Chapter 3 Water and Life

Chapter 3: Water and Life Chapter 3: Water and Life 3.1 Explain how hydrogen bonding results from polar covalent bonds. 3.2 Identify four properties of water that are important for life and describe how they result from hydrogen bonding. 3.3 Differentiate between an acid and a base; define pH and describe how it affects the processes of life. Living systems depend on properties of water that result from its polarity and hydrogen bonding. As you work through this chapter, you will come to understand how these properties impact life processes. Be sure you understand the concept of determining pH as well as its biological significance. Study Tip: Figure 3.1 in your text gets at the core of why water is such a remarkable compound. Read and study the entire figure, and then use the information to help you answer the questions that follow. Concept 3.1 Polar covalent bonds in water molecules result in hydrogen bonding LO 3.1: Explain how hydrogen bonding results from polar covalent bonds. 1. Study the water molecules below. On the central molecule, label oxygen (O) and hydrogen (H). Now, add + and − signs to indicate the charged regions of each molecule. Then, indicate the hydrogen bonds. The charged regions in a water molecule are due to its polar covalent bonds. Regions of neighboring water molecules with the opposite partial charges are attracted to each other, forming hydrogen bonds. + H O + H + Each water molecule can hydrogen-bond to several others; these associations are constantly changing. + Oxygen can form 2 hydrogen bonds. - 1 - Chapter 3: Water and Life 2. Water is considered a polar molecule. What does this mean? A polar molecule is a molecule whose overall charge is unevenly distributed. Water is considered polar because of its unequal sharing of electrons and its V-like shape-- the oxygen region of each molecule has a partial negative charge (-), and each hydrogen has a partial positive charge (+). 3. Explain hydrogen bonding. How many hydrogen bonds can a single water molecule form? Hydrogen bonding is when the slightly positive hydrogen of one molecule is slightly attracted to the slightly negative oxygen of a nearby molecule, holding them together. Hydrogen bonds are fragile and at any given moment a substantial percentage of all water molecules are hydrogen-bonded to their neighbors. Each water molecule can for 4 hydrogen bonds. Concept 3.2 Four emergent properties of water contribute to Earth’s suitability for life LO 3.2: Identify four properties of water that are important for life and describe how they result from hydrogen bonding. Hydrogen bonding accounts for the unique properties of water. Let’s look at several of these properties. 4. Distinguish between cohesion and adhesion. Cohesion is the action of hydrogen bonds holding water together (the clinging of LIKE molecules). Adhesion is the clinging of one substance to another (i.e. beads of water on a waxed car hood). 5. Which property explains the ability of a water strider to walk on water? Surface Tension. 6. Which property is demonstrated when paper towels absorb a water spill? Cohesion. 7. The calorie is a unit of heat. Define calorie. A calorie is the amount of heat it takes to raise the temperature of 1g of water by 1°C, and also the amount of heat that 1g of water releases when it cools by 1°C. - 2 - Chapter 3: Water and Life 8. Water has high specific heat. What does this mean? How does water’s specific heat compare to alcohol’s 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 by a temperature of 1°C. Water's specific heat is unusually high at 1 cal/g x °C, whereas alcohol's specific heat is 0.6 cal/g x °C. Water's high specific heat allows its temperature to change slower than other liquids/substances. 9. Explain how hydrogen bonding contributes to water’s high specific heat. Heat must be absorbed in order to break hydrogen bonds and heat is released when hydrogen bonds form. Lots of heat is used to disrupt hydrogen bonds before water molecules can move faster. the temperature of water goes down, lots of hydrogen bonds form, and release energy as heat. 10. Summarize how water’s high specific heat contributes to the moderation of temperature. How is this property important to life? With a high specific heat, a large body of water can absorb and store a huge amount of heat from the sun in the daytime and during summer while warming up only a few degrees. As this water generally cools during the night and during winter, it warms the air, contributing to milder climates in coastal regions. 11. Define evaporation. What is heat of vaporization? Explain at least three effects of this property on living organisms. Evaporation is the transformation of a liquid to a gas. The evaporation of water from leaves of a plant helps keep the tissues in the leaves from becoming too warm in the sunlight. The evaporation of sweat from human skin dissipates body heat and helps prevent overheating on a hot day or as a result of excess heat created by strenuous activity. The 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 heat of vaporization accounts for the severity of steam burns, burns caused by the heat energy released when steam condenses into liquid on the skin. 12. Ice floats! So what? Consider what would happen if ponds and other bodies of water accumulated ice at the bottom. Describe why this property of water is important. If ice sank, eventually ponds, lakes, and oceans would freeze solid and only a few inches at the top would thaw during the summer. Because ice floats, the upper frozen layer insulates the liquid water below, keeping it from freezing and allowing life to exist beneath the surface. Additionally, the frozen layer provides a solid habitat for some animals, such as polar bears and seals. 13. Now, explain why ice floats. Why is 4°C the critical temperature? As a result of hydrogen bonding, water expands as it solidifies. At 4°C water acts like other liquids, expanding as it warms and contracting as it cools. However, below 4°C water begins to freeze as more and more hydrogen molecules are moving to slowly to break the hydrogen bonds. Hydrogen bonds keep the molecules far enough apart to make ice about 10% less dense than water at 4°C; it is this lower density of ice that allows it to float. - 3 - Chapter 3: Water and Life 14. Review and define these terms: solvent The dissolving agent of a solution solution A liquid that is a completely homogeneous mixture of two or more substances solute The substance dissolved by the solvent aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending to the relevant chemical formula. For example, a solution of table salt, or sodium chloride, in water would be represented as Na⁺ + Cl⁻. 