Science of Everyday Things Vol. 2 - Physics Episode 7 pptx

183 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS GAS LAWS Gas Laws CONCEPT Gases respond more dramatically to temperature and pressure than do the other three basic types of matter (liquids, solids and plasma). For gases, temperature and pressure are closely related to volume, and this allows us to predict their behav- ior under certain conditions. These predictions can explain mundane occurrences, such as the fact that an open can of soda will soon lose its fizz, but they also apply to more dramatic, life- and-death situations. HOW IT WORKS Ordinary air pressure at sea level is equal to 14.7 pounds per square inch, a quantity referred to as an atmosphere (atm). Because a pound is a unit of force and a kilogram a unit of mass, the met- ric equivalent is more complex in derivation. A newton (N), or 0.2248 pounds, is the metric unit of force, and a pascal (Pa)—1 newton per square meter—the unit of pressure. Hence, an atmosphere, expressed in metric terms, is 1.013 ϫ 10 5 Pa. Gases vs. Solids and Liq- uids: A Strikingly Different Response Regardless of the units you use, however, gases respond to changes in pressure and temperature in a remarkably different way than do solids or liquids. Using a small water sample, say, 0.2642 gal (1 l), an increase in pressure from 1-2 atm will decrease the volume of the water by less than 0.01%. A temperature increase from 32° to 212°F (0 to 100°C) will increase its volume by only 2% The response of a solid to these changes is even less dramatic; however, the reaction of air (a combination of oxygen, nitrogen, and other gases) to changes in pressure and temperature is radically different. For air, an equivalent temperature increase would result in a volume increase of 37%, and an equivalent pressure increase will decrease the volume by a whopping 50%. Air and other gases also have a boiling point below room tempera- ture, whereas the boiling point for water is high- er than room temperature and that of solids is much higher. The reason for this striking differ- ence in response can be explained by comparing all three forms of matter in terms of their overall structure, and in terms of their molecular behav- ior. (Plasma, a gas-like state found, for instance, in stars and comets’ tails, does not exist on Earth, and therefore it will not be included in the com- parisons that follow.) Molecular Structure Deter- mines Reaction Solids possess a definite volume and a definite shape, and are relatively noncompressible: for instance, if you apply extreme pressure to a steel plate, it will bend, but not much. Liquids have a definite volume, but no definite shape, and tend to be noncompressible. Gases, on the other hand, possess no definite volume or shape, and are compressible. At the molecular level, particles of solids tend to be definite in their arrangement and close in proximity—indeed, part of what makes a solid “solid,” in the everyday meaning of that term, is the fact that its constituent parts are basically immovable. Liquid molecules, too, are close in proximity, though random in arrangement. Gas set_vol2_sec6 9/13/01 12:48 PM Page 183 Gas Laws 184 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS molecules, too, are random in arrangement, but tend to be more widely spaced than liquid mole- cules. Solid particles are slow moving, and have a strong attraction to one another, whereas gas particles are fast-moving, and have little or no attraction. (Liquids are moderate in both regards.) Given these interesting characteristics of gases, it follows that a unique set of parameters— collectively known as the “gas laws”—are needed to describe and predict their behavior. Most of the gas laws were derived during the eighteenth and nineteenth centuries by scientists whose work is commemorated by the association of their names with the laws they discovered. These men include the English chemists Robert Boyle (1627-1691), John Dalton (1766-1844), and William Henry (1774-1836); the French physi- cists and chemists J. A. C. Charles (1746-1823) and Joseph Gay-Lussac (1778-1850), and the Ital- ian physicist Amedeo Avogadro (1776-1856). Boyle’s, Charles’s, and Gay- Lussac’s Laws Boyle’s law holds that in isothermal conditions (that is, a situation in which temperature is kept constant), an inverse relationship exists between the volume and pressure of a gas. (An inverse relationship is