8.1.pdf

Chapter 8: Gases: 8.1 Pressure Book Title: Chemistry: An Atoms First Approach Printed By: Deysi Santana Fuentes ([email protected]) © 2019 Cengage Learning, Cengage Learning 8.1 Pressure A gas uniformly fills any container, is easily compressed, and mixes completely with any other gas. One of the mo...

Chapter 8: Gases: 8.1 Pressure Book Title: Chemistry: An Atoms First Approach Printed By: Deysi Santana Fuentes ([email protected]) © 2019 Cengage Learning, Cengage Learning 8.1 Pressure A gas uniformly fills any container, is easily compressed, and mixes completely with any other gas. One of the most obvious properties of a gas is that it exerts pressure on its surroundings. For example, when you blow up a balloon, the air inside pushes against the elastic sides of the balloon and keeps it firm. As mentioned earlier, the gases most familiar to us form the earth’s atmosphere. The pressure exerted by this gaseous mixture that we call air can be dramatically demonstrated by the experiment shown in Fig. 8.1. A small volume of water is placed in a metal can, and the water is boiled, which fills the can with steam. The can is then sealed and allowed to cool. Why does the can collapse as it cools? It is the atmospheric pressure that crumples the can. When the can is cooled after being sealed so that no air can flow in, the water vapor (steam) condenses to a very small volume of liquid water. As a gas, the water filled the can, but when it is condensed to a liquid, the liquid does not come close to filling the can. The molecules formerly present as a gas are now collected in a very small volume of liquid, and there are very few molecules of gas left to exert pressure outward and counteract the air pressure. As a result, the pressure exerted by the gas molecules in the atmosphere smashes the can. Figure 8.1 The pressure exerted by the gases in the atmosphere can be demonstrated by boiling water in a large metal can (a) and then turning off the heat and sealing the can. As the can cools, the water vapor condenses, lowering the gas pressure inside the can. This causes the can to crumple (b). Charles D. Winters A device to measure atmospheric pressure, the barometer (a device for measuring atmospheric pressure.) , was invented in 1643 by an Italian scientist named Evangelista Torricelli (1608–1647), who had been a student of Galileo. Torricelli’s barometer is constructed by filling a glass tube with liquid mercury and inverting it in a dish of mercury, as shown in Fig. 8.2. Notice that a large quantity of mercury stays in the tube. In fact, at sea level the height of this column of mercury averages mm. Why does this mercury stay in the tube, seemingly in defiance of gravity? Fig. 8.2 illustrates how the pressure exerted by the atmospheric gases on the surface of mercury in the dish keeps the mercury in the tube. Figure 8.2 A torricellian barometer. The tube, completely filled with mercury, is inverted in a dish of mercury. Mercury flows out of the tube until the pressure of the column of mercury (shown by the black arrow) “standing on the surface” of the mercury in the dish is equal to the pressure of the air (shown by the purple arrows) on the rest of the surface of the mercury in the dish. Atmospheric pressure results from the mass of the air being pulled toward the center of the earth by gravity—in other words, it results from the weight of the air. Changing weather conditions cause the atmospheric pressure to vary, so the height of the column of Hg supported by the atmosphere at sea level varies; it is not always mm. The meteorologist who says a “low” is approaching means that the atmospheric pressure is going to decrease. This condition often occurs in conjunction with a storm. Atmospheric pressure also varies with altitude. For example, when Torricelli’s experiment is done in Breckenridge, Colorado (elevation feet), the atmosphere supports a column of mercury only about mm high because the air is “thinner.” That is, there is less air pushing down on the earth’s surface at Breckenridge than at sea level. Chapter 8: Gases: 8.1 Pressure Book Title: Chemistry: An Atoms First Approach Printed By: Deysi Santana Fuentes ([email protected]) © 2019 Cengage Learning, Cengage Learning © 2023 Cengage Learning Inc. All rights reserved. No part of this work may by reproduced or used in any form or by any means graphic, electronic, or mechanical, or in any other manner - without the written permission of the copyright holder.