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Superset The Chemistry of Water 3 Figure 3.1 How does life on Earth depend on the chemistry of water? Key Concepts The Molecule That...

Superset The Chemistry of Water 3 Figure 3.1 How does life on Earth depend on the chemistry of water? Key Concepts The Molecule That Supports All of Life Life on Earth began in water and evolved there for 3 billion years before spreading 3.1 Polar covalent bonds in water molecules result in hydrogen onto land. Water is the substance that makes life possible as we know it here on Earth, bonding and possibly on other planets as well. All organisms familiar to us are made mostly of water and live in an environment dominated by water. 3.2 Four emergent properties of water contribute to Earth’s Three-quarters of Earth’s surface is covered by water. Although most of this water suitability for life is in liquid form, water is also present on Earth as a solid (ice) and a gas (water vapor). Water is the only common substance on Earth to exist in the natural environment in 3.3 Acidic and basic conditions affect living organisms all three physical states of matter. Furthermore, the solid form of water floats on the liquid form, a rare property emerging from the chemistry of the water molecule. As the Earth is warming from climate change (see Concept 1.1), the ratio of ice to liquid water is changing. Arctic sea ice and glaciers are melting, affecting life on, under, and around them. In the Arctic, warmer waters and the smaller ice pack are resulting in blooms of phytoplankton (microscopic aquatic photosynthetic organisms), seen from space as the “cloudy” seawater in Figure 3.1. Organisms that depend on Arctic ice, however, are suffering. For instance, a population of black guillemots in Alaska is declining due to the warming climate and reduction of Arctic sea ice. In this chapter, you will learn how the structure of a water molecule allows it to interact with other molecules, including other water molecules. This ability leads to water’s unique emergent properties that help make Earth suitable for life. When you see this blue icon, log in to MasteringBiology Get Ready for This Chapter and go to the Study Area for digital resources. Black guillemots, threatened by climate change 92 URRY0435_11_C03_GE_PRF.indd 92 12/21/16 8:52 PM  Concept 3.1 are hydrogen-bonded to their neighbors. The extraordinary properties of water emerge from this hydrogen bonding, which Polar covalent bonds in water organizes water molecules into a higher level of structural order. molecules result in hydrogen bonding Concept Check 3.1 Water is so familiar to us that it is easy to overlook its many 1. MAKE CONNECTIONS What is electronegativity, and extraordinary qualities. Following the theme of emergent how does it affect interactions between water molecules? (Review Figure 2.11.) properties, we can trace water’s unique behavior to the 2. VISUAL SKILLS Look at Figure 3.2 and explain why structure and interactions of its molecules. the central water molecule can hydrogen bond to four Studied on its own, the water molecule is deceptively (rather than three or five) other water molecules. simple. It is shaped like a wide V, with its two hydrogen atoms 3. Why is it unlikely that two neighboring water molecules joined to the oxygen atom by single covalent bonds. Oxygen would be arranged like this? is more electronegative than hydrogen, so the electrons of HH the covalent bonds spend more time closer to oxygen than to O O HH hydrogen; these are polar covalent bonds (see Figure 2.11). 4. WHAT IF? What would be the effect on the properties This unequal sharing of electrons and water’s V-like shape of the water molecule if oxygen and hydrogen had equal make it a polar molecule, meaning that its overall charge electronegativity? is unevenly distributed. In water, the oxygen of the molecule For suggested answers, see Appendix A. has two regions of partial negative charge (δ-), and each hydrogen has a partial positive charge (δ+). The properties of water arise from attractions between oppo- Concept 3.2 sitely charged atoms of different water molecules: The partially positive hydrogen of one molecule is attracted to the partially Four emergent properties of water negative oxygen of a nearby molecule. The two molecules are contribute to Earth’s suitability for life thus held together by a hydrogen bond (Figure 3.2). When We will examine four emergent properties of water that con- water is in its liquid form, its hydrogen bonds are very fragile, tribute to Earth’s suitability as an environment for life: cohe- each only about 1/20 as strong as a covalent bond. The hydro- sive behavior, ability to moderate temperature, expansion gen bonds form, break, and re-form with great frequency. Each upon freezing, and versatility as a solvent. lasts only a few trillionths of a second, but the molecules are constantly forming new hydrogen bonds with a succession of partners. Therefore, at any instant, most of the water molecules Cohesion of Water Molecules Water molecules stay close to each other as a result of hydro- gen bonding. Although the arrangement of molecules in a Figure 3.2 Hydrogen bonds between water molecules. sample of liquid water is constantly changing, at any given δ+ moment many of the molecules are linked by multiple hydro- H δ+ H The charged regions in a water gen bonds. These linkages make water more structured than O molecule are due to its polar most other liquids. Collectively, the hydrogen bonds hold covalent bonds. δ– the substance together, a phenomenon called cohesion. Because of its electron δ– Regions of neighboring water molecules with opposite partial Cohesion due to hydrogen bonding contributes to the arrangement, oxygen has two regions with charges are attracted to each other, transport of water and dissolved nutrients against gravity in δ+ partial negative charge. forming hydrogen bonds. plants. Water from the roots reaches the leaves through a net- H work of water-conducting cells (Figure 3.3). As water evapo- Each water molecule can hydrogen-bond to several rates from a leaf, hydrogen bonds cause water molecules O δ– others; these associations δ+ H leaving the veins to tug on molecules farther down, and the are constantly changing. δ– δ+ upward pull is transmitted through the water-conducting δ– cells all the way to the roots. Adhesion, the clinging of one δ+ substance to another, also plays a role. Adhesion of water by hydrogen bonds to the molecules of cell walls helps counter the downward pull of gravity (see Figure 3.3). Related to cohesion is surface tension, a measure of how difficult it is to stretch or break the surface of a liquid. At the Draw It Draw partial charges on the water molecule at the far left, and draw three more water molecules hydrogen-bonded to it. interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the