a situation involving two vari- ables, in which one of the two increases in direct proportion to the decrease in the other.) In this case, the greater the pressure, the less the volume and vice versa. Therefore the product of the vol- ume multiplied by the pressure remains constant in all circumstances. Charles’s law also yields a constant, but in this case the temperature and volume are allowed to vary under isobarometric conditions—that is, a situation in which the pressure remains the same. As gas heats up, its volume increases, and when it cools down, its volume reduces accord- ingly. Hence, Charles established that the ratio of temperature to volume is constant. By now a pattern should be emerging: both of the aforementioned laws treat one parameter (temperature in Boyle’s, pressure in Charles’s) as unvarying, while two other factors are treated as variables. Both in turn yield relationships between the two variables: in Boyle’s law, pres- sure and volume are inversely related, whereas in Charles’s law, temperature and volume are directly related. In Gay-Lussac’s law, a third parameter, vol- ume, is treated as a constant, and the result is a constant ratio between the variables of pressure and temperature. According to Gay-Lussac’s law, the pressure of a gas is directly related to its absolute temperature. Absolute temperature refers to the Kelvin scale, established by William Thomson, Lord Kelvin (1824-1907). Drawing on Charles’s dis- covery that gas at 0°C (32°F) regularly contracted by about 1/273 of its volume for every Celsius degree drop in temperature, Thomson derived the value of absolute zero (-273.15°C or -459.67°F). Using the Kelvin scale of absolute temperature, Gay-Lussac found that at lower temperatures, the pressure of a gas is lower, while at higher temperatures its pressure is higher. Thus, the ratio of pressure to temperature is a constant. Avogadro’s Law Gay-Lussac also discovered that the ratio in which gases combine to form compounds can be expressed in whole numbers: for instance, water is composed of one part oxygen and two parts hydrogen. In the language of modern science, this would be expressed as a relationship between molecules and atoms: one molecule of water contains one oxygen atom and two hydrogen atoms. In the early nineteenth century, however, sci- entists had yet to recognize a meaningful distinc- tion between atoms and molecules. Avogadro was the first to achieve an understanding of the difference. Intrigued by the whole-number rela- tionship discovered by Gay-Lussac, Avogadro reasoned that one liter of any gas must contain the same number of particles as a liter of anoth- er gas. He further maintained that gas consists of particles—which he called molecules—that in turn consist of one or more smaller particles. In order to discuss the behavior of mole- cules, it was necessary to establish a large quanti- ty as a basic unit, since molecules themselves are very small. For this purpose, Avogadro estab- lished the mole, a unit equal to 6.022137 ϫ 10 23 (more than 600 billion trillion) molecules. The term “mole” can be used in the same way we use the word “dozen.” Just as “a dozen” can refer to twelve cakes or twelve chickens, so “mole” always describes the same number of molecules. set_vol2_sec6 9/13/01 12:48 PM Page 184 Gas Laws Just as one liter of water, or one liter of mer- cury, has a certain mass, a mole of any given sub- stance has its own particular mass, expressed in grams. The mass of one mole of iron, for instance, will always be greater than that of one mole of oxygen. The ratio between them is exact- ly the same as the ratio of the mass of one iron atom to one oxygen atom. Thus the mole makes if possible to compare the mass of one element or one compound to that of another. Avogadro’s law describes the connection between gas volume and number of moles. According to Avogadro’s law, if the volume of gas is increased under isothermal and isobarometric conditions, the number of moles also increases. The ratio between volume and number of moles is therefore a constant. The Ideal Gas Law Once again, it is easy to see how Avogadro’s law can be related to the laws discussed earlier, since they each involve two or more of the four param- eters: temperature, pressure, volume, and quanti- ty of molecules (that is, number of moles). In fact, all the laws so far described are brought together in what is known as the ideal gas law, sometimes