Ch 02: The Chemical Context of Life

There are _______ naturally occurring elements.

A: 92 B: 4 C: 25 D: 108 E: 238 A There are 92 naturally occurring elements. These are the elements that occur in nature. This number is different from the total number of elements, which is larger because it includes man-made elements in addition to naturally occurring ones. Four of the naturally occurring elements (H, O, N, and C) make up 96 percent of living matter. Between 20 and 25 percent of the natural elements are essential elements, those that an organism needs to live a healthy life and reproduce. The essential elements are similar among organisms, but there is some variation.

Why is the increasing amount of carbon dioxide being taken up by the oceans a cause for concern?

A:More carbon dioxide causes an increase in carbonic acid (H2CO3), which leads to a decrease in the concentration of carbonate ion (CO32-). B: The buffering action of carbonic acid (H2CO3) causes the pH of seawater to rise. C: More carbon dioxide causes an increase in carbonic acid (H2CO3), which leads to an increase in the concentration of carbonate ion (CO32-). D: There is no cause for concern. Carbonic acid (H2CO3) is an excellent buffer and will help keep the pH of seawater constant. A The increasing amount of carbon dioxide being taken up by the oceans is a cause for concern because more carbon dioxide causes an increase in carbonic acid (H2CO3), which leads to a decrease in the concentration of carbonate ion (CO32-). When carbon dioxide dissolves in seawater, it reacts with water to form carbonic acid, which lowers ocean pH, a process known as ocean acidification. Based on measurements of carbon dioxide levels in air bubbles trapped in ice over thousands of years, scientists calculate that the pH of the oceans is 0.1 pH unit lower now than at any time in the past 420,000 years. Recent studies predict that it will drop another 0.3 to 0.5 pH unit by the end of this century. As seawater acidifies, the extra hydrogen ions combine with carbonate ions to form bicarbonate ions (HCO3-), thereby reducing the carbonate concentration. Carbonic acid lowers the pH of the oceans. It does not act as a buffer. An increase in carbon dioxide does cause an increase in carbonic acid. However, the increase in carbonic acid causes a decrease in the concentration of carbonate ion. When carbonic acid is present in equilibrium with bicarbonate ion, a buffer system exists. However, if carbonic acid is continually added to our oceans, the system is not in equilibrium, and there is cause for concern.

A solution at pH 6 contains ________________than the same amount of solution at pH 8.

A: 100 times more H+ B: 2 times more H+ C: 4 times more H+ D: 4 times less H+ E: 100 times less H+ A A solution at pH 6 contains 100 times more H+ than the same amount of solution at pH 8. The pH scale compresses the range of H+ and OH- concentrations by employing logarithms. The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = log [H+]. For a neutral aqueous solution, [H+] is 10-7 M, giving us log 10-7 = -(-7) = 7. Notice that pH declines as H+ concentration increases. Remember that each pH unit represents a tenfold difference in H+ and OH- concentrations. "2 times more H+" is incorrect because the pH scale is a base-10 logarithmic scale. Each unit represents a tenfold increase or decrease in the hydrogen ion concentration. "4 times more H+" is incorrect because the pH scale is a base-10 logarithmic scale. Each unit represents a tenfold increase or decrease in the hydrogen ion concentration. "4 times less H+" is incorrect because the pH scale is a base-10 logarithmic scale and a lower pH indicates a higher hydrogen ion concentration. "100 times less H+" is also incorrect because and a lower pH indicates a higher hydrogen ion concentration.

The tendency of water molecules to stay close to each other as a result of hydrogen bonding __________.