Animation: Polarity of Water water below, but not to the air above. This asymmetry gives chapter 3 The Chemistry of Water 93 URRY0435_11_C03_GE_PRF.indd 93 12/21/16 8:53 PM  Figure 3.3 Water transport in plants. Because of the properties Temperature and Heat of cohesion and adhesion, the tallest trees can transport water more than 100 m upward—approximately one-quarter the height of the Empire Anything that moves has kinetic energy, the energy of State Building in New York City. motion. Atoms and molecules have kinetic energy because Evaporation from leaves pulls water they are always moving, although not necessarily in any par- upward from the roots through ticular direction. The faster a molecule moves, the greater its water-conducting cells. kinetic energy. The kinetic energy associated with the random movement of atoms or molecules is called thermal energy. H 2O Adhesion of the water to cell walls Thermal energy is related to temperature, but they are not the by hydrogen bonds helps resist the downward pull of gravity. same thing. Temperature represents the average kinetic energy of the molecules in a body of matter, regardless of volume, Two types of whereas the thermal energy of a body of matter reflects the water-conducting total kinetic energy, and thus depends on the matter’s volume. cells When water is heated in a coffeemaker, the average speed of the molecules increases, and the thermometer records this as a rise in temperature of the liquid. The total amount of thermal energy also increases in this case. Note, however, that although the pot of coffee has a much higher temperature than, say, the water in a swimming pool, the swimming pool contains more thermal energy because of its much greater volume. Cohesion due to Whenever two objects of different temperature are brought Direction of water 300 μm hydrogen bonds together, thermal energy passes from the warmer to the cooler movement between water object until the two are the same temperature. Molecules in molecules helps hold together the cooler object speed up at the expense of the thermal energy the column of of the warmer object. An ice cube cools a drink not by adding H2O water within the cells. coldness to the liquid, but by absorbing thermal energy from H2O the liquid as the ice itself melts. Thermal energy in transfer from one body of matter to another is defined as heat. BioFlix® Animation: Adhesion and Cohesion in Plants One convenient unit of heat used in this book is the calorie Animation: Cohesion of Water (cal). A calorie is the amount of heat it takes to raise the tem- perature of 1 g of water by 1°C. Conversely, a calorie is also the water an unusually high surface tension, making it behave as amount of heat that 1 g of water releases when it cools by 1°C. though it were coated with an invisible film. You can observe A kilocalorie (kcal), 1,000 cal, is the quantity of heat required the surface tension of water by slightly overfilling a drink- to raise the temperature of 1 kilogram (kg) of water by 1°C. (The ing glass; the water will stand above the rim. The spider in “Calories” on food packages are actually kilocalories.) Another Figure 3.4 takes advantage of the surface tension of water energy unit used in this book is the joule ( J). One joule equals to walk across a pond without breaking the surface. 0.239 cal; one calorie equals 4.184 J. Moderation of Temperature by Water Water’s High Specific Heat Water moderates air temperature by absorbing heat from The ability of water to stabilize temperature stems from its rel- air that is warmer and releasing the stored heat to air that is atively high specific heat. The specific heat of a substance cooler. Water is effective as a heat bank because it can absorb is defined as the amount of heat that must be absorbed or lost or release a relatively large amount of heat with only a slight for 1 g of that substance to change its temperature by 1°C. We change in its own temperature. To understand this capability already know water’s specific heat because we have defined of water, let’s first look at temperature and heat. a calorie as the amount of heat that causes 1 g of water to change its temperature by 1°C. Therefore, the specific heat of Figure 3.4 Walking on water is 1 calorie per gram and per degree Celsius, abbreviated as 1 cal/(g # °C). Compared with most other substances, water water. The high surface tension of water, resulting from the collective strength of its hydrogen has an unusually high specific heat. For example, ethyl alcohol, bonds, allows this raft spider to the type of alcohol in alcoholic beverages, has a specific heat walk on the surface of a pond. of 0.6 cal/(g # °C); that is, only 0.6 cal is required to raise the temperature of 1 g of ethyl alcohol by 1°C. Because of the high specific heat of water relative to other materials, water will change its temperature less than other 94 Unit one The Role of Chemistry in Biology URRY0435_11_C03_GE_PRF.indd 94 12/21/16 8:53 PM  liquids when it absorbs or loses a given amount of heat. The overcome these attractions can depart the liquid and enter reason you can burn your fingers by touching the side of an the air as a gas (vapor). This transformation from a liquid to iron pot on the stove when the water in the pot is still luke- a gas is called vaporization, or evaporation. Recall that the warm is that the specific heat of water is ten times greater speed of molecular movement varies and that temperature than that of iron. In other words, the same amount of heat is the average kinetic energy of molecules. Even at low tem- will raise the temperature of 1 g of the iron much faster than peratures, the speediest molecules can escape into the air. it will raise the temperature of 1 g of the water. Specific heat Some evaporation occurs at any temperature; a glass of water can be thought of as a measure of how well a substance resists at room temperature, for example, will eventually evaporate changing its temperature when it absorbs or releases heat. completely. If a liquid is heated, the average kinetic energy of Water resists changing its temperature; when it does change molecules increases and the liquid evaporates more rapidly. its temperature, it absorbs or loses a relatively large quantity Heat of vaporization is the quantity of heat a liquid of heat for each degree of change. must absorb for 1 g of it to be converted from the liquid to the We can trace water’s high specific heat, like many of gaseous state. For the same reason that water has a high specific its other properties, to hydrogen bonding. Heat must be heat, it also has a high heat of vaporization relative to most absorbed in order to break hydrogen bonds; by the same other liquids. To evaporate 1 g of water at 25°C, about 580 cal of token, heat is released when hydrogen bonds form. A calorie heat is needed—nearly double the amount