called the combined gas law. The ideal gas law can be stated as a formula, pV = nRT, where p stands for pressure, V for vol- ume, n for number of moles, and T for tempera- ture. R is known as the universal gas constant, a figure equal to 0.0821 atm • liter/mole • K. (Like most terms in physics, this one is best expressed in metric rather than English units.) Given the equation pV = nRT and the fact that R is a constant, it is possible to find the value of any one variable—pressure, volume, number of moles, or temperature—as long as one knows the value of the other three. The ideal gas law also makes it possible to discern certain relations: thus if a gas is in a relatively cool state, the prod- uct of its pressure and volume is proportionately low; and if heated, its pressure and volume prod- uct increases correspondingly. Thus , where p 1 V 1 is the product of its initial pressure and its initial volume, T 1 its initial temperature, V = T 1 p 11 V T 2 p 22 185 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS p 2 V 2 the product of its final volume and final pressure, and T 2 its final temperature. Five Postulates Regarding the Behavior of Gases Five postulates can be applied to gases. These more or less restate the terms of the earlier dis- cussion, in which gases were compared to solids and liquids; however, now those comparisons can be seen in light of the gas laws. First, the size of gas molecules is minuscule in comparison to the distance between them, making gas highly compressible. In other words, there is a relatively high proportion of empty space between gas molecules. Second, there is virtually no force attracting gas molecules to one another. Third, though gas molecules move random- ly, frequently colliding with one another, their net effect is to create uniform pressure. A FIRE EXTINGUISHER CONTAINS A HIGH-PRESSURE MIX- TURE OF WATER AND CARBON DIOXIDE THAT RUSHES OUT OF THE SIPHON TUBE , WHICH IS OPENED WHEN THE RELEASE VALVE IS DEPRESSED. (Photograph by Craig Lovell/ Corbis. Reproduced by permission.) set_vol2_sec6 9/13/01 12:48 PM Page 185 Gas Laws 186 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS Fourth, the elastic nature of the collisions results in no net loss of kinetic energy, the ener- gy that an object possesses by virtue of its motion. If a stone is dropped from a height, it rapidly builds kinetic energy, but upon hitting a nonelastic surface such as pavement, most of that kinetic energy is transferred to the pavement. In the case of two gas molecules colliding, however, A HOT-AIR BALLOON FLOATS BECAUSE THE AIR INSIDE IT IS NOT AS DENSE THAN THE AIR OUTSIDE. THE WAY IN WHICH THE DENSITY OF THE AIR IN THE BALLOON IS REDUCED REFLECTS THE GAS LAWS . (Duomo/Corbis. Reproduced by permission.) set_vol2_sec6 9/13/01 12:48 PM Page 186 Gas Laws they simply bounce off one another, only to col- lide with other molecules and so on, with no kinetic energy lost. Fifth, the kinetic energy of all gas molecules is directly proportional to the absolute tempera- ture of the gas. Laws of Partial Pressure Two gas laws describe partial pressure. Dalton’s law of partial pressure states that the total pressure of a gas is equal to the sum of its par tial pressures—that is, the pressure exerted by each component of the gas mixture. As noted earlier, air is composed mostly of nitrogen and oxygen. Along with these are small compo- nents carbon dioxide and gases collectively known as the rare or noble gases: argon, helium, krypton, neon, radon, and xenon. Hence, the total pressure of a given quantity of air is equal to the sum of the pressures exerted by each of these gases. Henry’s law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the surface of the solution. This applies only to gases such as oxygen and hydrogen that do not react chemical- ly to liquids. On the other hand, hydrochloric acid will ionize when introduced to water: one or more of its electrons will be removed, and its atoms will convert to ions, which are either posi- tive or negative in charge. REAL-LIFE APPLICATIONS Pressure Changes OPENING A SODA CAN. Inside a can or bottle of carbonated soda is carbon diox- ide gas (CO 2 ), most of which is dissolved in the drink itself. But some of it is in the space (some- times referred to as “head space”) that makes up the difference between the volume of the soft drink and the volume of the container. At the bottling plant, the soda manufacturer