Atoms, Bonding, and Water

Atoms, Bonding, and Water
Chapter 2 The Chemistry of Life Atoms, Bonding, and Water

Chemicals are the stuff that make up
Introduction Chemicals are the stuff that make up our bodies, the bodies of other organisms, and the physical environment. Biology is a multidisciplinary science, and concepts of both chemistry and physics apply. © 2012 Pearson Education, Inc.

Life’s chemistry is tied to water.
Introduction Life’s chemistry is tied to water. Life first evolved in water. All living organisms require water. The chemical reactions of your body occur in cells consisting of 70–95% water. Biology is a multidisciplinary science, and concepts of both chemistry and physics apply. © 2012 Pearson Education, Inc.

ELEMENTS, ATOMS, AND COMPOUNDS
© 2012 Pearson Education, Inc.

2.1 Organisms are composed of elements, in combinations called compounds
Living organisms are composed of matter, which is anything that occupies space and has mass (weight). Matter is composed of chemical elements. An element is a substance that cannot be broken down to other substances. There are 92 elements in nature—only a few exist in a pure state. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed (for example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen). Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. © 2012 Pearson Education, Inc.

About 25 elements are essential to life. Four elements make up about 96% of the weight of most living organisms. These are oxygen, carbon, hydrogen, and nitrogen. Trace elements are essential but are only needed in minute quantities. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed (for example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen). 2. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. © 2012 Pearson Education, Inc.

Table 2.1 Table 2.1 Elements In the Human Body 7

2.2 CONNECTION: Trace elements are common additives to food and water
Some trace elements are required to prevent disease. Without iron, your body cannot transport oxygen. An iodine deficiency prevents production of thyroid hormones, resulting in goiter. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. 2. Many breakfast cereals are fortified with iron (see Figure 2.2c). As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect an abstract concept to a concrete example. © 2012 Pearson Education, Inc.

Figure 2.2A Figure 2.2A Goiter, a symptom of iodine deficiency, in a Burmese woman 9

Several chemicals are added to food to help preserve it, make it more nutritious, and/or make it look better. Check out the “Nutrition Facts” label on foods and drinks you purchase. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. 2. Many breakfast cereals are fortified with iron (see Figure 2.2c). As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect an abstract concept to a concrete example. © 2012 Pearson Education, Inc.

Figure 2.2C Figure 2.2C Nutrition facts from a fortified cereal 11

Fluoride is added to municipal water and dental products to help reduce tooth decay. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. 2. Many breakfast cereals are fortified with iron (see Figure 2.2c). As noted in Module 2.2, you can crush the cereal and extract distinct iron particles with a magnet. An overhead projector or video imaging device should clearly reveal the iron particles stuck to the magnet. This short practical demonstration can help connect an abstract concept to a concrete example. © 2012 Pearson Education, Inc.

2.3 Atoms consist of protons, neutrons, and electrons
Each element consists of one kind of atom. An atom is the smallest unit of matter that still retains the properties of an element. Three subatomic particles in atoms are relevant to our discussion of the properties of elements. Protons are positively charged. Electrons are negatively charged. Neutrons are electrically neutral. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.) The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of a baseball stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts. After sharing teaching tips 4 and 5 above, consider asking your students to compare the mass of the gnat orbiting a baseball stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons? The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. © 2012 Pearson Education, Inc.

Neutrons and protons are packed into an atom’s nucleus. Electrons orbit the nucleus. The negative charge of electrons and the positive charge of protons keep electrons near the nucleus. Student Misconceptions and Concerns   The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips  Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.)  The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of a baseball stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts.  Consider asking your students to compare the mass of the gnat orbiting a baseball stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons?   The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. © 2012 Pearson Education, Inc.

Protons ( charge) determine element
Figure 2.UN01 Nucleus Electrons ( charge) form negative cloud and determine chemical behavior Protons ( charge) determine element Neutrons (no charge) determine isotope Figure 2.UN01 Reviewing the Concepts, 2.3 Atom 15

The number of protons is the atom’s atomic number. An atom’s mass number is the sum of the number of protons and neutrons in the nucleus. The atomic mass is approximately equal to its mass number. Student Misconceptions and Concerns   The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips  Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.)  The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of a baseball stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts.  Consider asking your students to compare the mass of the gnat orbiting a baseball stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons?   The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. © 2012 Pearson Education, Inc.