needed to vaporize a of heat causes a relatively small change in the temperature of gram of alcohol or ammonia. Water’s high heat of vaporization water because much of the heat is used to disrupt hydrogen is another emergent property resulting from the strength of its bonds before the water molecules can begin moving faster. hydrogen bonds, which must be broken before the molecules And when the temperature of water drops slightly, many can exit from the liquid in the form of water vapor. additional hydrogen bonds form, releasing a considerable The high amount of energy required to vaporize water has amount of energy in the form of heat. a wide range of effects. On a global scale, for example, it helps What is the relevance of water’s high specific heat to life moderate Earth’s climate. A considerable amount of solar heat on Earth? A large body of water can absorb and store a huge absorbed by tropical seas is consumed during the evaporation amount of heat from the sun in the daytime and during sum- of surface water. Then, as moist tropical air circulates poleward, mer while warming up only a few degrees. At night and dur- it releases heat as it condenses and forms rain. On an organis- ing winter, the gradually cooling water can warm the air. This mal level, water’s high heat of vaporization accounts for the capability of water serves to moderate air temperatures in severity of steam burns. These burns are caused by the heat coastal areas (Figure 3.5). The high specific heat of water also energy released when steam condenses into liquid on the skin. tends to stabilize ocean temperatures, creating a favorable As a liquid evaporates, the surface of the liquid that environment for marine life. Thus, because of its high specific remains behind cools down (its temperature decreases). heat, the water that covers most of Earth keeps temperature This evaporative cooling occurs because the “hottest” fluctuations on land and in water within limits that permit molecules, those with the greatest kinetic energy, are the life. Also, because organisms are made primarily of water, most likely to leave as gas. It is as if the 100 fastest runners they are better able to resist changes in their own temperature at a college transferred to another school; the average speed than if they were made of a liquid with a lower specific heat. of the remaining students would decline. Evaporative cooling of water contributes to the stability of Evaporative Cooling temperature in lakes and ponds and also provides a mecha- Molecules of any liquid stay close together because they are nism that prevents terrestrial organisms from overheating. attracted to one another. Molecules moving fast enough to For example, evaporation of water from the leaves of a plant helps keep the tissues in the leaves from becoming too warm in the sunlight. Evaporation of sweat from human skin dis- Figure 3.5 Temperatures for the Pacific Ocean and Southern California on an August day. sipates body heat and helps prevent overheating on a hot day or when excess heat is generated by strenuous activity. High Burbank San Bernardino humidity on a hot day increases discomfort because the high Santa Barbara 73° 100° 90° concentration of water vapor in the air inhibits the evapora- Los Angeles Riverside 96° (Airport) 75° Santa Ana tion of sweat from the body. 84° Palm Springs 70s (°F) 106° 80s Pacific Ocean 68° Floating of Ice on Liquid Water 90s Water is one of the few substances that are less dense as a solid 100s San Diego 72° 40 miles than as a liquid. In other words, ice floats on liquid water. While INTERPRET THE DATA Explain the pattern of temperatures shown other materials contract and become denser when they solidify, in this diagram. water expands. The cause of this exotic behavior is, once again, chapter 3 The Chemistry of Water 95 URRY0435_11_C03_GE_PRF.indd 95 12/21/16 8:53 PM  Figure 3.6 Ice: crystalline structure Hydrogen bond and floating barrier. In ice, each molecule Liquid water: Hydrogen bonds is hydrogen-bonded to four neighbors in break and re-form a three-dimensional crystal. Because the crystal is spacious, ice has fewer molecules than an equal volume of liquid water. In other words, ice is less dense than liquid water. Floating ice becomes a barrier that insulates the liquid water below from the colder air. The marine organism shown here is a type of shrimp called krill; it was photographed beneath floating ice in the Southern Ocean near Antarctica. WHAT IF? If water did not form hydrogen Ice: bonds, what would happen to the shrimp’s Hydrogen bonds habitat, shown here? are stable hydrogen bonding. At temperatures above 4°C, water behaves Many scientists are worried that these bodies of ice are at risk like other liquids, expanding as it warms and contracting as it of disappearing. Global warming, which is caused by carbon cools. As the temperature falls from 4°C to 0°C, water begins to dioxide and other “greenhouse” gases in the atmosphere (see freeze because more and more of its molecules are moving too Figure 56.28), is having a profound effect on icy environments slowly to break hydrogen bonds. At 0°C, the molecules become around the globe. In the Arctic, the average air temperature locked into a crystalline lattice, each water molecule hydrogen- has risen 2.2°C just since 1961. This temperature increase has bonded to four partners (Figure 3.6). The hydrogen bonds affected the seasonal balance between Arctic sea ice and liquid keep the molecules at “arm’s length,” far enough apart to make water, causing ice to form later in the year, to melt earlier, and ice about 10% less dense (10% fewer molecules in the same vol- to cover a smaller area. The rate at which glaciers and Arctic sea ume) than liquid water at 4°C. When ice absorbs enough heat ice are disappearing is posing an extreme challenge to animals for its temperature to rise above 0°C, hydrogen bonds between that depend on ice for their survival (Figure 3.7). molecules are disrupted. As the crystal col- lapses, the ice melts and molecules have Figure 3.7 Effects of climate change on the Arctic. Warmer temperatures in the Arctic fewer hydrogen bonds, allowing them cause more sea ice to melt in the summer, benefiting some organisms and harming others. to slip closer together. Water reaches its greatest density at 4°C and then begins to Species that are benefiting from loss of ice: expand as the molecules move faster. Even More light and warmer Bowhead Some fish species, in liquid water, many of the molecules are waters result in more whales, which feed such as capelin, connected by hydrogen bonds, though phytoplankton, which on plankton they benefit from only transiently: The hydrogen bonds are are eaten by filter, are thriving. having more other organ- plankton to constantly breaking and re-forming. isms. eat. The ability of ice to float due to its Species being harmed lower