A: All of the listed responses are correct. B: provides the surface tension that allows leaves to float on water C: is called cohesion D: keeps water moving through the vessels in a tree trunk E: acts to moderate temperature A All of the listed responses are correct. The tendency of water molecules to stay close to each other as a result of hydrogen bonding gives us the emergent properties of water. Hydrogen bonds holding water together is called cohesion. The hydrogen bonds in water give it an unusually high surface tension, making it behave as though it were coated with an invisible film. This allows leaves or other small objects that are denser than water to sometimes remain on the surface. Hydrogen bonding is instrumental in the movement of water through a tree's vessels. Water from the roots reaches the leaves through a network of water-conducting cells. As water evaporates from a leaf, hydrogen bonds cause water molecules leaving the veins to tug on molecules farther down, and the upward pull is transmitted through the water-conducting cells all the way to the roots. The tendency of water molecules to stay close to each other as a result of hydrogen bonding acts to moderate temperature. Heat must be absorbed in order to break hydrogen bonds; by the same token, heat is released when hydrogen bonds form.

Which of the following is a trace element?

A: Copper B: Hydrogen C: Oxygen D: Nitrogen E: Carbon A Copper is a trace element. Trace elements are elements needed by an organism only in trace, or very small, amounts. As such, a trace element cannot be one of the four major components of living matter. Copper accounts for less than 0.01 percent of the mass of a human body. Oxygen, nitrogen, hydrogen, and carbon are the four major components of living matter and together make up 96 percent of all living things.

Which of the following has negligible mass?

A: Electron B: Proton C: Neutron D: Atom E: Element A An electron has negligible mass, meaning a mass so low that it is not taken into account in ordinary calculations of atomic mass. The mass of a proton is about 1,200 times that of an electron. Protons are important components of atomic mass, and each weighs about 1.7 x 1024 grams, equal to about 1 dalton, the standard unit of measurement of atomic mass. The mass of a neutron is also about 1,200 times that of an electron. Atoms are made of subatomic particles (protons, neutrons, and electrons). The atomic mass of an atom is equal to the sum of the masses of its protons and its neutrons. Each element consists of a certain type of atom that is different from the atoms of any other element. An atom is the smallest unit of matter that still retains the properties of an element.

What are the four most abundant elements found in living organisms?

A: Hydrogen, oxygen, nitrogen, and carbon B: Hydrogen, oxygen, calcium, and nitrogen C: Nitrogen, carbon, sulfur, and oxygen D: Hydrogen, oxygen, nitrogen, and iron E: Iodine, boron, silicon, and zinc A The four most abundant elements in living organisms are hydrogen, oxygen, nitrogen, and carbon. Together these make up about 96 percent of the mass of living organisms. Other elements in living organisms are present in much smaller quantities. For example, in humans calcium makes up about 1.5 percent of the mass of the body. Sulfur makes up about 0.3 percent of the human body. Only about 0.1 percent of the mass of the human body is magnesium. Trace elements are required by an organism in only minute quantities. Iodine, boron, silicon, and zinc are all trace elements.

When the number of protons and electrons possessed by an atom are unequal, the atom __________.

A: Is an ion B: forms a covalent bond with another atom C: becomes part of a molecule D: gains or loses a proton E: gains or loses a neutron A When the number of protons and electrons possessed by an atom are unequal, the atom is an ion. Protons have a positive charge, and electrons have a negative charge. If an atom has an equal number of protons and neutrons, it is electrically neutral. An ion has an electrical charge. A positively charged ion is called a cation, while a negatively charged ion is called an anion. Covalent bonds form when electrons are shared, not when there are different numbers of protons and electrons. An atom becomes part of a molecule when it covalently bonds to another atom by sharing electrons. The gain or loss of a proton would cause the atom to become a different element. Gain or loss of a neutron results in an isotope.

Which of the following molecules has the shape of a completed tetrahedron?

A: Methane (CH4) B: Water (H2O) C: Hydrogen gas (H2) D: Oxygen gas (O2) E: Glucose (C6H12O6) A Methane (CH4) has the shape of a completed tetrahedron. Molecular shape is crucial in biology: It determines how biological molecules recognize and respond to one another with specificity. Methane's four covalent bonds radiating out to hydrogen nuclei give it its tetrahedron shape, a pyramid with a triangular base. Water (H2O) is shaped roughly like a V, with its two covalent bonds spread apart at an angle of 104.5°. A molecule consisting of two atoms, such as hydrogen gas (H2) or oxygen (O2), is always linear. Molecules containing many carbon atoms, such as glucose (C6H12O6), have more complex shapes than that of a completed tetrahedron.

