chapter 14 Solids, Liquids, and Gases section 3 Behavior of Gases What You ll Learn how a gas exerts pressure on its container how changing pressure, temperature, or volume affect a gas Before You Read The air that you breathe is a gas. List other gases that you have heard about in a science class. Study Coach Writing for Understanding As you read this section, write down each question heading. After each heading, write the answer to the question. D Understanding Cause and Effect Make the following Foldable to help you understand the cause and effect relationship of gases. Gas: Change in Pressure Gas: Change in Temperature Gas: Change in Volume Read to Learn Pressure You learned from the kinetic theory that gas particles are moving constantly and bumping into anything in their path. The collisions of these particles in the air result in pressure. Pressure is the amount of force exerted per unit of area. Pressure can be expressed by the formula P F/A. Often, gases are placed in containers. A balloon and a bicycle tire are gas containers. The balloon and the tire stay inflated because of collisions the air particles have with the walls of the container. The collisions of the particles cause a group of forces to push the walls of the container outward in every direction. If more air is pumped into the balloon, the number of air particles inside the balloon increases. This causes more collisions with the walls of the container and the balloon expands even more. Bicycle tires cannot expand as much as balloons. When more air is pumped into a bicycle tire, the tire s pressure increases. 260 CHAPTER 14 Solids, Liquids, and Gases
How is pressure measured? Pressure is measured in a unit called pascal, the SI unit of pressure. The abbreviation for pascal is Pa. Because pressure is the amount of force divided by area, one pascal of pressure is one Newton per square meter, or 1N/m 2. Because this is a very small pressure unit, most pressures are given in kilopascals, or 1,000 pascals. The abbreviation for kilopascals is kpa. At sea level, atmospheric pressure is 101.3 kpa. This means that at Earth s surface at sea level, the atmosphere exerts a force of about 101,300 N on every square meter. This is about the weight of a large truck. The air in the atmosphere gets thinner as distance from sea level increases. This means that the air pressure decreases as you travel higher above sea level. E Organize Make the following Foldable to help you organize the important information about each law. Boyle s Law Pressure- Temperature Relationship Charles s Law Boyle s Law You have learned how gas creates pressure in a container. Suppose the container gets smaller. What happens to the gas pressure? The pressure of a gas depends on how often its particles hit the walls of the container. If you squeeze gas into a smaller space, its particles will strike the walls of their container more often. This increases the pressure. The opposite is true, too. If you give the gas particles more space, they will hit the walls less often. Gas pressure is reduced. Robert Boyle was a British scientist who lived from 1627 to 1691. Boyle explained the property of gases described above. According to Boyle s law, if you decrease the volume of a container of gas and keep the temperature the same, the pressure of the gas will increase. If you increase the volume of the container and keep the temperature the same, the pressure of the gas will decrease. Pressure Outside an Object Affects Volume Boyle s law explains the behavior of weather balloons. Weather balloons are large balloons that carry instruments to high altitudes. The instruments detect weather information and send the data back to Earth. The balloons are inflated near Earth s surface with a low-density gas. As a balloon rises, the atmospheric pressure outside the balloon decreases. This allows the balloon to slowly get bigger. It can reach a volume of 30 to 200 times its original size. Eventually, the balloon expands so much it breaks. Boyle s law states that as pressure outside an object is decreased, the volume increases. The weather balloon demonstrates this law. The graph on the next page shows that the opposite is also true. As the pressure outside an object is increased, the volume will decrease. 1. Apply If you increase the size of a container, will the pressure of the gas inside it increase or decrease? Reading Essentials 261
Applying Math 2. Interpret a Graph What is the pressure of the gas at 300 L? Volume (L) 600 500 400 300 200 100 Volume v. Pressure for a Fixed Amount of Gas at Constant Temperature 0 50 100 150 200 250 300 350 400 450 500 Pressure (kpa) Applying Math 3. Solve an Equation Use the equation P i V i P f V f. Assume that P i 101 kpa and V i 10.0 L. If P f 43.0 kpa, what is V f? Show your work. Using Boyle s law Using Boyle s law, you will find that the pressure inside an object multiplied by the volume of the object is always equal to the same number, or a constant. This is true only if the temperature stays the same. As the pressure and volume change, the constant number remains the same. You can use the equations P i V i constant P f V f to express this mathematically. In this equation, P i and V i stand for the initial pressure and volume. P f and V f stand for the final pressure and final volume. The equation shows us that the product of the initial pressure and volume is equal to the product of the final pressure and volume. If you know three of these values, you can find the unknown fourth value. The Pressure-Temperature Relationship Have you ever seen the words keep away from heat on a spray can? If you heat a gas inside a closed container, the particles of gas will strike the walls of the container more often. Because the container is rigid, its volume cannot increase. Instead, its pressure increases. The pressure can become greater than the container can hold. If this happens, the container will explode. If the volume stays the same, an increase in temperature results in an increase in pressure. The opposite is also true. That is, a decrease in temperature results in a decrease in pressure, if the volume stays the same. Suppose the same spray can is placed in a very low temperature. The gas particles will slow down and decrease in pressure. The pressure decrease can buckle or crumple the container. 262 CHAPTER 14 Solids, Liquids, and Gases
Charles s Law Have you ever seen a hot-air balloon being inflated? You know that gases expand when they are heated. Because particles in the hot air are farther apart than particles in the cool air, the hot air is less dense than the cool air. This difference in density allows the hot-air balloon to rise. Jacques Charles was a French scientist who studied gases. He lived from 1746 to 1823. According to Charles s law, the volume of a gas increases with increasing temperature. This only happens if pressure does not change. The graph shows that the volume of a gas increases as temperature increases, but only at a constant pressure. As with Boyle s law, the reverse is true, also. The volume of a gas decreases as temperature decreases at a constant pressure. Volume (L) 100 80 60 40 Temperature v. Volume for a Fixed Amount of Gas at Constant Pressure Extrapolation Gas A Gas B Applying Math 4. Interpret a Graph About how much did the volume of Gas B increase when the temperature rose from 100 C to 200 C? 20 273 Gas C 200 100 0 100 200 300 Temperature ( C) Charles s law can be explained using the kinetic theory of matter. As a gas is heated, its particles move faster and faster and its temperature increases. Because the gas particles move faster, they begin to strike the walls of their container more often and with more force. In the hot-air balloon, the walls have room to expand. Instead of increased pressure, the volume of the gas increases. How is Charles s law used? In the same way that there is a formula for the relationship of pressure and volume, there is a formula for the relationship of temperature to volume. The temperature is given in kelvins. The formula for that relationship is V i /T i V f /T f. In this formula, V i and T i stand for initial volume and temperature. V f and T f stand for final volume and temperature. The pressure must be kept constant when you use Charles s law. 5. Infer When the temperature of the air in a hot-air balloon increases, what happens to the volume of the gas? Reading Essentials 263
After You Read Mini Glossary pascal: the SI unit of pressure 1. Review the term pascal and its definition in the Mini Glossary. Use the term in a sentence that shows that you understand its definition. 2. Complete the graphic organizer to compare Boyle s law and Charles s law. Boyle s Law States The volume of a gas decreases when the pressure increases. Equation Assumes 264 CHAPTER 14 Solids, Liquids, and Gases
Charles s Law States Assumes Constant pressure Equation 3. Study Coach As you read this section, you wrote down the question headings and their answers. How could you use this to study for a quiz on this section? End of Section Reading Essentials 265