Helium Nucleus 2e Electron cloud 2 Protons Mass number  4 2 Neutrons
Figure 2.3A Helium Nucleus 2e Electron cloud Figure 2.3A Two models of a helium atom 2 Protons Mass number  4 2 Neutrons 2 Electrons 17

Carbon Electron cloud 6e Nucleus 6 Protons Mass number  12 6
Figure 2.3B Carbon Electron cloud 6e Nucleus Figure 2.3B Model of a carbon atom 6 Protons Mass number  12 6 Neutrons 6 Electrons 18

Although all atoms of an element have the same atomic number, some differ in mass number. Different isotopes of an element have the same number of protons, but different numbers of neutrons. Different isotopes of an element behave identically in chemical reactions. In radioactive isotopes, the nucleus decays spontaneously, giving off particles and energy. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Students with limited backgrounds in chemistry and physics might struggle with basic concepts of mass, weight, compounds, elements, and isotopes. It may also be early in the semester when mature study habits have not yet developed. Consider passing along basic studying advice and tips to help students master these early chemistry concepts. In-class quizzes (graded or not) or a few homework problems will also provide reinforcing practice. Teaching Tips  Here is a comparison that helps make the point about the differences in mass of protons and electrons. If a proton were as massive as a bowling ball, an electron would be the mass of a Lifesaver. (This is calculated by considering a 15-pound bowling ball, a Lifesaver with a mass of 0.12 ounces, and the mention in Module 2.3 that an electron is about 1/2,000 the mass of a proton.)  The text in Module 2.3 makes an analogy regarding the size of a helium atom. The text notes that if a helium atom were the size of a baseball stadium, the nucleus would be about the size of a fly in center field, and the two electrons would be like tiny gnats buzzing around the stadium. This analogy helps to relate the great distances between parts of an atom. Consider modifying the analogy to any local stadium in your region. Such concrete examples help to relate abstract concepts.  Consider asking your students to compare the mass of the gnat orbiting a baseball stadium to the mass of the fly in center field. If a proton or neutron is about 2,000 times more massive than an electron, how does the mass of a helium nucleus compare to the mass of one of its electrons?   The text notes the use of radioactive isotopes in dating fossils but references Module 15.5 for further discussion. If your course does not include Chapter 15, consider explaining this process at this point in your course. © 2012 Pearson Education, Inc.

Table 2.3 Table 2.3 Isotopes of Carbon 20

2.4 CONNECTION: Radioactive isotopes can help or harm us
Living cells cannot distinguish between isotopes of the same element. Therefore, radioactive compounds in metabolic processes can act as tracers. This radioactivity can be detected by instruments. Using these instruments, the fate of radioactive tracers can be monitored in living organisms. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals.  Teaching Tips  The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly.   Depending upon where you are teaching, radon in homes may be a common problem and significant health risk. If you are in a high radon region, consider adding details about home remediation methods and expenses or having students research the topic and report back. © 2012 Pearson Education, Inc.

Radioactive tracers are frequently used in medical diagnosis. Sophisticated imaging instruments are used to detect them. An imaging instrument that uses positron-emission tomography (PET) detects the location of injected radioactive materials. PET is useful for diagnosing heart disorders, cancer, and in brain research. Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals.  Teaching Tips  The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly.   Depending upon where you are teaching, radon in homes may be a common problem and significant health risk. If you are in a high radon region, consider adding details about home remediation methods and expenses or having students research the topic and report back. © 2012 Pearson Education, Inc.