adds high-pressure carbon dioxide to the head space in order to ensure that more CO 2 will be absorbed into the soda itself. This is in accor- dance with Henry’s law: the amount of gas (in this case CO 2 ) dissolved in the liquid (soda) is directly proportional to the partial pressure of the gas above the surface of the solution—that is, the CO 2 in the head space. The higher the pres- sure of the CO 2 in the head space, the greater the amount of CO 2 in the drink itself; and the greater the CO 2 in the drink, the greater the “fizz” of the soda. Once the container is opened, the pressure in the head space drops dramatically. Once again, Henry’s law indicates that this drop in pressure will be reflected by a corresponding drop in the amount of CO 2 dissolved in the soda. Over a period of time, the soda will release that gas, and will eventually go “flat.” FIRE EXTINGUISHERS. A fire extinguisher consists of a long cylinder with an operating lever at the top. Inside the cylinder is a tube of carbon dioxide surrounded by a quantity of water, which creates pressure around the CO 2 tube. A siphon tube runs vertically along the length of the extinguisher, with one opening near the bottom of the water. The other end opens in a chamber containing a spring mechanism attached to a release valve in the CO 2 tube. The water and the CO 2 do not fill the entire cylinder: as with the soda can, there is “head space,” an area filled with air. When the operating lever is depressed, it activates the spring mecha- nism, which pierces the release valve at the top of the CO 2 tube. When the valve opens, the CO 2 spills out in the “head space,” exerting pressure on the water. This high-pressure mixture of water and carbon dioxide goes rushing out of the siphon tube, which was opened when the release valve was depressed. All of this happens, of course, in a fraction of a second—plenty of time to put out the fire. AEROSOL CANS. Aerosol cans are similar in structure to fire extinguishers, though with one important difference. As with the fire extinguisher, an aerosol can includes a nozzle that depresses a spring mechanism, which in turn allows fluid to escape through a tube. But instead of a gas cartridge surrounded by water, most of the can’s interior is made up of the product (for instance, deodorant), mixed with a liquid pro- pellant. The “head space” of the aerosol can is filled with highly pressurized propellant in gas form, and in accordance with Henry’s law, a correspon- ding proportion of this propellant is dissolved in the product itself. When the nozzle is depressed, 187 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec6 9/13/01 12:48 PM Page 187 Gas Laws the pressure of the propellant forces the product out through the nozzle. A propellant, as its name implies, propels the product itself through the spray nozzle when the latter is depressed. In the past, chlorofluorocar- bons (CFCs)—manufactured compounds con- taining carbon, chlorine, and fluorine atoms— were the most widely used form of propellant. Concerns over the harmful effects of CFCs on the environment, however, has led to the develop- ment of alternative propellants, most notably hydrochlorofluorocarbons (HCFCs), CFC-like compounds that also contain hydrogen atoms. When the Temperature Changes A number of interesting things, some of them unfortunate and some potentially lethal, occur when gases experience a change in temperature. In these instances, it is possible to see the gas laws—particularly Boyle’s and Charles’s— at work. There are a number of examples of the dis- astrous effects that result from an increase in the temperature of a product containing com- bustible gases, as with natural gas and petrole- um-based products. In addition, the pressure on the gases in aerosol cans makes the cans highly explosive—so much so that discarded cans at a city dump may explode on a hot summer day. Yet there are other instances when heating a gas can produce positive effects. A hot-air balloon, for instance, floats because the air inside it is not as dense than the air outside. By itself, this fact does not depend on any of the gas laws, but rather reflects the concept of buoyancy. However, the way in which the den- sity of the air in the balloon is reduced does indeed reflect the gas laws. According to Charles’s law, heating a gas will increase its volume. Also, as noted in the first and second propositions regarding the behavior of