density is an important factor by loss of ice: in the suitability of the environment Russia Loss of ice has reduced feeding Arctic for life. If ice sank, then eventually all opportunities for ocean ponds, lakes, and even oceans would polar bears, who Extent of sea ice in Sept. 2014 freeze solid, making life as we know it hunt from the ice. Extent of sea ice in Sept. 1979 impossible on Earth. During summer, Bering only the upper few inches of the ocean Strait The Pacific walrus depends North Pole would thaw. Instead, when a deep body on the ice to rest; its fate is uncertain. Greenland of water cools, the floating ice insulates the liquid water below, preventing it Black guillemots in Alaska from freezing and allowing life to exist cannot fly from their nests Alaska under the frozen surface, as shown in the on land to their fishing grounds at the edge photo in Figure 3.6. Besides insulating of the ice, which is the water below, ice also provides a solid now too far from land; Canada habitat for some animals, such as polar young birds are starving. bears and seals. Sea ice in Sept. 2014 Ice lost from Sept. 1979 to Sept. 2014 96 Unit one The Role of Chemistry in Biology URRY0435_11_C03_GE_PRF.indd 96 12/21/16 8:53 PM  Water: The Solvent of Life Figure 3.9 A water-soluble protein. Human lysozyme is a protein found in tears and saliva that has antibacterial action (see A sugar cube placed in a glass of water will dissolve with a little Figure 5.16). This model shows the lysozyme molecule (purple) in an stirring. The glass will then contain a uniform mixture of sugar aqueous environment. Ionic and polar regions on the protein’s surface and water; the concentration of dissolved sugar will be the same attract the partially charged regions on water molecules. everywhere in the mixture. A liquid that is a completely homo- This oxygen is attracted to a slight geneous mixture of two or more substances is called a solution. positive charge on The dissolving agent of a solution is the solvent, and the sub- the lysozyme stance that is dissolved is the solute. In this case, water is the molecule. solvent and sugar is the solute. An aqueous solution is one in which the solute is dissolved in water; water is the solvent. δ+ Water is a very versatile solvent, a quality we can trace to δ– δ– the polarity of the water molecule. Suppose, for example, that a spoonful of table salt, the ionic compound sodium δ+ chloride (NaCl), is placed in water (Figure 3.8). At the sur- face of each crystal of salt, the sodium and chloride ions are exposed to the solvent. These ions and regions of the water This hydrogen is attracted to a slight molecules are attracted to each other due to their opposite negative charge on charges. The oxygens of the water molecules have regions of the lysozyme partial negative charge that are attracted to sodium cations. molecule. The hydrogen regions are partially positively charged and are attracted to chloride anions. As a result, water molecules sur- such as the sugar in the sugar cube mentioned earlier, are also round the individual sodium and chloride ions, separating water-soluble. Such compounds dissolve when water mol- and shielding them from one another. The sphere of water ecules surround each of the solute molecules, forming hydro- molecules around each dissolved ion is called a hydration gen bonds with them. Even molecules as large as proteins shell. Working inward from the surface of each salt crystal, can dissolve in water if they have ionic and polar regions on water eventually dissolves all the ions. The result is a solu- their surface (Figure 3.9). Many different kinds of polar com- tion of two solutes, sodium cations and chloride anions, pounds are dissolved (along with ions) in the water of such homogeneously mixed with water, the solvent. Other ionic biological fluids as blood, the sap of plants, and the liquid compounds also dissolve in water. Seawater, for instance, within all cells. Water is the solvent of life. contains a great variety of dissolved ions, as do living cells. A compound does not need to be ionic to dissolve in water; Hydrophilic and Hydrophobic Substances many compounds made up of nonionic polar molecules, Any substance that has an affinity for water is said to be hydrophilic (from the Greek hydro, water, and philos, loving). In some cases, substances can be hydrophilic with- Negative oxygen regions out actually dissolving. For example, some molecules in of polar water _ cells are so large that they do not dissolve. Another example molecules are Na+ +_ of a hydrophilic substance that does not dissolve is cotton, attracted to sodium + cations (Na+). _ a plant product. Cotton consists of giant molecules of cellu- + _ _ lose, a compound with numerous regions of partial positive Positive Na+ and partial negative charges that can form hydrogen bonds hydrogen regions _ of water molecules + + with water. Water adheres to the cellulose fibers. Thus, a cot- are attracted Cl– Cl– ton towel does a great job of drying the body, yet it does not to chloride + _ _ dissolve in the washing machine. Cellulose is also present in anions (Cl–). + the walls of water-conducting cells in a plant; you read earlier – + how the adhesion of water to these hydrophilic walls helps _ water move up the plant against gravity. _ There are, of course, substances that do not have an affinity for water. Substances that are nonionic and nonpolar (or oth- Figure 3.8 Table salt dissolving erwise cannot form hydrogen bonds) actually seem to repel in water. A sphere of water molecules, called a hydration shell, surrounds each water; these substances are said to be hydrophobic (from solute ion. the Greek phobos, fearing). An example from the kitchen is WHAT IF? What would happen if you vegetable oil, which, as you know, does not mix stably with heated this solution for a long time? water-based substances such as vinegar. The hydrophobic chapter 3 The Chemistry of Water 97 URRY0435_11_C03_GE_PRF.indd 97 12/21/16 8:53 PM  behavior of the oil molecules results from a prevalence of rela- Water’s capacity as a versatile solvent complements the tively nonpolar covalent bonds, in this case bonds between other properties discussed in this chapter. Since these remark- carbon and hydrogen, which share electrons almost equally. able properties allow water to support life on Earth so well, Hydrophobic molecules related to oils are major ingredients scientists who seek life elsewhere in the universe look for of cell membranes. (Imagine what would happen to a cell if its water as a sign that a planet might sustain life. membrane dissolved!) MP3 Tutor: The Properties of Water Solute Concentration in Aqueous Solutions Possible Evolution of Life on Other Planets Most of the chemical reactions in organisms involve solutes dissolved in water. To understand such reactions, we must Evolution Biologists who look