Figure 2.4A Figure 2.4A Technician monitoring the output of a PET scanner 23

Healthy person Alzheimer’s patient Figure 2.4B
Figure 2.4B PET images of brains of a healthy person (left), and a person with Alzheimer’s disease (right) Healthy person Alzheimer’s patient 24

In addition to benefits, there are also dangers associated with using radioactive substances. Uncontrolled exposure can cause damage to some molecules in a living cell, especially DNA. Chemical bonds are broken by the emitted energy, which causes abnormal bonds to form. Student Misconceptions and Concerns   The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals.  Teaching Tips  The half-lives of many radioactive substances, especially those used for dating fossils, might lead some students to expect very long periods of decay for any radioactive substance. This might even be alarming if students are someday asked to consume a radioactive substance for a medical test. However, some medically significant isotopes have relatively short half-lives. Radioactive iodine-131 is often used to diagnose or treat certain thyroid problems. Its half-life of eight days means that it will decay quickly.   Depending upon where you are teaching, radon in homes may be a common problem and significant health risk. If you are in a high radon region, consider adding details about home remediation methods and expenses or having students research the topic and report back. © 2012 Pearson Education, Inc.

2.5 The distribution of electrons determines an atom’s chemical properties
Of the three subatomic particles—protons, neutrons, and electrons—only electrons are directly involved in chemical activity. Electrons occur in energy levels called electron shells. Information about the distribution of electrons is found in the periodic table of the elements. Teaching Tips Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) © 2012 Pearson Education, Inc.

Figure 2.5 Hydrogen Helium First shell Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Second shell Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Argon Third shell Figure 2.5 The electron distribution diagrams of the first 18 elements in the periodic table 28

An atom may have one, two, or three electron shells surrounding the nucleus. The number of electrons in the outermost shell determines the chemical properties of the atom. Atoms whose outer shells are not full tend to interact with other atoms, participating in chemical reactions. Teaching Tips  Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) © 2012 Pearson Education, Inc.

Atoms with incomplete outer shells tend to react so that both atoms end up with completed outer shells. These atoms may react with each other by sharing, donating, or receiving electrons. These interactions usually result in atoms staying close together, held by attractions called chemical bonds. Teaching Tips  Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) © 2012 Pearson Education, Inc.

2.6 Covalent bonds join atoms into molecules through electron sharing
The strongest kind of chemical bond is a covalent bond in which two atoms share one or more outer-shell electrons. Two or more atoms held together by covalent bonds form a molecule. Student Misconceptions and Concerns Students with limited backgrounds in chemistry will benefit from a discussion of Table 2.6 and the differences and limitations of representing atomic structure. The contrast in Table 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips 1. Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) 2. Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) 3. Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. Animation: Covalent Bonds © 2012 Pearson Education, Inc.

A covalent bond connects two hydrogen atoms in a molecule of the gas H2. There are four alternative ways to represent common molecules. Student Misconceptions and Concerns Students with limited backgrounds in chemistry will benefit from a discussion of Table 2.6 and the differences and limitations of representing atomic structure. The contrast in Table 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3. Teaching Tips 1. Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) 2. Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.) 3. Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. © 2012 Pearson Education, Inc.

Table 2.6 Table 2.6 Alternative ways to represent four common molecules 33

Table 2.6_1 Table 2.6_1 Alternative ways to represent four common molecules (part 1) 34

Table 2.6_2 Table 2.6_2 Alternative ways to represent four common molecules (part 2) 35

Atoms in a covalently bonded molecule continually compete for shared electrons. The attraction (pull) for shared electrons is called electronegativity. More electronegative atoms pull harder. Student Misconceptions and Concerns Students with limited backgrounds in chemistry will benefit from a discussion of Table 2.6 and the differences and limitations of representing atomic structure. The contrast in Table 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3.   Teaching Tips  Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.)  Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.)  Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. © 2012 Pearson Education, Inc.

In molecules of only one element, the pull toward each atom is equal, because each atom has the same electronegativity. The bonds formed are called nonpolar covalent bonds. Student Misconceptions and Concerns Students with limited backgrounds in chemistry will benefit from a discussion of Table 2.6 and the differences and limitations of representing atomic structure. The contrast in Table 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3.   Teaching Tips  Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.)  Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.)  Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. © 2012 Pearson Education, Inc.