gases, gas molecules are highly nonattractive to one another, and therefore, there is a great deal of space between them. The increase in volume makes that space even greater, leading to a signif- icant difference in density between the air in the balloon and the air outside. As a result, the bal- loon floats, or becomes buoyant. Although heating a gas can be beneficial, cooling a gas is not always a wise idea. If someone were to put a bag of potato chips into a freezer, thinking this would preserve their flavor, he would be in for a disappointment. Much of what maintains the flavor of the chips is the pressur- ization of the bag, which ensures a consistent internal environment in which preservative chemicals, added during the manufacture of the chips, can keep them fresh. Placing the bag in the freezer causes a reduction in pressure, as per Gay- Lussac’s law, and the bag ends up a limp version of its earlier self. Propane tanks and tires offer an example of the pitfalls that may occur by either allowing a gas to heat up or cool down by too much. Because most propane tanks are made according to strict regulations, they are generally safe, but it is not entirely inconceivable that an extremely hot summer day could cause a defective tank to burst. Certainly the laws of physics are there: an increase in temperature leads to an increase in pressure, in accordance with Gay-Lussac’s law, and could lead to an explosion. Because of the connection between heat and pressure, propane trucks on the highways during the summer are subjected to weight tests to ensure that they are not carrying too much of the gas. On the other hand, a drastic reduction in temperature could result in a loss in gas pressure. If a propane tank from Florida were transported by truck during the winter to northern Canada, the pressure would be dramatically reduced by the time it reached its destination. Gas Reactions That Move and Stop a Car In operating a car, we experience two examples of gas laws in operation. One of these, common to everyone, is that which makes the car run: the combustion of gases in the engine. The other is, fortunately, a less frequent phenomenon—but it can and does save lives. This is the operation of an air bag, which, though it is partly related to laws of motion, depends also on the behaviors explained in Charles’s law. With regard to the engine, when the driver pushes down on the accelerator, this activates a throttle valve that sprays droplets of gasoline mixed with air into the engine. (Older vehicles used a carburetor to mix the gasoline and air, but most modern cars use fuel-injection, which sprays the air-gas combination without requiring an intermediate step.) The mixture goes into the 188 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set_vol2_sec6 9/13/01 12:48 PM Page 188 Gas Laws cylinder, where the piston moves up, compress- ing the gas and air. While the mixture is still compressed (high pressure, high density), an electric spark plug produces a flash that ignites it. The heat from this controlled explosion increases the volume of air, which forces the piston down into the cylinder. This opens an outlet valve, causing the piston to rise and release exhaust gases. As the piston moves back down again, an inlet valve opens, bringing another burst of gaso- line-air mixture into the chamber. The piston, whose downward stroke closed the inlet valve, now shoots back up, compressing the gas and air to repeat the cycle. The reactions of the gasoline and air are what move the piston, which turns a crankshaft that causes the wheels to rotate. So much for moving—what about stopping? Most modern cars are equipped with an airbag, which reacts to sudden impact by inflating. This protects the driver and front-seat passenger, who, even if they are wearing seatbelts, may otherwise be thrown against the steering wheel or dash- board But an airbag is much more complicated than it seems. In order for it to save lives, it must deploy within 40 milliseconds (0.04 seconds). Not only that, but it has to begin deflating before the body hits it. An airbag does not inflate if a car simply goes over a bump; it only operates in sit- 189 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS IN CASE OF A CAR COLLISION, A SENSOR TRIGGERS THE AIR BAG TO INFLATE RAPIDLY WITH NITROGEN GAS. BEFORE YOUR BODY REACHES THE BAG , HOWEVER, IT HAS ALREADY BEGUN DEFLATING. (Illustration by Hans & Cassidy. The