for life elsewhere in the uni- know how many atoms and molecules are involved and cal- verse (known as astrobiologists) have concentrated their search culate the concentration of solutes in an aqueous solution on planets that might have water. More than 800 planets have (the number of solute molecules in a volume of solution). been found outside our solar system, and there is evidence for When carrying out experiments, we use mass to calculate the the presence of water vapor on a few of them. In our own solar number of molecules. We must first calculate the molecular system, Mars has been a focus of study. Like Earth, Mars has mass, which is the sum of the masses of all the atoms in a an ice cap at both poles. Images from spacecraft sent to Mars molecule. As an example, let’s calculate the molecular mass of showed that ice is present just under the surface of Mars and table sugar (sucrose), C12H22O11, by multiplying the number of enough water vapor exists in its atmosphere for frost to form. atoms by the atomic mass of each element (see Appendix B). In 2015, scientists found evidence of water flowing on Mars In round numbers of daltons, the mass of a carbon atom is 12, (Figure 3.10), and other studies suggested conditions existed the mass of a hydrogen atom is 1, and the mass of an oxygen that could have supported microorganismal life. Drilling be- atom is 16. Thus, sucrose has a molecular mass of (12 * 12) + low the surface may be the next step in the search for signs of (22 * 1) + (11 * 16) = 342 daltons. Because we can’t weigh out life on Mars. If any life-forms or fossils are found, their study small numbers of molecules, we usually measure substances in will shed light on the process of evolution from an entirely units called moles. Just as a dozen always means 12 objects, a new perspective. mole (mol) represents an exact number of objects: 6.02 * 1023, Figure 3.10 Evidence for liquid water on Mars. Water which is called Avogadro’s number. Because of the way in appears to have helped form these dark streaks that run downhill which Avogadro’s number and the unit dalton were originally on Mars during the summer. NASA scientists also found evidence defined, there are 6.02 * 1023 daltons in 1 g. Once we deter- of hydrated salts, indicating water is present. (This digitally treated photograph was taken by the Mars Reconnaissance Orbiter.) mine the molecular mass of a molecule such as sucrose, we can use the same number (342), but with the unit gram, to repre- sent the mass of 6.02 * 1023 molecules of sucrose, or 1 mol of sucrose (this is sometimes called the molar mass). To obtain Dark streaks 1 mol of sucrose in the lab, therefore, we weigh out 342 g. The practical advantage of measuring a quantity of chemi- cals in moles is that a mole of one substance has exactly the same number of molecules as a mole of any other substance. If the molecular mass of substance A is 342 daltons and that of substance B is 10 daltons, then 342 g of A will have the same number of molecules as 10 g of B. A mole of ethyl alco- hol (C2H6O) also contains 6.02 * 1023 molecules, but its mass is only 46 g because the mass of a molecule of ethyl alcohol is Concept Check 3.2 less than that of a molecule of sucrose. Measuring in moles 1. Describe how properties of water contribute to the makes it convenient for scientists working in the laboratory upward movement of water in a tree. to combine substances in fixed ratios of molecules. 2. Explain the saying “It’s not the heat; it’s the humidity.” How would we make a liter (L) of solution consisting of 3. How can the freezing of water crack boulders? 1 mol of sucrose dissolved in water? We would measure out 4. WHAT IF? Dishwashing soaps are made up of molecules 342 g of sucrose and then gradually add water, while stirring, with one hydrophilic end and one hydrophobic end. How until the sugar was completely dissolved. We would then add does this help to clean greasy utensils? What would happen if soaps lose their hydrophobic property? enough water to bring the total volume of the solution up to 5. INTERPRET THE DATA The concentration of the appetite- 1 L. At that point, we would have a 1-molar (1 M) solution of regulating hormone ghrelin is about 1.3 * 10-10 M in the sucrose. Molarity—the number of moles of solute per liter blood of a fasting person. How many molecules of ghrelin of solution—is the unit of concentration most often used by are in 1 L of blood? biologists for aqueous solutions. For selected answers, see Appendix A. 98 Unit one The Role of Chemistry in Biology URRY0435_11_C03_GE_PRF.indd 98 12/21/16 8:53 PM  Concept 3.3 Acids and Bases What would cause an aqueous solution to have an imbal- Acidic and basic conditions affect ance in H+ and OH- concentrations? When acids dissolve in living organisms water, they donate additional H+ to the solution. An acid is a substance that increases the hydrogen ion concentration of a Occasionally, a hydrogen atom participating in a hydrogen solution. For example, when hydrochloric acid (HCl) is added bond between two water molecules shifts from one mol- to water, hydrogen ions dissociate from chloride ions: ecule to the other. When this happens, the hydrogen atom HCl S H+ + Cl- leaves its electron behind, and what is actually transferred is a hydrogen ion (H+), a single proton with a charge of 1+. This source of H+ (dissociation of water is the other source) The water molecule that lost a proton is now a hydroxide results in an acidic solution—one having more H+ than OH-. ion (OH-), which has a charge of 1-. The proton binds to the A substance that reduces the hydrogen ion concentration other water molecule, making that molecule a hydronium of a solution is called a base. Some bases reduce the H+ con- ion (H3O+). We can picture the chemical reaction as follows: centration directly by accepting hydrogen ions. Ammonia (NH3), for instance, acts as a base when the unshared electron + – pair in nitrogen’s valence shell attracts a hydrogen ion from H H the solution, resulting in an ammonium ion (NH4+): NH3 + H+ L NH4+ O H O O H + O H H H H 2 H2O Other bases reduce the H+ concentration indirectly by dissoci- Hydronium Hydroxide ion (H3O+) ion (OH–) ating to form hydroxide ions, which combine with hydrogen ions and form water. One such base is sodium hydroxide Animation: Dissociation of Water Molecules (NaOH), which in water dissociates into its ions: NaOH S Na+ + OH- + By convention, H (the hydrogen ion) is used to represent In either case, the base reduces the H+ concentration. H3O+ (the hydronium ion), and we follow that practice in this Solutions with a higher concentration of OH- than H+ are book. Keep in mind, though, that H+ does not exist on its own known as basic solutions. A solution in which the H+ and in an aqueous solution. It is always associated with a water OH- concentrations are equal is said to be