Water has atoms with different electronegativities. Oxygen attracts the shared electrons more strongly than hydrogen. So, the shared electrons spend more time near oxygen. The oxygen atom has a slightly negative charge and the hydrogen atoms have a slightly positive charge. The result is a polar covalent bond. Because of these polar covalent bonds, water is a polar molecule. Student Misconceptions and Concerns   Students with limited backgrounds in chemistry will benefit from a discussion of Table 2.6 and the differences and limitations of representing atomic structure. The contrast in Table 2.6 is a good beginning for such a discussion. In addition to comparing how the positions of electrons are depicted, note the problems with the sense of scale as discussed in Module 2.3.   Teaching Tips  Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.)  Have your students try to calculate the number of covalent bonds possible for a variety of atoms. (Carbon, for example, can form up to four covalent bonds.) Then provide the students with a list of elements and the number of outer electrons for each and have them make predictions about the chemical formula for many types of molecules. (For example, carbon could form covalent bonds with four hydrogen atoms.)  Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. © 2012 Pearson Education, Inc.

(slightly ) (slightly ) (slightly ) Figure 2.6
Figure 2.6 A water molecule, with polar covalent bonds (slightly ) (slightly ) 39

2.7 Ionic bonds are attractions between ions of opposite charge
An ion is an atom or molecule with an electrical charge resulting from gain or loss of electrons. When an electron is lost, a positive charge results. When an electron is gained, a negative charge results. Two ions with opposite charges attract each other. When the attraction holds the ions together, it is called an ionic bond. Salt is a synonym for an ionic compound. Teaching Tips Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example.Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) Animation: Ionic Bonds © 2012 Pearson Education, Inc.

Na Sodium atom Cl Chlorine atom
Figure 2.7A_s2 Transfer of electron Na Sodium atom Cl Chlorine atom Figure 2.7A_s1 Formation of an ionic bond, producing sodium chloride (step 1) 41

Na Sodium atom Cl Chlorine atom Na Sodium ion Cl Chloride ion
Figure 2.7A_s2 Transfer of electron Na Sodium atom Cl Chlorine atom Na Sodium ion Cl Chloride ion Figure 2.7A_s2 Formation of an ionic bond, producing sodium chloride (step 2) Sodium chloride (NaCl) 42

Figure 2.7B Cl Na Figure 2.7B A crystal of sodium chloride 43

A compound is a substance consisting of two or more different elements in a fixed ratio. Compounds are more common than pure elements. Sodium chloride, table salt, is a common compound of equal parts of sodium (Na) and chlorine (Cl). Student Misconceptions and Concerns The dangers posed by certain chemicals in our food and broader environment have sometimes misled people to associate chemicals with harm. People might not want chemicals added to their food or in their environment. Students often fail to appreciate the chemical nature of our bodies and our world and the potential harm or benefits of naturally occurring chemistry. They often fail to understand why “natural” does not necessarily mean good. (Consider presenting a long list of naturally occurring toxins to make this point.) Your class may benefit from a class discussion of these misconceptions about our attitudes toward chemicals. Teaching Tips 1. The text notes the unique properties of pure sodium, pure chlorine, and the compound sodium chloride formed when the two bond together. Consider challenging your students to think of other simple examples of new properties that result when a compound is formed (for example, water, formed from hydrogen and oxygen, and rust, formed from iron and oxygen). 2. Students might be interested in the following aside: One of the challenges of raising captive, exotic animals is meeting the unique dietary requirements of a species. A zoo might have trouble keeping a particular animal because zoologists have not identified all of the trace elements required in the animal’s diet. © 2012 Pearson Education, Inc.