Gale Group.) set_vol2_sec6 9/13/01 12:48 PM Page 189 Gas Laws 190 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS ABSOLUTE TEMPERATURE: Te m - perature in relation to absolute zero (-273.15°C or -459.67°F). Its unit is the Kelvin (K), named after William Thomson, Lord Kelvin (1824-1907), who created the scale. The Kelvin and Celsius scales are directly related; hence, Celsius tempera- tures can be converted to Kelvins (for which neither the word or symbol for “degree” are used) by adding 273.15. AVOGADRO’S LAW: A statement, derived by the Italian physicist Amedeo Avogadro (1776-1856), which holds that as the volume of gas increases under isother- mal and isobarometric conditions, the number of molecules (expressed in terms of mole number), increases as well. Thus the ratio of volume to mole number is a constant. BOYLE’S LAW: A statement, derived by English chemist Robert Boyle (1627- 1691), which holds that for gases in isothermal conditions, an inverse relation- ship exists between the volume and pres- sure of a gas. This means that the greater the pressure, the less the volume and vice versa, and therefore the product of pres- sure multiplied by volume yields a constant figure. CHARLES’S LAW: A statement, derived by French physicist and chemist J. A. C. Charles (1746-1823), which holds that for gases in isobarometric conditions, the ratio between the volume and temper- ature of a gas is constant. This means that the greater the temperature, the greater the volume and vice versa. DALTON’S LAW OF PARTIAL PRES- SURE: A statement, derived by the English chemist John Dalton (1766-1844), which holds that the total pressure of a gas is equal to the sum of its partial pres- KEY TERMS uations when the vehicle experiences extreme deceleration. When this occurs, there is a rapid transfer of kinetic energy to rest energy, as with the earlier illustration of a stone hitting concrete. And indeed, if you were to smash against a fully inflated airbag, it would feel like hitting con- crete—with all the expected results. The airbag’s sensor contains a steel ball attached to a permanent magnet or a stiff spring. The spring holds it in place through minor mishaps in which an airbag would not be war- ranted—for instance, if a car were simply to be “tapped” by another in a parking lot. But in a case of sudden deceleration, the magnet or spring releases the ball, sending it down a smooth bore. It flips a switch, turning on an electrical circuit. This in turn ignites a pellet of sodium azide, which fills the bag with nitrogen gas. The events described in the above illustra- tion take place within 40 milliseconds—less time than it takes for your body to come flying for- ward; and then the airbag has to begin deflating before the body reaches it. At this point, the high- ly pressurized nitrogen gas molecules begin escaping through vents. Thus as your body hits the bag, the deflation of the latter is moving it in the same direction that your body is going—only much, much more slowly. Two seconds after impact, which is an eternity in terms of the processes involved, the pressure inside the bag has returned to 1 atm. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison- Wesley, 1991. “Chemistry Units: Gas Laws.” (Web site). <http://bio.bio.rpi.edu/MS99/ausemaW/chem/gases. hmtl> (February 21, 2001). set_vol2_sec6 9/13/01 12:48 PM Page 190 Gas Laws 191 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS Laws of Gases. New York: Arno Press, 1981. Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Mebane, Robert C. and Thomas R. Rybolt. Air and Other Gases. Illustrations by Anni Matsick. New York: Twenty-First Century Books, 1995. “Tutorials—6.” <http://www.chemistrycoach.com/tutori- als-6.html> (February 21, 2001). sures—that is, the pressure exerted by each component of the gas mixture. GAY-LUSSAC’S LAW: A statement, derived by the French physicist and chemist Joseph Gay-Lussac (1778-1850), which holds that the pressure of a gas is directly related to its absolute temperature. Hence the ratio of pressure to absolute temperature is a constant. HENRY’S LAW: A statement, derived by the English chemist William Henry (1774-836), which holds that the amount of gas dissolved in a liquid is directly pro- portional to the partial pressure of the gas above the solution. This holds true only for gases, such as hydrogen and oxygen, that are capable of dissolving in water without undergoing ionization. IDEAL