How many electrons would be present in the valence shell of a sulfur atom (atomic number 16, mass number 32)?

A: Six electrons B: One electron C: Two electrons D: Four electrons E: Eight electrons A Six electrons would be present in the valence shell of a sulfur atom (atomic number 16, mass number 32). To answer the question, you need to know how many electrons the sulfur atom has as well as how many electrons are needed to fill each shell. If the atomic number of a neutral atom is 16, then there are 16 protons and 16 electrons. Electrons, like all matter, tend to exist in the lowest available state of potential energy. In an atom, this state is in the first shell. However, the first shell can hold no more than two electrons. In an atom with more than two electrons, the additional electrons must occupy higher shells because the first shell is full. The second shell holds a maximum of eight electrons. Electrons remaining after the first two shells are full will occupy the third shell, which also holds a maximum of eight electrons. The 16 electrons of a sulfur atom will fill the shells as follows: two in the first shell, eight in the second shell, and the remainder, six, in the third, or valence, shell.

You can fill a glass of water to just slightly above the rim without it spilling over. What property of water best explains this phenomenon?

A: Surface tension B: Adhesion C: Heat of vaporization D: Evaporative cooling E: None of the listed responses is correct. A You can fill a glass of water to just slightly above the rim without it spilling over. The property of water that best explains this phenomenon is surface tension. Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. Water has a greater surface tension than most other liquids. At the interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below. This makes the water behave as though coated with an invisible film. Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state. Water has a high heat of vaporization, but that is not the property that best explains the ability of a volume of water to extend slightly above the walls of its container. Adhesion is the clinging of one substance to another. Water would adhere to both the inside and outside of a glass. Evaporative cooling is the cooling of a liquid's surface as some of the liquid evaporates. A glass of water at room temperature will eventually evaporate completely, so this is not a property keeping water in the glass.

Cells are surrounded by water, and cells themselves consist of about 70 to 95 percent water. As a result, ___________.

A: all of the listed responses are correct B: he temperature of living things tends to change relatively slowly C: a variety of nutrient molecules is readily available as dissolved solutes D: waste products produced by cell metabolism can be easily removed E: dissolved substances can be easily transported within a cell or between cells in multicellular organisms A All of the listed responses are correct. The emergent properties of water help make Earth suitable for life. Water is an effective heat bank because it can absorb or release a relatively large amount of heat with only a slight change in temperature. Because cells are surrounded by water and cells themselves consist of about 70 to 95 percent water, the temperature of living things tends to change very slowly. Many different kinds of polar compounds are dissolved (along with ions) in the water of such biological fluids as blood, the sap of plants, and the liquid within all cells. Water is the solvent of life; therefore, a variety of nutrient molecules is readily available as dissolved solutes. Cohesion due to hydrogen bonding contributes to the transport of water and dissolved nutrients against gravity in plants; therefore, dissolved substances can be easily transported within a cell or between cells in multicellular organisms, and waste products produced by cell metabolism can be easily removed.

Isotopes of an element will always differ in ______________.

A: atomic mass B: atomic number C: number of electrons D: number of protons E: symbol A Isotopes of an element will always differ in atomic mass, the sum of the number of protons and neutrons in the nucleus. Isotopes are different atomic forms of the same element. Although the isotopes of an element have slightly different masses, they behave identically in chemical reactions. Isotopes of an element do not differ in the number of protons but do differ in the number of neutrons. An atom's atomic number is the number of protons. All atoms of a particular element have the same atomic number. The atomic number tells us the number of protons and the number of electrons in an electrically neutral atom, but not all atoms are electrically neutral. Atoms with the same number of protons but different numbers of electrons carry a charge; they are called ions of the element. Each element has a symbol, usually the first letter or two of its name. All isotopes of a given element are represented by the same symbol.