Sodium Chlorine Sodium chloride Figure 2.1
Figure 2.1 The emergent properties of the edible compound sodium chloride Sodium Chlorine Sodium chloride 45

2.8 Hydrogen bonds are weak bonds important in the chemistry of life
Most large molecules are held in their three-dimensional functional shape by weak bonds. Hydrogen, as part of a polar covalent bond, has a partial positive charge. The charged regions on molecules are electrically attracted to oppositely charged regions on neighboring molecules. Because the positively charged region is always a hydrogen atom, the bond is called a hydrogen bond. Teaching Tips 1. Consider challenging your students to suggest relationships in human lives that are analogous to each of the three types of chemical bonds (covalent, ionic, and hydrogen). Evaluating the accuracy of potential analogies requires careful analysis of the chemical bonding relationships and practices critical thinking skills. Small groups might provide immediate critiques before passing along analogies for the entire class to consider. The following is one example. Ionic and covalent bonds are different types of relationships. Consider this analogy. A woman taking out a loan has a specific relationship to her bank. She owes the bank money, something she got from the bank. A man shares an office with another man. Both look out the same window and answer the same phone. Ionic bonds are like a bank loan, in which something is borrowed. Covalent bonds are like sharing an office, with items (electrons) shared by both members of the relationship. After presenting this analogy, ask your students to modify the office analogy to represent a polar covalent bond. (Perhaps one man in the office sits closer to the window and the phone.) 2. Modules 2.6 and 2.8 discuss the special bonding in and between water molecules. Many students do not appreciate the importance of weak chemical bonds in water and cellular chemistry. Extra time and attention may be required to address this special aspect of chemistry. © 2012 Pearson Education, Inc.

Figure 2.8 Hydrogen bond Figure 2.8 Hydrogen bonds between water molecules 47

2.9 Chemical reactions make and break chemical bonds
Remember that the structure of atoms and molecules determines the way they behave. Remember that atoms combine to form molecules. Hydrogen and oxygen can react to form water: 2H2 + O H2O Student Misconceptions and Concerns 1. Students may misunderstand the chemical shorthand equation of photosynthesis presented in Module 2.9. As noted in the text, this overall equation does not include many smaller steps and reactions that occur in photosynthesis. If you discuss greater details of photosynthesis in your course, you might mention that you will address the details at a later time. 2. A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips As noted in the text, chemical reactions do not create or destroy matter. Instead, they rearrange the structure and form new relationships. This is much like shuffling and dealing cards. When playing poker, cards are not created nor destroyed. Instead, new combinations are formed as the cards are dealt to the players. 2. The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions. Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) © 2012 Pearson Education, Inc.

The formation of water from hydrogen and oxygen is an example of a chemical reaction. The reactants (H2 and O2) are converted to H2O, the product. Chemical reactions do not create or destroy matter. Chemical reactions only rearrange matter. Student Misconceptions and Concerns Students may misunderstand the chemical shorthand equation of photosynthesis presented in Module 2.9. As noted in the text, this overall equation does not include many smaller steps and reactions that occur in photosynthesis. If you discuss greater details of photosynthesis in your course, you might mention that you will address the details at a later time.  A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips  As noted in the text, chemical reactions do not create or destroy matter. Instead, they rearrange the structure and form new relationships. This is much like shuffling and dealing cards. When playing poker, cards are not created nor destroyed. Instead, new combinations are formed as the cards are dealt to the players.  The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions. Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) © 2012 Pearson Education, Inc.

2 H2 O2 2 H2O Reactants Products Figure 2.9
Figure 2.9 Breaking and making of bonds in a chemical reaction Reactants Products 50

Photosynthesis is a chemical reaction that is essential to life on Earth. Carbon dioxide (from the air) reacts with water. Sunlight powers the conversion to produce the products glucose and oxygen. Student Misconceptions and Concerns Students may misunderstand the chemical shorthand equation of photosynthesis presented in Module 2.9. As noted in the text, this overall equation does not include many smaller steps and reactions that occur in photosynthesis. If you discuss greater details of photosynthesis in your course, you might mention that you will address the details at a later time.  A common student misconception is that energy is produced by a chemical reaction. When introducing chemical reactions, consider addressing the conservation of energy (the first law of thermodynamics) and the investment and release of energy in the creation and breaking of chemical bonds. Teaching Tips  As noted in the text, chemical reactions do not create or destroy matter. Instead, they rearrange the structure and form new relationships. This is much like shuffling and dealing cards. When playing poker, cards are not created nor destroyed. Instead, new combinations are formed as the cards are dealt to the players.  The overall reaction of photosynthesis illustrates the investment and release of energy by chemical reactions. Consider discussing the investment of sunlight energy to create chemical bonds and the release of energy in the form of heat when plant materials are burned. (Animals invest some of the energy released by the breakdown of sugars to form new chemical bonds, such as those in ATP.) © 2012 Pearson Education, Inc.