GAS LAW: A proposition, also known as the combined gas law, that draws on all the gas laws. The ideal gas law can be expressed as the formula pV = nRT, where p stands for pressure, V for volume, n for number of moles, and T for temperature. R is known as the universal gas constant, a figure equal to 0.0821 atm • liter/mole • K. INVERSE RELATIONSHIP: A situa- tion involving two variables, in which one of the two increases in direct proportion to the decrease in the other. IONIZATION: A reaction in which an atom or group of atoms loses one or more electrons. The atoms are then converted to ions, which are either wholly positive or negative in charge. ISOTHERMAL: Referring to a situa- tion in which temperature is kept constant. ISOBAROMETRIC: Referring to a sit- uation in which pressure is kept constant. MOLE: A unit equal to 6.022137 ϫ 10 23 molecules. KEY TERMS CONTINUED set_vol2_sec6 9/13/01 12:48 PM Page 191 192 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS MOLECULAR DYNAMICS Molecular Dynamics CONCEPT Physicists study matter and motion, or matter in motion. These forms of matter may be large, or they may be far too small to be seen by the most high-powered microscopes available. Such is the realm of molecular dynamics, the study and sim- ulation of molecular motion. As its name sug- gests, molecular dynamics brings in aspects of dynamics, the study of why objects move as they do, as well as thermodynamics, the study of the relationships between heat, work, and energy. Existing at the borders between physics and chemistry, molecular dynamics provides under- standing regarding the properties of matter— including phenomena such as the liquefaction of gases, in which one phase of matter is trans- formed into another. HOW IT WORKS Molecules The physical world is made up of matter, physical substance that has mass; occupies space; is com- posed of atoms; and is, ultimately, convertible to energy. On Earth, three principal phases of mat- ter exist, namely solid, liquid, and gas. The differ- ences between these three are, on the surface at least, easily perceivable. Clearly, water is a liquid, just as ice is a solid and steam a gas. Yet, the ways in which various substances convert between phases are often complex, as are the interrela- tions between these phases. Ultimately, under- standing of the phases depends on an awareness of what takes place at the molecular level. An atom is the smallest particle of a chemi- cal element. It is not, however, the smallest thing in the universe; atoms are composed of subatom- ic particles, including protons, neutrons, and electrons. These subatomic particles are dis- cussed in the context of the structure of matter elsewhere in this volume, where they are exam- ined largely with regard to their electromagnetic properties. In the present context, the concern is primarily with the properties of atomic and molecular particles, in terms of mechanics, the study of bodies in motion, and thermodynamics. An atom must, by definition, represent one and only one chemical element, of which 109 have been identified and named. It should be noted that the number of elements changes with continuing research, and that many of the ele- ments, particularly those discovered relatively recently—as, for instance, meitnerium (No. 109), isolated in the 1990s—are hardly part of every- day experience. So, perhaps 100 would be a bet- ter approximation; in any case, consider the mul- titude of possible ways in which the elements can be combined. Musicians have only seven tones at their dis- posal, and artists only seven colors—yet they manage to create a seemingly infinite variety of mutations in sound and sight, respectively. There are only 10 digits in the numerical system that has prevailed throughout the West since the late Middle Ages, yet it is possible to use that system to create such a range of numbers that all the books in all the libraries in the world could not contain them. This gives some idea of the range of combinations available using the hundred- odd chemical elements nature has provided—in other words, the number of possible molecular combinations that exist in the universe. set_vol2_sec6 9/13/01 12:48 PM Page 192 [...]... temperature is -4 49.9°F ( -2 6 7. 