Chapter 3 Review
Concept 3.1· The polarity of water molecules results in hydrogen bonding.
· A hydrogen bond forms when the oxygen of one water molecule is electrically attracted to the hydrogen of a nearby molecule.
· Hydrogen bonding between water molecules is the basis for water′s unusual properties.
· Four emergent properties of water contribute to Earth′s fitness for life.
· Cohesion: Hydrogen bonding keeps water molecules close to each other, and this cohesion helps pull water upward in the microscopic vessels of plants. Hydrogen bonding is also responsible for water′s surface tension.
· Moderation of Temperature: Hydrogen bonding gives water a high specific heat. Heat is absorbed when hydrogen bonds break and is released when hydrogen bonds form, helping minimize temperature fluctuations to within limits that permit life. Evaporative cooling is based on water′s high heat of vaporization. Water molecules must have a relatively high kinetic energy to break hydrogen bonds. The evaporative loss of these energetic water molecules cools a surface.
· Insulation of Bodies of Water by Floating Ice: Ice is less dense than liquid water because its more organized hydrogen bonding causes expansion into a crystal formation. The lower density causes ice to float, which allows life to exist under the frozen surfaces of lakes and polar seas.
· The Solvent of Life: Water is an unusually versatile solvent because its polar molecules are attracted to charged and polar substances. Ions or polar substances surrounded by water molecules dissolve and are called solutes. Hydrophilic substances have an affinity for water; hydrophobic substances do not. Molarity, the number of moles of solute per liter of solution, is used as a measure of solute concentration in solutions. A mole is a certain number of molecules of a substance. The mass of a mole of the substance in grams is the same as the molecular mass in daltons.
· Dissociation of water molecules leads to acidic and basic conditions that affect living organisms.
· Effects of Changes in pH: Water can dissociate into H+ and OH−. The concentration of H+ is expressed as pH, where pH = −log [H+].
· Acids donate additional H+.
· Bases donate OH− or accept H+.
· In a neutral solution at 25°C, [H+] = [OH−] = 10−7, and pH = 7.
· In an acidic solution, [H+] is greater than [OH−], and the pH is less than 7.
· In a basic solution, [H+] is less than [OH−], and the pH is greater than 7.
· Buffers in biological fluids resist changes in pH. A buffer consists of an acid–base pair that combines reversibly with hydrogen ions.
· The Threat of Acid Precipitation Acid precipitation is rain, snow, or fog with a pH below 5.6. It often results from a reaction in the air between water vapor and sulfur oxides and nitrogen oxides produced by the combustion of fossil fuels.
1. What is the best explanation of the phrase “fitness of the environment,” as used in this chapter?
a. Earth′s environment is constant.
b. It is the physical environment, not life, that has changed.
c. The environment of Earth has adapted to life.
d. Life as we know it depends on certain environmental qualities on Earth.
e. Water and other aspects of Earth′s environment exist because they make the planet more suitable for life.
2. Many mammals control their body temperature by sweating. Which property of water is most directly responsible for the ability of sweat to lower body temperature?
a. water′s change in density when it condenses
b. water′s ability to dissolve molecules in the air
c. the release of heat by the formation of hydrogen bonds
d. the absorption of heat by the breaking of hydrogen bonds
e. water′s high surface tension
3. For two bodies of matter in contact, heat always flows from
a. the body with greater heat to the one with less heat.
b. the body of higher temperature to the one of lower temperature.
c. the denser body to the less dense body.
d. the body with more water to the one with less water. e. the larger body to the smaller body.
4. A slice of pizza has 500 kcal. If we could burn the pizza and use all the heat to warm a 50–L container of cold water, what would be the approximate increase in the temperature of the water? (Note: A liter of cold water weighs about 1 kg.)
a. 50°C b. 5°C c. 10°C d. 100°C e. 1°C
5. The bonds that are broken when water vaporizes are
a. ionic bonds. b. bonds between water molecules.
c. bonds between atoms within individual water molecules. d. polar covalent bonds.
e. nonpolar covalent bonds.
6. Which of the following is an example of a hydrophobic material?
a. paper b. table salt c. wax d. sugar e. pasta
7. We can be sure that a mole of table sugar and a mole of vitamin C are equal in their
a. mass in daltons. b. mass in grams. c. number of molecules.
d. number of atoms. e. volume.
8. How many grams of acetic acid (C2H4O2) would you use to make 10 L of a 0.1 M aqueous solution of acetic acid? (Note: The atomic masses, in daltons, are approximately 12 for carbon, 1 for hydrogen, and 16 for oxygen.)
a. 10.0 g b. 0.1 g c. 6.0 g d. 60.0 g e. 0.6 g
9. Acid precipitation has lowered the pH of a particular lake to 4.0. What is the hydrogen ion concentration of the lake?
a. 4.0 M b. 10−10M c. 10−4M d. 104M e. 4%
10. What is the hydroxide ion concentration of the lake described in question 9?
a. 10−7M b. 10−4M c. 10−10M d. 10−14 M e. 10 M
The surface of the planet Mars has many landscape features reminiscent of those formed by flowing water on Earth, including what appear to be meandering channels and outwash areas. Recent probes sent to Mars have revealed strong evidence that liquid water was once present on its surface. Ice exists at the Martian poles today, and some scientists suspect a great deal more water may be present beneath the Martian surface. Why has there been so much interest in the presence of water on Mars? Does the presence of water make it more likely that life had evolved there? What other physical factors might also be important?
1. Design a controlled experiment to test the hypothesis that acid precipitation inhibits the growth of Elodea, a common freshwater plant.
2. In agricultural areas, farmers pay close attention to the weather forecast. Right before a predicted overnight freeze, farmers spray water on crops to protect the plants. Use the properties of water to explain how this works. Be sure to mention why hydrogen bonds are responsible for this phenomenon. Investigation How Does Acid Precipitation Affect Trees?
Agriculture, industry, and the growing populations of cities all compete, through political influence, for water. If you were in charge of water resources in an arid region, what would your priorities be for allocating the limited water supply for various uses? How would you try to build consensus among the different special–interest groups?