Ionic bonds form as a result of ______.

A: attraction between ions that have opposite charges B: attraction between hydrogen and other atoms that share electrons unequally C: sharing of electron pairs between atoms D: unequal sharing of electrons between atoms E: chance accumulations of electrons as they move around atomic nuclei A Ionic bonds form as a result of attraction between ions that have opposite charges. In some cases, two atoms are so unequal in their attraction for valence electrons that the more electronegative atom strips an electron completely away from its partner. The two resulting oppositely charged atoms (or molecules) are called ions. A positively charged ion is called a cation, while a negatively charged ion is called an anion. Because of their opposite charges, cations and anions attract each other. Electrons are shared in covalent bonds but not in ionic bonds. Unequal sharing of electrons occurs in a polar covalent bond A hydrogen bond is formed as a result of noncovalent attraction between a hydrogen atom and an atom with a partial negative charge. Van der Waals interactions involve chance accumulations of electrons as they move around atomic nuclei. These ever-changing regions of positive and negative charge, even across molecules with nonpolar covalent bonds, enable all molecules and atoms to stick together.

An atom that normally has _______________in its outer shell would not form chemical bonds with other atoms.

A: eight electrons B: one electron C: three electrons D: four electrons E:six electrons A An atom that normally has eight electrons in its outer shell would not form chemical bonds with other atoms. The chemical behavior of an atom depends mostly on the number of electrons in its outermost shell. We call those outer electrons valence electrons and the outermost electron shell the valence shell. Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior. For example, fluorine (F) and chlorine (Cl) both have seven valence electrons, and both form compounds when combined with the element sodium (Na): Sodium fluoride (NaF) is commonly added to toothpaste to prevent tooth decay, and NaCl is table salt. An atom with a completed valence shell is unreactive; that is, it will not interact readily with other atoms. Helium, neon, and argon have full valence shells and are said to be inert, meaning chemically unreactive.

The number of protons in an uncharged atom ____________.

A: equals the number of electrons B: equals the number of neutrons C: varies with the different isotopes D: equals the number of electrons in the outer orbital of the atom E: determines its mass number A The number of protons in an uncharged atom equals the number of electrons. An uncharged atom is an atom that lacks an electrical charge: It is electrically neutral. An uncharged atom must have an equal number of protons, which carry a positive charge, and electrons, which carry a negative charge. Neutrons are electrically neutral, and therefore the number of neutrons does not contribute to the atom's charge. An atom may or may not have the same number of protons and neutrons. Isotopes of an element have the same number of protons and electrons but different numbers of neutrons. The number of protons and the total number of electrons, not just the number in the outer orbital, are equal in an uncharged atom. The number of protons determines an element's atomic number, not the mass number, which equals the sum of the number of protons and the number of neutrons.

Hydrophilic substances, but not hydrophobic substances, _______________________.

A: have charges or partial charges to which water molecules can adhere B: have a higher bond energy than water C: give up electrons to solvents D: accept electrons from solvents E: are repelled by water A Hydrophilic substances, but not hydrophobic substances, have charges or partial charges to which water molecules can adhere. Hydrophilic substances are "water loving" and have an affinity for water. For example, cotton consists of giant molecules of cellulose, a compound with numerous regions of partial positive and partial negative charges that can form hydrogen bonds with water. Water adheres to the cellulose fibers. Thus, a cotton towel does a great job of drying the body, yet it does not dissolve in the washing machine. Bond energy does not relate to how hydrophilic a substance is, a characteristic arising from an unequal sharing of electrons between atoms of a molecule or the presence of more fleeting partial positive and negative charges. Substances need not give up electrons to a solvent in order to be hydrophilic. Unequal sharing of electrons resulting in a polar molecule leads to an affinity for polar water molecules as does the loss or gain of an electron to or from any substance. For the same reason, substances need not accept electrons from a solvent in order to be hydrophilic. Hydrophobic substances, not hydrophilic substances, are repelled by water.