neutral. molecule in the form of H3O+. Notice that single arrows were used in the reactions for As indicated by the double arrows, this is a reversible HCl and NaOH. These compounds dissociate completely reaction that reaches a state of dynamic equilibrium when when mixed with water, so hydrochloric acid is called a water molecules dissociate at the same rate that they are strong acid and sodium hydroxide a strong base. In contrast, being reformed from H+ and OH-. At this equilibrium point, ammonia is a weak base. The double arrows in the reaction the concentration of water molecules greatly exceeds the for ammonia indicate that the binding and release of hydro- concentrations of H+ and OH-. In pure water, only one gen ions are reversible reactions, although at equilibrium water molecule in every 554 million is dissociated; the there will be a fixed ratio of NH4+ to NH3. concentration of H+ and of OH- in pure water is therefore Weak acids are acids that reversibly release and accept back 10-7 M (at 25°C). This means there is only one ten-millionth hydrogen ions. An example is carbonic acid: L of a mole of hydrogen ions per liter of pure water and an equal number of hydroxide ions. (Even so, this is a huge H2CO3 HCO3- + H+ number—over 60,000 trillion—of each ion in a liter of Carbonic acid Bicarbonate ion Hydrogen ion pure water.) Here the equilibrium so favors the reaction in the left direction Although the dissociation of water is reversible and that when carbonic acid is added to pure water, only 1% of the statistically rare, it is exceedingly important in the chem- molecules are dissociated at any particular time. Still, that is istry of life. H+ and OH- are very reactive. Changes in their enough to shift the balance of H+ and OH- from neutrality. concentrations can drastically affect a cell’s proteins and other complex molecules. As we have seen, the concentra- The pH Scale tions of H+ and OH- are equal in pure water, but adding In any aqueous solution at 25°C, the product of the H+ and certain kinds of solutes, called acids and bases, disrupts OH- concentrations is constant at 10-14. This can be written this balance. Biologists use something called the pH scale to describe how acidic or basic (the opposite of acidic) a [H+][OH-] = 10-14 solution is. In the remainder of this chapter, you will learn (The brackets indicate molar concentration.) As previously about acids, bases, and pH and why changes in pH can mentioned, in a neutral solution at 25°C, [H+] = 10-7 and adversely affect organisms. [OH-] = 10-7. Therefore, the product of [H+] and [OH-] in a chapter 3 The Chemistry of Water 99 URRY0435_11_C03_GE_PRF.indd 99 12/21/16 8:53 PM  neutral solution at 25°C is 10-14. If enough acid is added to a The pH of a solution is defined as the negative logarithm solution to increase [H+] to 10-5 M, then [OH-] will decline by (base 10) of the hydrogen ion concentration: an equivalent factor to 10-9 M (note that 10-5 * 10-9 = 10-14). pH = -log [H+] This constant relationship expresses the behavior of acids and bases in an aqueous solution. An acid not only adds For a neutral aqueous solution, [H+] is 10-7 M, giving us hydrogen ions to a solution, but also removes hydroxide -log 10-7 = -(-7) = 7 ions because of the tendency for H+ to combine with OH-, Notice that pH decreases as H+ concentration increases (see forming water. A base has the opposite effect, increasing Figure 3.11). Notice, too, that although the pH scale is based OH- concentration but also reducing H+ concentration by on H+ concentration, it also implies OH- concentration. the formation of water. If enough of a base is added to raise A solution of pH 10 has a hydrogen ion concentration of the OH- concentration to 10-4 M, it will cause the H+ con- 10-10 M and a hydroxide ion concentration of 10-4 M. centration to drop to 10-10 M. Whenever we know the con- The pH of a neutral aqueous solution at 25°C is 7, the centration of either H+ or OH- in an aqueous solution, we can midpoint of the pH scale. A pH value less than 7 denotes an deduce the concentration of the other ion. acidic solution; the lower the number, the more acidic the Because the H+ and OH- concentrations of solutions can solution. The pH for basic solutions is above 7. Most biologi- vary by a factor of 100 trillion or more, scientists have devel- cal fluids, such as blood and saliva, are within the range of oped a way to express this variation more conveniently than pH 6–8. There are a few exceptions, however, including the in moles per liter. The pH scale (Figure 3.11) compresses the strongly acidic digestive juice of the human stomach (gastric range of H+ and OH- concentrations by employing logarithms. juice), which has a pH of about 2. Remember that each pH unit represents a tenfold differ- Figure 3.11 The pH scale and pH values of some aqueous solutions. ence in H+ and OH- concentrations. It is this mathematical feature that makes the pH scale so compact. A solution of pH Scale 0 pH 3 is not twice as acidic as a solution of pH 6, but 1,000 times (10 * 10 * 10) more acidic. When the pH of a solu- 1 tion changes slightly, the actual concentrations of H+ and Battery acid OH- in the solution change substantially. 2 Gastric juice (in stomach), H+ lemon juice Buffers Increasingly Acidic + H + [H+] > [OH–] OH– H 3 Vinegar, wine, + H – H+ The internal pH of most living cells is close to 7. Even a slight OH H+ cola H+ H+ change in pH can be harmful because the chemical processes Acidic 4 Tomato juice of the cell are very sensitive to the concentrations of hydrogen solution Beer and hydroxide ions. The pH of human blood is very close to Black coffee 5 7.4, which is slightly basic. A person cannot survive for more Rainwater than a few minutes if the blood pH drops to 7 or rises to 7.8, 6 Urine and a chemical system exists in the blood that maintains OH– Saliva a stable pH. If 0.01 mol of a strong acid is added OH– – Neutral H+ H+ OH [H+] = [OH–] 7 Pure water to a liter of pure water, the pH drops from – OH– OH + H+ H Human blood, tears 7.0 to 2.0. If the same amount of acid is added H+ 8 Seawater to a liter of blood, however, the pH decrease Neutral solution Inside small intestine is only from 7.4 to 7.3. Why does the addition 9 of acid have so much less of an effect on the pH of blood than it does on the pH of water? Increasingly Basic [H+] < [OH–] 10 The presence of substances called buffers allows biological OH – Milk of magnesia fluids to maintain a relatively constant pH despite the addi- OH– – 11 tion of acids or bases. A buffer is a substance that minimizes OH– OH + H OH– OH – Household ammonia changes in the concentrations of H+ and OH- in a solution. It H+ OH– 12 does so by accepting hydrogen ions from the solution when Basic they are in excess and donating hydrogen ions to the solution solution Household 13 bleach when they have been depleted. Most buffer solutions