element differ in the number of the neutrons in their nuclei.
2.2
1. How are atoms, molecules, and compounds related?
Two or more atoms bonded together form a molecule. A compound is a molecule made
of atoms of two or more different elements.
2. How does the number of valence electrons determine an atom’s tendency to form
bonds?
An atom is most stable if its valence (outermost) shell is full. The number of electrons
and number of “vacancies” in the valence shell determine how many electrons an atom
will gain, lose, or share to achieve a full valence shell. Some atoms, such as neon and
argon, do not form bonds because they already have a full valence shell.
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3. Explain how electronegativity differences between atoms result in each type of
chemical bond.
Ionic bonds form between pairs of atoms that have extremely different
electronegativities; in that case, one atom takes one or more electrons from the other.
Polar covalent bonds form between atoms that have moderately different
electronegativities; such atoms share electrons unequally and thus have partial positive or
partial negative charges. Nonpolar covalent bonds form between atoms that have similar
electronegativities and therefore share electrons approximately equally. The partial
charges on molecules with polar covalent bonds may lead to hydrogen bonds between
adjacent molecules.
2.3
1. How are cohesion and adhesion important to life?
Cohesion is water’s tendency to stick to itself; adhesion is water’s tendency to stick to
other substances. Cohesion means water is liquid at most temperatures on Earth’s
surface; it also allows small insects to “skate” on the water surface. Both cohesion and
adhesion help water move from roots to leaves in plants.
2. Distinguish between a solute and a solvent and between a hydrophilic and a
hydrophobic molecule.
A solute (such as salt) is a substance that dissolves into a liquid solvent (such as water).
Hydrophilic molecules will dissolve in water, whereas hydrophobic molecules will not.
3. How does water help an organism regulate its body temperature?
Hydrogen bonds between water molecules prevent water from changing temperatures
quickly, even if the external temperature rises or drops. Sweat is an adaptation that cools
the body as water evaporates from the skin.
4. How does the density difference between ice and water affect life?
Because ice is less dense than liquid water, a layer of ice forms at the top of a lake in cold
temperatures. Insulated from the cold air above, the liquid water below the ice layer is
less likely to freeze. Aquatic organisms that cannot tolerate freezing can therefore survive
the cold season.
5. What happens in a chemical reaction?
In a chemical reaction, reactants swap atoms to form products. As atoms are swapped,
chemical bonds are broken, energy is released or used in the reaction, and new chemical
bonds are formed.
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6. How does water participate in the chemistry of life?
Because cells are mostly made of water and because most cells are surrounded by water,
water is the medium in which nearly all of life’s chemical reactions occur. Water is either
a reactant in or a product of many of the chemical reactions that sustain life, including
respiration and photosynthesis.
2.4
1. How do acids and bases affect a solution’s H+ concentration?
An acid adds H+ to the solution, whereas a base absorbs H+ or releases OH-.
2. How do the values of 0, 7, and 14 relate to the pH scale?
0 and 14 on the pH scale represent the strongest acidic and basic solutions respectively,
whereas 7 is the pH of a neutral solution.
3. How do organisms maintain their pH within certain limits?
Buffer systems allow organisms to maintain their pH within certain limits. In a buffer,
pairs of weak acids and bases consume or release H+. Adding an acid or base therefore
does not affect the pH of a buffered solution.
2.5
1. Distinguish between hydrolysis and dehydration synthesis.
Hydrolysis reactions break covalent bonds in polymers, releasing individual monomers.
The opposite reaction, dehydration synthesis, forms covalent bonds between monomers,
producing a polymer.
2. Compare and contrast the structures of polysaccharides, proteins, and nucleic acids.
All are long polymers built of monomers. The monomers that form polysaccharides are
monosaccharides; the monomers that form proteins are amino acids; and the monomers
that form nucleic acids are nucleotides.
3. What is the significance of a protein’s shape, and how can that shape be destroyed?
A protein’s shape determines its function. That shape can be destroyed by exposure to
heat, strong acids, or strong bases; each of these can denature a protein. Also, genetic
mutations can change the amino acid sequence of a protein, perhaps altering its overall
shape.
4. What are some differences between RNA and DNA?
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RNA is typically a single strand of nucleotides; DNA is a double strand of nucleotides.