7°C), or just 5.3K Half a century after Faraday, French physicist Louis Paul Cailletet (18 3 2- 1913) and Swiss chemist Raoul Pierre Pictet (184 6-1 929 ) developed the nozzle and porous-plug methods of liquefaction This, in turn, made it possible to liq- 20 0 A P P L I CAT I O N S O F GAS L I Q U E FAC T I O N Liquefied natural gas (LNG) VOLUME 2: REAL-LIFE PHYSICS S C I E... temperature of the gas GAS E S A N D A B S O LU T E T E M P E RAT U R E The term “absolute tempera- ture” refers to the Kelvin scale, established by William Thomson, Lord Kelvin (1 824 -1 9 07) Drawing on Charles’s discovery that gas at 0°C ( 32 F) regularly contracts by about 1 / 27 3 of its volume for every Celsius degree drop in temperature, Thomson derived the value of absolute zero ( -2 7 3.15°C or -4 59. 67 F)... Matter New York: DK Publishing, 1999 “Kinetic Theory of Gases: A Brief Review” University of Virginia Department of Physics (Web site) (April 15, 20 01) 20 2 VOLUME 2: REAL-LIFE PHYSICS “The Kinetic Theory Page” (Web site) (April 15, 20 01) Medoff, Sol and John Powers The Student Chemist Explores... contributed to the development of quantum theory Because of its unique atomic structure, the Bose-Einstein Condensate has been dubbed a “new” form of matter It represents a quantum mechanical effect, relating to a cutting-edge area of physics devoted to studying the properties of VOLUME 2: REAL-LIFE PHYSICS 20 1 Molecular Dynamics subatomic particles and the interaction of matter with radiation Thus... carbon monoxide By the end of the nineteenth century, physicists were able to liquefy the gases with the lowest critical temperatures James Dewar of Scotland (18 4 2- 1 923 ) liquefied hydrogen, whose critical temperature is -3 99.5°F ( -2 3 9 .7 C) Some time later, Dutch physicist Heike Kamerlingh Onnes (185 3-1 926 ) successfully liquefied the gas with the lowest critical temperature of them all: helium, which,... kinetic theory of matter HEAT: CHANGE OF PHASE: The transition CURVE: The boundary A phase of matter in which mole- Internal thermal energy that flows from one body of matter to another from one phase of matter to another KELVIN SCALE: Established by A sub- William Thomson, Lord Kelvin (1 824 - stance made up of atoms of more than one 19 07) , the Kelvin scale measures tempera- chemical element These atoms are... practical comparisons of mass Hence, the mole, a quantity equal to “Avogadro’s number.” The latter, named after Avogadro though not derived by him, is equal to 6. 022 1 37 x 1 023 (more than 600 billion trillion) molecules S C I E N C E O F E V E RY DAY T H I N G S VOLUME 2: REAL-LIFE PHYSICS Structure of Matter AT O M I C AND MOLECULAR T H E O RY The idea of atoms is not new More 20 5 The term “mole” can... 1 8 27 , Scottish botanist Robert Brown ( 177 3-1 858) was studying pollen grains under a microscope, when he noticed that the grains underwent a curious zigzagging motion in the water The pollen assumed the shape of a colloid, a pattern that occurs when particles of one substance are dispersed—but not dissolved—in another substance Another example of a colloidal pattern is a puff of smoke Kinetic theory offered... Molecular Dynamics KEY TERMS CONTINUED The MOLE: A unit equal to 6. 022 1 37 ϫ 1 023 application of the kinetic theory of gases to (more than 600 billion trillion) molecules all forms of matter Since particles of liq- Since their size makes it impossible to uids and solids move much more slowly weigh molecules in relatively small quanti- than do gas particles, kinetic theory is not ties; hence, the mole,... the physicist Amedeo Avogadro ( 177 6-1 856), proposition that the internal energy of any facilitates comparisons of mass between substance is at least partly related to the substances kinetic energies of its molecules helps MOLECULAR DYNAMICS: explain much about the behavior of and simulation of molecular motion KINETIC THEORY OF MATTER: matter MOLECULE: LIQUID: A phase of matter in which molecules exert . properties of the gas, such an 198 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS set _vol2 _sec6 9/13/01 12: 49 PM Page 198 Molecular Dynamics 199 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS ABSOLUTE. initial volume, T 1 its initial temperature, V = T 1 p 11 V T 2 p 22 185 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS p 2 V 2 the product of its final volume and final pressure, and T 2 its. Gale Group.) set _vol2 _sec6 9/13/01 12: 48 PM Page 189 Gas Laws 190 SCIENCE OF EVERYDAY THINGS VOLUME 2: REAL-LIFE PHYSICS ABSOLUTE TEMPERATURE: Te m - perature in relation to absolute zero ( -2 7 3.15°C or -4 59. 67 F).