15. Consider coffee to which you have added sugar. Which of these is the solvent? Which is the solute? Coffee is the solvent and sugar is the solute. 16. Explain why water is such a fine solvent. The fact that water is such a versatile solvent can be traced back to the polarity of the water molecule. Water can dissolve ionic compounds, many compounds made up of non-ionic polar molecules, as well as molecules such as proteins if they have polar and ionic regions on their surface. 17. Distinguish between hydrophobic and hydrophilic substances. Give an example of each. Any substance that has an affinity for water is said to be hydrophilic, whereas substances that are non-ionic and nonpolar and cannot form hydrogen bonds and therefore seem to repel water and are called hydrophobic. An example of a hydrophilic substance is cotton, whereas an example of a hydrophobic substance is vegetable oil. 18. You already know that some materials, such as olive oil, will not dissolve in water. In fact, oil will float on top of water. Explain this property in terms of hydrogen bonding. Oil molecules cannot form hydrogen bonds with water and thus behave hydrophobically, because there is a prevalence of relatively non-polar covalent bonds—in this case, bonds between the carbon and hydrogen, which share electrons almost equally. - 4 - Chapter 3: Water and Life 19. Now, let’s do a little work that will enable you to prepare solutions. Read the section on solute concentrations carefully and show your calculations here for preparing a 1-molar (1-M) solution of sucrose. Steps to help you do this follow. The first step is done for you. Fill in the steps below. Steps to Prepare a Solution a. Write the molecular formula. (C12H22O11) b. Use the periodic table (Appendix B in the textbook) to calculate the atomic mass of each element. Multiply by the number of atoms of the element. (For example, oxygen has a mass of 16. Therefore, oxygen has a total mass of 16 × 11 = 176 grams.) c. Add the masses of each element in the molecule. 144 + 22 + 176 = 342 g d. Add water to the total molar mass you determined and bring the solution to a volume of 1 L. This makes a liter of a 1-M solution. 656g water + 342g M= 1L 20. Can you prepare 1 L of a 0.5-M solution of glucose? Show your work here. You can check the answer at the end of this chapter. = (6 x 12) = 72 = (12 x 1) = 12 = (6 x 16) = 96 = 180 g = 1 mole 21. 0.5-molar glucose = 90 g. Add this mass of the compound to water bring it to volume of 1 liter. This makes 1 liter of 0.5 M solution. Define molarity. Molarity is the number of moles of solute per liter of solution—is the unit of concentration most often used by biologists for aqueous solutions. - 5 - Chapter 3: Water and Life Concept 3.3 Acidic and basic conditions affect living organisms LO 3.3: Differentiate between an acid and a base; define pH and describe how it affects the processes of life. 22. What two ions form when water dissociates? Hydronium and Hydroxide ions (You should have answered “hydronium (H3O+) and hydroxide ions (OH−)” in the preceding question. However, by convention, we will represent the hydronium ion as H+.) 23. Draw and label the chemical reaction that occurs when water dissociates. Hydronium Ion Hydroxide Ion 24. What is the concentration of H+ and OH− in pure water at 25°C? M (at 25°C) 25. Water has a pH of 7. pH is defined as the negative log of the hydrogen ion concentration [H+]. Explain why water is assigned a pH of 7. Although this question is rhetorical, the concentration of [H+] = 26. and the log of is 7. To go a step further, the product of H+ and OH− concentrations is constant at 10−14. [H+][OH−] = 10−14 Water, which is neutral with a pH of 7, has an equal number of H+ and OH− ions. Now, define acid A substance that increases the hydrogen ion concentration of a solution base A substance that reduces the hydrogen ion concentration of a solution - 6 - Chapter 3: Water and Life 27. Because the pH scale is logarithmic, each numerical change represents a 10× change in ion concentration. a. How many times more acidic is a pH of 3 compared to a pH of 5? b. How many times more basic is a pH of 12 compared to a pH of 8? c. Explain the difference between a pH of 8 and a pH of 12 in terms of H+ concentration. The H+ concentration of a pH 12 is 28. On the pH scale, label pH 1–14. Label neutral, acid, and base. Indicate the locations of pure water, urine, gastric juice, and bleach. Pure Bleach Gastric Juice Increasingly Acidic 29. times greater than that of a pH of 8. Urine Water Neutral Increasingly Basic Even a slight change in pH can be harmful! How do buffers moderate pH change? A buffer is a substance that minimizes changes in the concentration of H+ and OH- in a solution. 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. Most buffer solutions contain a weak acid and its corresponding base, which combine reversibly with hydrogen ions. 30. Exercise will result in the production of CO2, which will acidify the blood. Explain the buffering system that minimizes blood pH changes. Carbonic acid is formed when reacts with water in the blood plasma. Carbonic acid dissociates to yield a bicarbonate ion ( ) and a hydrogen ion (H+). The chemical equilibrium between carbonic acid and bicarbonate acts as a pH regulator, the reaction shifting from left or right as other processes in the solution add or remove hydrogen ions. - 7 - Chapter 3: Water and Life 31. Using Figure 3.12 in the text as a starting point, discuss how CO2 emissions affect marine life and ecosystems. CO2 dissolves in seawater creating carbonic acid, which lowers the pH of the ocean causing ocean acidification. Ocean acidification decreases the carbonate concentration, which is required for calcification by many marine organisms. Additionally, CO2 emissions create acid precipitation that damages life in streams and lakes and adversely affects life on land by altering soil chemistry. Test Your Understanding, p. 55 Now you should be ready to test your knowledge. Place your answers here: 1. C 2. D 3. C 4. A 5. D Answer to Question 20: The formula for glucose is C6H12O6 which gives a molar mass of 180 g. A 0.5M solution would require 90 g of glucose, adding water to bring the total volume to 1 L. - 8 -