Sweating has a cooling effect because of water's high _______________.

A: heat of vaporization B: buffering capacity C: surface tension D: specific heat E: density A Sweating has a cooling effect because of water's high heat of vaporization. Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state. As a liquid evaporates, the surface of the liquid that remains behind cools down (its temperature decreases). This evaporative cooling occurs because the "hottest" molecules, those with the greatest kinetic energy, are the ones most likely to leave as gas. Evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity. Buffers resist changes in pH but do not have an effect on water's cooling capacity. Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. Water has high surface tension, but this does not directly contribute to the ability of sweating to cool the body. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1°C. Coastal climates are more moderate than inland climates primarily because of water's high specific heat. Water's density is higher than that of some other liquids, but this is not related to cooling the body through sweating.

The chemical characteristics or reactivity of an element depend mostly on the _______.

A: number of electrons in its outermost shell B: number of electron shells C:mean energy level of its electrons D:degree to which it has more or fewer electrons than protons E: number of protons plus the number of neutrons A The chemical characteristics or reactivity of an element depend mostly on the number of electrons in its outermost shell. These are the electrons that are most free to interact chemically with the electrons of other atoms. We call those outer electrons valence electrons and the outermost electron shell the valence shell. Atoms with the same number of electrons in their valence shells exhibit similar chemical behavior. The number of electron shells is not an important determinant of an atom's reactivity. The number of protons plus the number of neutrons is the element's mass number; it does not determine the element's reactivity. The mean energy level of the electrons is not of primary importance in determining the atom's reactivity. The difference in the number of protons versus electrons affects the atom's charge, but not its reactivity.

A covalent bond will be polar if _________________.

A: one of the atoms sharing electrons is more electronegative B: the two atoms sharing electrons are equally electronegative C: the atoms sharing electrons are both highly electronegative D: the two atoms sharing electrons are of the same element E: it is between two atoms that are both very strong electron donors A A covalent bond will be polar if one of the atoms sharing electrons is more electronegative. In a polar covalent bond, one of the atoms in the bond is more electronegative than the other and therefore pulls the shared electrons toward itself. Such bonds vary in their polarity, depending on the relative electronegativity of the two atoms. For example, the bonds between the oxygen and hydrogen atoms of a water molecule are quite polar. Oxygen is one of the most electronegative elements, attracting shared electrons much more strongly than hydrogen does. In a covalent bond between oxygen and hydrogen, the electrons spend more time near the oxygen nucleus than they do near the hydrogen nucleus. If the atoms sharing electrons are equally electronegative, the electrons are shared equally and the bond is nonpolar. This would be the case, for example, for two atoms of the same element. Two strong electron acceptors (high electronegativity) or electron donors (low electronegativity) would not necessarily form a polar covalent bond; it is the difference in electronegativity between two atoms that determines polarity.

Coastal climates are more moderate than inland climates primarily because of water's high __________.

A: specific heat B: heat of fusion C: surface tension D: heat of vaporization E:density A Coastal climates are more moderate than inland climates primarily because of water's high specific heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1°C. Because of its high specific heat, water's temperature will change less when it absorbs or loses a given amount of heat. Thus, the oceans absorb and store a huge amount of heat and release it when the temperature of the air has dropped. Heat of fusion is the heat required to convert a solid into a liquid with no temperature change. Surface tension is a measure of how difficult it is to stretch or break the surface of a liquid. Water has high surface tension, but this does not directly contribute to its ability to moderate climate. Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state. Water has high heat of vaporization, but this is not the feature of hydrogen bonding most directly related to water's ability to moderate climate. Water's density is higher than that of some other liquids, but this is not related to its ability to moderate climate.

An acid is __________.