RNA nucleotides include ribose; DNA nucleotides include deoxyribose. RNA has the
nitrogenous base uracil; DNA has thymine.
5. What are the components of a triglyceride?
The components of a triglyceride are a glycerol molecule and three fatty acids.
6. Explain why fats are essential to a healthful diet.
Fats are an excellent source of energy, providing more than twice as many Calories per
gram as carbohydrates or proteins. Dietary fat is essential for growth in animals. It slows
digestion and is required for the use of some vitamins and minerals. Stored body fat
cushions organs and forms insulating layers that help regulate body temperature.
7. List an example of a carbohydrate, protein, nucleic acid, and lipid, and name the
function of each.
Answers will vary, but here are some possibilities. An example of a carbohydrate is
cellulose, which makes up a plant’s cell wall. An example of a protein is an enzyme,
which helps speed chemical reactions. An example of a nucleic acid is DNA, which
stores and transmits genetic information. An example of a lipid is a fat, which stores
energy.
Answers to Write It Out Questions
1. Describe how the number of protons, neutrons, and electrons in an atom affects its
atomic number, mass number, and charge.
The atomic number equals the number of protons. The mass number is the number of
protons and neutrons added together. If an atom has more electrons than protons, it has a
negative charge. If it has more protons than electrons, it has a positive charge.
2. The vitamin biotin contains 10 atoms of carbon, 16 of hydrogen, 3 of oxygen, 2 of
nitrogen, and 1 of sulfur. What is its molecular formula?
Its formula is C10H16O3N2S.
3. Consider the following atomic numbers: oxygen (O) = 8; fluorine (F) = 9; neon (Ne) =
10; magnesium (Mg) = 12. Draw the electron shells of each atom, and then predict how
many bonds each atom should form.
See models in text. Oxygen: 2 bonds. Fluorine: 1 bond. Neon: 0 bonds (its outermost
shell is full). Magnesium: 2 bonds.
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4. Distinguish between nonpolar covalent bonds, polar covalent bonds, and ionic bonds.
Nonpolar covalent bonds are bonds in which both atoms exert approximately equal pull
on the shared electrons. In a polar covalent bond, one nucleus exerts a stronger pull on
the shared electrons than does the other nucleus. An ionic bond results from the electrical
attraction between two ions with opposite charges.
5. If oxygen strongly attracts electrons, why is a covalent bond between two oxygen
atoms considered nonpolar?
The nucleus of each oxygen atom exerts an equal pull on the shared electrons, so the
bond is nonpolar.
6. Can nonpolar molecules such as CH4 participate in hydrogen bonds? Why or why not?
Nonpolar molecules cannot form hydrogen bonds because the atoms do not have partial
charges.
7. How does electronegativity explain whether a covalent bond is polar or nonpolar?
A polar covalent bond forms between two atoms of different electronegativities (e.g., O
and H) because one atom attracts the shared electrons more strongly than the other. In
contrast, a nonpolar covalent bond forms between two atoms of nearly equal
electronegativities (e.g., C and H) because the electrons are shared equally between the
bonded atoms.
8. Give an example from everyday life of each of the following properties of water:
cohesion, adhesion, ability to dissolve solutes, resistance to temperature change.
A water droplet combining with another droplet on a windshield shows cohesion. A
splash of water sticks to the wall with adhesion. Mixing instant coffee into hot water
illustrates water’s ability to dissolve solutes. Coastal climates are relatively mild because
the vast ocean is extremely resistant to temperature change.
9. Draw from memory a diagram showing the interactions among a few water molecules.
The diagram should resemble components of figure 2.10. Hydrogen atoms should have
partial positive charges that are attracted to the partial negative charges on the oxygen
atoms of adjacent water molecules.
10. Why would the formation of ice crystals inside a cell cause the cell to rupture?
Most of a cell’s cytoplasm consists of water, which expands as it freezes. The resulting
increase in volume is likely to rupture the delicate cell membrane.
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11. How do hydrogen ions relate to the pH scale?
The concentration of hydrogen ions determines a solution’s pH. The higher the
concentration, the lower the pH.
12. Sketch a monosaccharide, amino acid, nucleotide, glycerol molecule, and fatty acid.
Then show how those smaller molecules form carbohydrates, proteins, nucleic acids, or
fats.