WATER’S LIFE-SUPPORTING PROPERTIES
© 2012 Pearson Education, Inc.

2.10 Hydrogen bonds make liquid water cohesive
The tendency of molecules of the same kind to stick together is cohesion. Cohesion is much stronger for water than other liquids. Most plants depend upon cohesion to help transport water and nutrients from their roots to their leaves. The tendency of two kinds of molecules to stick together is adhesion. Student Misconceptions and Concerns Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips 1. Here is a way to help your students think about the sticky nature of water in their lives. Ask them to consider the need for a towel after a shower or a bath. Once we get out of the shower or bath, we have left the source of water. So why do we need the towel? A towel helps us dry off water that is still clinging to our bodies because water molecules are polar. The molecules on cell surfaces are also polar, so our skin and the water stick to each other. 2. Some students may be intrigued if you tell them that you too can stand on the surface of water—when it is frozen. Thus, it is necessary to note a liquid water surface when discussing surface tension. © 2012 Pearson Education, Inc.

2.10 Hydrogen bonds make liquid water cohesive
Cohesion is related to surface tension—a measure of how difficult it is to break the surface of a liquid. Hydrogen bonds give water high surface tension, making it behave as if it were coated with an invisible film. Water striders stand on water without breaking the water surface. Student Misconceptions and Concerns Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis.   Teaching Tips  Here is a way to help your students think about the sticky nature of water in their lives. Ask them to consider the need for a towel after a shower or a bath. Once we get out of the shower or bath, we have left the source of water. So why do we need the towel? A towel helps us dry off water that is still clinging to our bodies because water molecules are polar. The molecules on cell surfaces are also polar, so our skin and the water stick to each other.  Some students may be intrigued if you tell them that you too can stand on the surface of water—when it is frozen. Thus, it is necessary to note a liquid water surface when discussing surface tension. Animation: Water Transport © 2012 Pearson Education, Inc.

Figure 2.10 Figure 2.10 Surface tension allows a water strider to walk on water. 55

2.11 Water’s hydrogen bonds moderate temperature
Because of hydrogen bonding, water has a greater ability to resist temperature change than other liquids. Heat is the energy associated with movement of atoms and molecules in matter. Temperature measures the intensity of heat. Heat is released when hydrogen bonds form. Heat must be absorbed to break hydrogen bonds. Student Misconceptions and Concerns Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips 1. Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature. 2. The following analogies may help students to understand the relationships between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students didn’t take the test. (b) The authors note that the performance of a track team would drop if the fastest runners did not compete. In both analogies, removing the top performers lowers the average, just as the evaporation of the most active water molecules cools the evaporative surface. 3. It’s not the heat, it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. © 2012 Pearson Education, Inc.

2.11 Water’s hydrogen bonds moderate temperature
When a substance evaporates, the surface of the liquid that remains behind cools down, in the process of evaporative cooling. This cooling occurs because the molecules with the greatest energy leave the surface. Student Misconceptions and Concerns Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips 1. Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature. 2. The following analogies may help students to understand the relationships between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students didn’t take the test. (b) The authors note that the performance of a track team would drop if the fastest runners did not compete. In both analogies, removing the top performers lowers the average, just as the evaporation of the most active water molecules cools the evaporative surface. 3. It’s not the heat, it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. © 2012 Pearson Education, Inc.