A: substance that donates hydrogen ions to a solution B: any substance with a pH C: any substance that accepts hydrogen ions D: a substance that resists changes in the pH of a solution E: a solution with a pH between 7 and 14 A An acid is a substance that donates hydrogen ions to a solution. When acids are added to water, they donate additional H+ to the solution. For example, when hydrochloric acid (HCl) is added to water, hydrogen ions dissociate from chloride ions: HCl H+ + Cl-. This source of H+ (dissociation of water is the other source) results in an acidic solution—one having more H+ than OH-. The pH can be measured for any solution, not just acids. Bases, not acids, accept hydrogen ions. Bases thereby decrease the hydrogen ion concentration of solutions. Basic, not acid, solutions have a pH between 7 and 14. A buffer is a substance that minimizes changes in the concentrations of H+ and OH− in a solution and therefore resists changes in pH. It does so by accepting hydrogen ions from the solution when they are in excess and donating hydrogen ions to the solution when they have been depleted.

Adhesion is best described as __________.

A: the clinging of one substance to another substance B: a property of water that helps moderate Earth's temperature C: the process by which a crystalline lattice forms D: the property that contributes to the transport of water and dissolved nutrients in plants by causing water molecules to tug on other water molecules A Adhesion is best described as the clinging of one substance to another substance. The properties of water, including adhesion, arise from attractions between oppositely charged atoms of different molecules. In water, the slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a nearby molecule. The two molecules are thus held together by a hydrogen bond. In adhesion, hydrogen bonding occurs between water and some other substance. Water transport in plants is aided by the adhesion of the water to cell walls. This helps resist the downward pull of gravity. A property of water that helps moderate Earth's temperature is its high specific heat. The specific heat of a substance is defined as the amount of heat required to change the temperature of 1 g of that substance by 1˚C. Freezing is the process by which water molecules become locked in a crystalline lattice. Each water molecule becomes hydrogen-bonded to four other water molecules. Cohesion is the phenomenon by which hydrogen bonds link water molecules together. This hydrogen bonding contributes to the transport of water and dissolved nutrients in plants by causing water molecules to tug on other water molecules.

The amount of heat required to convert 1 g of any substance from the liquid to the gaseous state is defined as __________.

A: the heat of vaporization of that substance B: the specific heat of that substance C: 1 calorie D: the heat of fusion of that substance E: molecular cohesion A The amount of heat required to convert 1 g of any substance from the liquid to the gaseous state is defined as the heat of vaporization of that substance. Heat of vaporization is the quantity of heat a liquid must absorb for 1 g of it to be converted from the liquid to the gaseous state. "The specific heat of that substance" is not the correct answer because the specific heat of a substance is defined as the amount of heat required to change the temperature of 1 g of that substance by 1˚C. "1 calorie" is not the correct answer because this is the amount of energy it takes to raise the temperature of 1 g of water by 1˚C. The amount of energy required to raise 1 g of other substances by 1˚C varies. "The heat of fusion of that substance" is not the correct answer because the heat of fusion is the heat required to convert a solid into a liquid with no temperature change. "Molecular cohesion" is not the correct answer because this refers to the tendency of like substances to stick together, not to an amount of energy.

The amount of heat required to change the temperature of 1 g of a substance by 1°C is defined as ____________.

A: the specific heat of that substance B: 1 calorie C: the heat of vaporization of that substance D: 1 kilocalorie E: cohesion A The amount of heat required to change the temperature of 1 g of a substance by 1°C is defined as the specific heat of that substance. Water has a high specific heat. It is this property of water, related to hydrogen bonding between water molecules, that leads to its moderating effect on environmental temperatures. Coastal climates are more moderate than inland climates primarily because of water's high specific heat. One calorie is defined as the amount of energy it takes to raise the temperature of 1 g of water by 1°C. One kilocalorie is the amount of energy it takes to raise the temperature of 1000 g of water by 1°C. Cohesion refers to the tendency of like substances to stick together, not to an amount of energy. The heat of vaporization is the amount of energy required to convert 1 g of a liquid to the gaseous state.