Answers will be visual. Consult the following figures to check your work: 2.19, 2.20,
2.24, and 2.26.
13. How is an amino acid’s R group analogous to a nucleotide’s nitrogenous base?
The R group is the variable portion of an amino acid, just as the nitrogenous base is the
variable portion of a nucleotide.
14. Refer to figure 2.23 and explain why regulating body temperature is essential to
survival.
Excessive heat causes proteins to become denatured, which ruins their shapes. If a
protein’s shape is destroyed, it can no longer function. Since proteins do nearly all the
work in living cells, excessive heat can be deadly.
15. Complete and explain the following analogy: a protein is to a knitted sweater as a
denatured protein is to a ____.
A protein is to a knitted sweater as a denatured protein is to a tangle of unraveled yarn. A
protein is a highly structured chain made of amino acids, just as a sweater is a structured
garment made of yarn. When unraveled, a sweater is just a mass of yarn; when denatured,
a protein is just a mass of amino acids with no function.
16. A topping for ice cream contains fructose, hydrogenated soybean oil, salt, and
cellulose. What types of chemicals are in it?
The topping contains simple sugars, complex carbohydrates, NaCl, and triglycerides
(lipids).
17. Three very different proteins are silk, hair, and collagen. Chemically how are they
similar, and how are they different?
All proteins are composed of amino acids. The proteins are different in the order of the
amino acids in the sequence, which means their shapes are different as well.
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18. Why are proteins extremely varied in organisms, but carbohydrates and lipids are
not?
Proteins consist of 20 different types of amino acids; in contrast, only a few types of
subunits make up carbohydrates and lipids.
19. A man on a very low-fat diet proclaims to his friend, “I’m going to get my cholesterol
down to zero!” Why is achieving this goal impossible (and undesirable)?
The liver produces cholesterol from other raw materials, so the man’s body would make
cholesterol even on an extremely low-fat diet. The goal is undesirable because
cholesterol is an essential component of animal cell membranes, and it is a precursor for
other essential molecules such as sex hormones.
20. You eat a sandwich made of starchy bread, ham, and cheese. What types of chemicals
are in it?
The bread is mostly starch, although it may also contain some fiber (cellulose) if it is
made with whole wheat; both starch and cellulose are polysaccharides. The ham and
cheese are composed mainly of protein and fat.
21. Name three examples of emergent properties (see chapter 1) in chemistry.
Answers will vary but could include: bonding of atoms into chemically unique
molecules; a protein’s function emerging when amino acids are linked together; surface
tension arising when many water molecules interact; or protein-encoding sequences in
DNA.
23. Flask A contains 100 milliliters of a solution with pH 5. After you add 100 ml of
solution from Flask B, the pH rises to 7. What was the pH of the solution in Flask B?
The solution in Flask B has a pH of 9.
Answers to Pull It Together Questions
1. How do ions and isotopes fit into this concept map?
“Atoms” could connect with the phrase “that have gained or lost electrons are called” to
“Ions.” “Atoms” could connect with the phrase “with different numbers of neutrons are
called” to “Isotopes.”
2. Besides water, what other molecules are essential to life?
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McGraw-Hill Education.
Besides water, life requires organic molecules such as carbohydrates (energy source),
proteins (many essential functions), nucleic acids (store and express genetic information),
and lipids (energy storage and membrane structure). Oxygen is essential to many but not
all organisms.
3. Add hydrogen bonds to this concept map and explain the relationship between
hydrogen bonds and polar covalent bonds.
Polar covalent bonds form between atoms that have different electronegativities and
therefore do not share the electrons equally. Polar covalent bonds create areas of partial
and negative charges within a molecule. A hydrogen bond forms if a partially positive
part of one molecule attracts a partially negative part of another molecule.
4. Add monomers, polymers, dehydration synthesis, and hydrolysis to this concept map.
“Carbohydrates,” “Proteins,” and “Nucleic acids” could connect with the phrase “are” to
“Polymers,” which in turn could connect with the phrase “are made of subunits called” to
“Monomers.” “Monomers” could connect with the phrase “are joined together by” to
“Dehydration synthesis.” “Polymers” could connect with the phrase “are broken down
by” to “Hydrolysis.”
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McGraw-Hill Education.