Figure 2.11 Figure 2.11 Evaporative cooling occurs as sweat dries. 58

2.12 Ice is less dense than liquid water
Water can exist as a gas, liquid, or solid. Water is less dense as a solid than a liquid because of hydrogen bonding. When water freezes, each molecule forms a stable hydrogen bond with its neighbors. As ice crystals form, the molecules are less densely packed than in liquid water. Because ice is less dense than water, it floats. Student Misconceptions and Concerns 1. Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. 2. Students at all levels struggle with the distinction between heat and temperature. Students might also expect that all ice is about the same temperature, 0 ºC. Redefining and correcting misunderstandings often takes more class time and energy than introducing previously unknown concepts. Teaching Tips 1. Ask your students if the ocean levels would change if ice did not float. They can try this experiment to find out, or you can begin class with the demonstration and watch the progress throughout the class period. Place several large chunks of ice in a glass and fill the glass up completely with water to the top rim. Thus, the ice cubes should be sticking up above the top of the filled glass. Will the glass overflow when the ice melts? (No.) This phenomenon is important when we consider the potential consequences of global warming. If floating glaciers melt, ocean levels will not be affected. However, if the ice over land melts, we can expect higher ocean levels. 2. Module 2.12 notes the insulating effect of ice forming at the surface of a lake. This phenomenon would not occur if ice were denser than water. Challenge students to think of other consequences from the expansion of water when it forms ice. (These include the ability to widen cracks in rocks, roads, and sidewalks!) © 2012 Pearson Education, Inc.

Ice Hydrogen bonds are stable.
Figure 2.12 Ice Hydrogen bonds are stable. Hydrogen bond Figure 2.12 Hydrogen bonds between water molecules in ice and water Liquid water Hydrogen bonds constantly break and re-form. 60

Ice Hydrogen bonds are stable.
Figure 2.12 Ice Hydrogen bonds are stable. Figure 2.12 Hydrogen bonds between water molecules in ice and water Liquid water Hydrogen bonds constantly break and re-form. 61

2.13 Water is the solvent of life
A solution is a liquid consisting of a uniform mixture of two or more substances. The dissolving agent is the solvent. The substance that is dissolved is the solute. An aqueous solution is one in which water is the solvent. Student Misconceptions and Concerns Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis. Teaching Tips A simple demonstration of a solute dissolving in a solvent can focus students’ attention on the process when discussing solutions. Using colored water and white sugar or salt may make it easier to see and reference while you are discussing the process. Such simple visual aids add life to a lecture. (You might also add corn oil to the top of the solution to demonstrate the properties of hydrophobic substances, and challenge your class to explain why oil and water do not mix.) © 2012 Pearson Education, Inc.

2.13 Water is the solvent of life
Water’s versatility as a solvent results from the polarity of its molecules. Polar or charged solutes dissolve when water molecules surround them, forming aqueous solutions. Table salt is an example of a solute that will go into solution in water. Student Misconceptions and Concerns Students are unlikely to have carefully considered the four special properties of water that are apparent in our world. However, these properties are of great biological significance and are often familiar parts of our lives. The connections between these properties and personal experiences can invest great meaning into a discussion of water’s properties. A homework assignment asking for examples of each of these properties in each student’s experiences will require reflection and may produce meaningful illustrations. Similarly, quizzes or exam questions matching examples to a list of the properties may require high-level evaluative analysis.   Teaching Tips  Have students compare the seasonal ranges of temperatures of Anchorage and Fairbanks, Alaska. (Many websites, such as www. weather.com, provide weather information about various cities.) These two northern cities have large differences in their annual temperature ranges. Make the point that the coastal location of Anchorage moderates the temperature.  The following analogies may help students to understand the relationships between evaporation, heat, and temperature. (a) Ask students how the average on an exam would be affected if the brightest students didn’t take the test. (b) The authors note that the performance of a track team would drop if the fastest runners did not compete. In both analogies, removing the top performers lowers the average, just as the evaporation of the most active water molecules cools the evaporative surface.  It’s not the heat, it’s the humidity. The efficiency of evaporative cooling is affected by humidity. As humidity rises, the rate of evaporation decreases, making it more difficult to cool our heat-generating bodies on a warm and humid summer day. © 2012 Pearson Education, Inc.