con- tain a weak acid and its corresponding base, which combine Oven cleaner 14 reversibly with hydrogen ions. Several buffers contribute to pH stability in human blood Animation: Acids, Bases, and pH and many other biological solutions. One of these is carbonic 100 Unit one The Role of Chemistry in Biology URRY0435_11_C03_GE_PRF.indd 100 12/21/16 8:53 PM  acid (H2CO3), which is formed when CO2 reacts with water in Figure 3.12 Atmospheric CO2 from human activities blood plasma. As mentioned earlier, carbonic acid dissociates and its fate in the ocean. to yield a bicarbonate ion (HCO3-) and a hydrogen ion (H+): Response to a rise in pH ∆ Some carbon dioxide (CO2) in the atmo- H2CO3 ∆ HCO 3 -    + H + CO2 sphere dissolves in H + donor Response to H + acceptor Hydrogen the ocean, where it (acid) a drop in pH (base) ion reacts with water to form carbonic acid The chemical equilibrium between carbonic acid and bicar- (H2CO3). bonate acts as a pH regulator, the reaction shifting left or right as other processes in the solution add or remove hydro- Carbonic acid CO2 + H2O H2CO3 dissociates into gen ions. If the H+ concentration in blood begins to fall (that hydrogen ions (H+) is, if pH rises), the reaction proceeds to the right and more and bicarbonate ions carbonic acid dissociates, replenishing hydrogen ions. But H2CO3 H+ + HCO3– (HCO3–). when the H+ concentration in blood begins to rise (when The added H+ pH drops), the reaction proceeds to the left, with HCO3- (the combines with H+ + CO32– HCO3– base) removing the hydrogen ions from the solution and carbonate ions (CO32–), forming forming H2CO3. Thus, the carbonic acid–bicarbonate buffer- more HCO3–. ing system consists of an acid and a base in equilibrium with CO32– + Ca2+ CaCO3 each other. Most other buffers are also acid-base pairs. Less CO32– is avail- able for calcification Acidification: A Threat to Our Oceans —the formation of calcium carbonate Among the many threats to water quality posed by human (CaCO3)—by marine organisms such as activities is the burning of fossil fuels, which releases CO2 corals. into the atmosphere. The resulting increase in atmospheric CO2 levels has caused global warming and other aspects of VISUAL SKILLS Looking at all the chemical ABC News Video: climate change (see Concept 56.4). In addition, about 25% of equations above, summarize the effect of Ocean Acidification adding excess CO2 to the oceans on the human-generated CO2 is absorbed by the oceans. In spite of calcification process in the final equation. the huge volume of water in the oceans, scientists worry that the absorption of so much CO2 will harm marine ecosystems. The disappearance of coral reef ecosystems would be a tragic Recent data have shown that such fears are well founded. loss of biological diversity. When CO2 dissolves in seawater, it reacts with water to form If there is any reason for optimism about the future qual- carbonic acid, which lowers ocean pH. This process, known as ity of water resources on our planet, it is that we have made ocean acidification, alters the delicate balance of conditions progress in learning about the delicate chemical balances in for life in the oceans (Figure 3.12). Based on measurements of oceans, lakes, and rivers. Continued progress can come only CO2 levels in air bubbles trapped in ice over thousands of years, from the actions of informed individuals, like yourselves, who scientists calculate that the pH of the oceans is 0.1 pH unit are concerned about environmental quality. This requires lower now than at any time in the past 420,000 years. Recent understanding the crucial role that water plays in the suitabil- studies predict that it will drop another 0.3–0.5 pH unit by the ity of the environment for continued life on Earth. end of this century. As seawater acidifies, the extra hydrogen ions com- Concept Check 3.3 bine with carbonate ions (CO32-) to form bicarbonate ions (HCO3-), thereby reducing the carbonate ion concentration 1. Compared with a basic solution at pH 9, the same volume of an acidic solution at pH 4 has _________ times as many (see Figure 3.12). Scientists predict that ocean acidification hydrogen ions (H+). will cause the carbonate ion concentration to decrease by 2. Is the pH of 0.01 M HCl the same as that of 0.001 M HCl? 40% by the year 2100. This is of great concern because car- Explain. bonate ions are required for calcification, the production 3. Acetic acid (CH3COOH) can be a buffer, similar to car- of calcium carbonate (CaCO3) by many marine organisms, bonic acid. Write the dissociation reaction, identifying the acid, base, H+ acceptor, and H+ donor. including reef-building corals and animals that build shells. 4. WHAT IF? What would happen to a solution of acetic The Scientific Skills Exercise allows you to work with data acid if a strong base like NaOH is added to it? Use the from an experiment examining the effect of carbonate ion reaction from question 3 to explain the result. concentration on coral reefs. Coral reefs are sensitive eco- For suggested answers, see Appendix A. systems that act as havens for a great diversity of marine life. chapter 3 The Chemistry of Water 101 URRY0435_11_C03_GE_PRF.indd 101 12/21/16 8:53 PM  Scientific Skills Exercise Interpreting a Scatter Plot with a Regression Line How Does the Carbonate Ion Concentration of Seawater Affect the Calcification Rate of a Coral Reef? Scientists predict that acidification of the ocean due to higher levels of atmospheric CO2 will lower the concentration of dissolved carbonate ions, which living corals use to build calcium carbonate reef structures. In this exercise, [mmol CaCO3 /(m2 day)] you will analyze data from a controlled experiment that examined the 20 Calcification rate effect of carbonate ion concentration ([CO32-]) on calcium carbonate deposition, a process called calcification. How the Experiment Was Done For several years, scientists con- 10 ducted research on ocean acidification using a large coral reef aquar- ium at Biosphere 2 in Arizona. They measured the rate of calcification by the reef organisms and examined how the calcification rate changed with differing amounts of dissolved carbonate ions in the seawater. 