Ion in solution Salt crystal Figure 2.13
Figure 2.13 A crystal of salt (NaCl) dissolving in water Salt crystal 64

2.14 The chemistry of life is sensitive to acidic and basic conditions
In aqueous solutions, a small percentage of water molecules break apart into ions. Some are hydrogen ions (H+). Some are hydroxide ions (OH–). Both types are very reactive. Teaching Tips Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in lab, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). © 2012 Pearson Education, Inc.

A compound that releases H+ to a solution is an acid. A compound that accepts H+ is a base. The pH scale describes how acidic or basic a solution is. The pH scale ranges from 0 to 14, with zero the most acidic and 14 the most basic. Each pH unit represents a tenfold change in the concentration of H+. Teaching Tips  Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in lab, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). © 2012 Pearson Education, Inc.

A buffer is a substance that minimizes changes in pH. Buffers accept H+ when it is in excess and donate H+ when it is depleted. Teaching Tips  Discussions of pH are enhanced by lab activities that permit students to test the pH of everyday items (foods and household solutions). If students do not have opportunities to conduct such tests in lab, consider testing a few items during your class (pH paper or a basic pH meter will, of course, be necessary). © 2012 Pearson Education, Inc.

Figure 2.14 pH scale Battery acid Lemon juice, gastric juice Acidic solution Vinegar, cola Increasingly ACIDIC (Higher H concentration) Tomato juice Rainwater Human urine Saliva NEUTRAL [H][OH] Pure water Neutral solution Human blood, tears Seawater Figure 2.14 The pH scale reflects the relative concentrations of H and OH. Increasingly BASIC (Higher OH concentration) Milk of magnesia Household ammonia Basic solution Household bleach Oven cleaner 68

Increasingly ACIDIC (Higher H concentration)
Figure 2.14_1 pH scale Battery acid Lemon juice, gastric juice Vinegar, cola Increasingly ACIDIC (Higher H concentration) Tomato juice Figure 2.14_1 The pH scale reflects the relative concentrations of H and OH (part 1). Rainwater Human urine Saliva NEUTRAL [H][OH] Pure water 69

Increasingly BASIC (Higher OH concentration)
Figure 2.14_2 pH scale NEUTRAL [H][OH] Pure water Human blood, tears Seawater Increasingly BASIC (Higher OH concentration) Milk of magnesia Figure 2.14_2 The pH scale reflects the relative concentrations of H and OH (part 2). Household ammonia Household bleach Oven cleaner 70

Acidic solution Neutral solution Basic solution
Figure 2.14_3 Acidic solution Neutral solution Basic solution Figure 2.14_3 The pH scale reflects the relative concentrations of H and OH (part 3). 71

2.15 CONNECTION: Acid precipitation and ocean acidification threaten the environment
When we burn fossil fuels (coal, oil, and gas), air-polluting compounds and CO2 are released into the atmosphere. Sulfur and nitrous oxides react with water in the air to form acids. These acids fall to Earth as acid precipitation, which is rain, snow, or fog with a pH lower than 5.2. CO2 dissolving in seawater lowers ocean pH in a process known as ocean acidification. Teaching Tips The Environmental Protection Agency (EPA) website, includes useful information about acid rain and related science experiments. 2. In Module 2.15, the authors note that threats to water quality are addressed in Chapter 38. If your course does not include Chapter 38, consider covering some of those threats in your discussion of acid precipitation. © 2012 Pearson Education, Inc.

Figure 2.15 Figure 2.15 Coral reefs are threatened by ocean acidification. 73

2.16 EVOLUTION CONNECTION: The search for extraterrestrial life centers on the search for water
The emergent properties of water support life on Earth. When astrobiologists search for signs of extraterrestrial life on distant planets, they look for evidence of water. The National Aeronautics and Space Administration (NASA) has found evidence that water was once abundant on Mars. Teaching Tips The SETI (Search For Extraterrestrial Intelligence) Institute’s Mission is to “explore, understand, and explain the origin, nature, and prevalence of life in the universe.” Your students might enjoy exploring this respected scientific organization’s website at © 2012 Pearson Education, Inc.

Figure 2.16 Figure 2.16 Robotic arm of the Phoenix Mars Lander probing for evidence of water 75

Atoms, Bonding, and Water

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