0 220 240 260 280 Data from the Experiment The black data points in the graph – form a scatter plot. The red line, known as a linear regression line, [CO32 ] (μmol/kg of seawater) is the best-fitting straight line for these points. Data from C. Langdon et al., Effect of calcium carbonate saturation state on Interpret the Data the calcification rate of an experimental coral reef, Global Biogeochemical Cycles 1. When presented with a graph of experimental data, the first step 14:639–654 (2000). in analysis is to determine what each axis represents. (a) In words, explain what is being shown on the x-axis. Be sure to include the carbonate ion concentration is 250 µmol/kg, what is the approxi- units. (b) What is being shown on the y-axis (including units)? mate rate of calcification, and approximately how many days (c) Which variable is the independent variable—the variable that would it take 1 square meter of reef to accumulate 30 mmol of was manipulated by the researchers? (d) Which variable is the calcium carbonate? (c) If carbonate ion concentration decreases, dependent variable—the variable that responded to or depended how does the calcification rate change, and how does that affect on the treatment, which was measured by the researchers? (For the time it takes coral to grow? additional information about graphs, see the Scientific Skills 4. (a) Referring to the equations in Figure 3.12, determine which Review in Appendix F.) step of the process is measured in this experiment. (b) Are the 2. Based on the data shown in the graph, describe in words the rela- results of this experiment consistent with the hypothesis that tionship between carbonate ion concentration and calcification rate. increased atmospheric [CO2] will slow the growth of coral reefs? 3. (a) If the seawater carbonate ion concentration is 270 µmol/kg, Why or why not? what is the approximate rate of calcification, and approximately how many days would it take 1 square meter of reef to accumu- Instructors: A version of this Scientific Skills Exercise late 30 mmol of calcium carbonate (CaCO3)? (b) If the seawater can be assigned in MasteringBiology. 3 Chapter Review Go to MasteringBiology™ for Videos, Animations, Vocab Self-Quiz, Practice Tests, and more in the Study Area. Draw It Label a hydrogen bond and a polar covalent bond in the dia- Summary of Key Concepts gram of five water molecules. Is a hydrogen bond a covalent bond? Explain. Concept 3.1 Concept 3.2 Polar covalent bonds in water Four emergent properties of water contribute molecules result in hydrogen to Earth’s suitability for life (pp. 93–98) bonding (p. 93) δ– VOCAB SELF-QUIZ Hydrogen bonding keeps water molecules close to each other, δ+ goo.gl/Rn5Uax Water is a polar molecule. A hydrogen bond giving water cohesion. Hydrogen bonding is also responsible for H forms when a partially negatively charged water’s surface tension. region on the oxygen of one δ– O Water has a high specific heat: Heat is absorbed when hydrogen δ+ H water molecule is attracted to the δ– δ+ bonds break and is released when hydrogen bonds form. This partially positively charged hydro- δ– helps keep temperatures relatively steady, within limits that δ+ gen of a nearby water molecule. Hydrogen permit life. Evaporative cooling is based on water’s high heat bonding between water molecules is the of vaporization. The evaporative loss of the most energetic basis for water’s properties. water molecules cools a surface. 102 Unit ONE  The Role of Chemistry in Biology URRY0435_11_C03_GE_PRF.indd 102 12/21/16 8:53 PM  Ice floats because it is less dense than liquid water. This property method works. Be sure to mention why hydrogen bonds are allows life to exist under the frozen surfaces of lakes and polar seas. responsible for this phenomenon. Water is an unusually versatile solvent because its polar mol- ecules are attracted to ions and polar substances that can form 8. MAKE CONNECTIONS What do climate change (see hydrogen bonds. Hydrophilic substances have an affinity for Concept 1.1 and Concept 3.2) and ocean acidification water; hydrophobic substances do not. Molarity, the number have in common? of moles of solute per liter of solution, is used as a measure of 9. EVOLUTION CONNECTION This chapter explains how the solute concentration in solutions. A mole is a certain number emergent properties of water contribute to the suitability of molecules of a substance. The mass of a mole of a substance in of the environment for life. Until fairly recently, scientists grams is the same as the molecular mass in daltons. assumed that other physical requirements for life included The emergent properties of water support life on Earth and may a moderate range of temperature, pH, atmospheric pressure, contribute to the potential for life to have evolved on other planets. and salinity, as well as low levels of toxic chemicals. That view has changed with the discovery of organisms known ? Describe how different types of solutes dissolve in water. Explain what a solution is. as extremophiles, which have been found flourishing in hot, acidic sulfur springs, around hydrothermal vents deep in the ocean, and in soils with high levels of toxic Concept 3.3 metals. Why would astrobiologists be interested in studying Acidic and basic conditions affect living extremophiles? What does the existence of life in such organisms (pp. 99–102) extreme environments say about the possibility of life on other planets? A water molecule can transfer an H+ to another water molecule to form H3O+ (represented simply by H+) and OH-. 10. SCIENTIFIC INQUIRY Design a controlled experiment to test The concentration of H+ is the hypothesis that water acidification caused by acidic rain 0 would inhibit the growth of Elodea, a freshwater plant (see expressed as pH; pH = -log [H+]. Acidic A buffer consists of an acid-base Figure 2.17). [H+] > [OH–] pair that combines reversibly Acids donate H+ in 11. WRITE ABOUT A THEME: ORGANIZATION A sudden change with hydrogen ions, allowing it aqueous solutions. in pH in a cell can be threatening to its survival. In a short essay to resist pH changes. (100–150 words), describe how a cell can resist pH changes. The burning of fossil fuels Neutral [H+] = [OH–] 7 increases the amount of CO2 12. SYNTHESIZE YOUR KNOWLEDGE  in the atmosphere. Some CO2 Bases donate OH– How do cats drink? dissolves in the oceans, causing or accept H+ in Scientists using high- ocean acidification, which Basic aqueous solutions. speed video have has potentially grave conse- [H+] < [OH–] shown that cats use an quences for marine organisms 14 interesting technique that rely on calcification. to drink aqueous ? Explain what happens to the concentration of hydrogen ions in an substances like water aqueous solution when you add a base and cause the concentration and milk. Four times a of OH - to rise to 10-3. What is the pH of this solution? second, the cat touches the tip of its tongue to the water and draws Test Your Understanding a column of water up into its mouth (as Multiple-choice Self-Quiz questions 1–5 can be found you can see in the in the Study Area in MasteringBiology. photo), which then shuts before gravity can pull the water back down. Describe how the properties of 6. DRAW IT Draw the hydration shells that form water allow cats to drink in this fashion, including how water’s around a potassium ion and a chloride ion when molecular structure contributes to the process. potassium chloride (KCl) dissolves in water. Label the positive, negative, and partial charges on the For selected answers, see Appendix A. at