SAM Teachers Guide Gas Laws Overview Students explore how volume, pressure, temperature and density of gas molecules interrelate. They cover Boyle's, Charles's, Gay-Lussac's and Avogadro's laws. Students are then are asked to solve a challenge. Learning Objectives Students will be able to: Describe gas pressure as it relates to collisions of molecules on a surface of an object. Explore the relationship between pressure, volume, temperature, and number of particles as shown in the Ideal Gas Law (PV = nRT). Describe the relationship between pressure and volume in a gas. (Boyle’s law) Describe the effect of varying the temperature on the volume of a gas. (Charles’s law) Describe the relationship between temperature and pressure in a gas. (GayLussac’s law) Explain the relationship between the number of particles and volume of a gas as demonstrated by Avogadro’s law. Create a model demonstrating why the soda can collapsed and explain the model using the gas laws. Possible Student Pre/Misconceptions Gases have no mass. Gases move in a set path. Gases are invisible and cannot be contained. Therefore, they cannot exert pressure. Models to Highlight and Possible Discussion Questions After completion of the activity: Models to Highlight: Page 2 – Balloon Cross Section o Highlight what happens as atoms are added to and removed from inside the balloon and use this as a way to review student understanding of gas pressure. Page 3 – Pressure / Volume Relationship Model o Highlight that when temperature is kept constant, pressure and volume exhibit a clear relationship. As volume increases, pressure decreases. As volume decreases, pressure increases. Page 4 – Temperature / Volume Relationship Model o Highlight that when pressure is kept constant, as temperature increases, volume increases. As temperature decreases, volume decreases. They are directly proportional. o Link to other SAM activities: Heat and Temperature. Discuss how change in temperature affects the motion of atoms. Page 5 – Temperature / Pressure Relationship Model o Highlight that when volume is kept constant, as temperature increases, pressure increases. As temperature decreases, pressure does so as well. Page 6 – Number of Particles / Volume Relationship Model o Highlight that temperature and pressure were kept constant in this model. When this is the case, increasing the number of particles is directly related to increasing the volume. Page 7 – Soda Can Demonstration Model o Have students share the models they created and the challenges they faced while creating the models. Did it work on the first try? Or were they able to determine the relationship at work by using the model itself? Possible Discussion Questions: Demonstration/Laboratory Ideas: Have students watch the Soda Can Demonstration and / or the Liquid Nitrogen Plus Balloon Demonstration in the lab. This will evoke class discussion and give students a firsthand account of what happened. What is temperature? [Link back to the previous SAM activity: Heat and Temperature.] How does the model of the balloon on page 2 relate to Newton’s third law? [Link back to previous SAM activity: Newton’s Laws at the Atomic Scale.] If scientists wanted to know the relationship between all of the factors in the gas law (pressure, volume, temperature, number of particles), why do they set one (or more) as constants to study these relationships? How does this relate to the scientific method? Brainstorm other phenomena that exist in the world around us and explain them in terms of the gas laws. Connections to Other SAM Activities This activity focuses on the relationships between pressure, volume, temperature, and number of particles in a gas. This activity is supported by the Heat and Temperature activity, which illustrates how a change in temperature is related to an increase in kinetic energy. Students learn that it is the increased collisions of particles with that container that builds pressure. Newton’s Laws at the Atomic Scale illustrates how particles move in a straight line until they encounter collisions. Finally, Phase Change introduces the molecular arrangement of atoms in different states. This activity supports Diffusion, Osmosis, and Active Transport. Here, dissolved substances behave in a manner that is very similar to that of gas particles. They spread out to fill their container. In addition, osmotic pressure is very similar to pressure created by gas molecules as they collide into the walls of their surroundings. Activity Answer Guide Page 1: Introduction, no questions Page 2: 1. Remove atoms inside the balloon until there are fewer than 20 atoms. Run the model for a minute, take a snapshot, and drag the image into the box below. 3. How does adding and removing atoms affect the position of the balloon's skin (the rectangle)? When atoms are removed, the balloon's skin moves to the left or starts to be pushed inward. There are more gas particles outside the balloon exerting pressure than inside. When atoms are added to the balloon, the balloon's skin moves to the right. The increased pressure inside the balloon causes the skin to expand. 4. Can you think of other ways of increasing the number of collisions on the inner surface of the balloon without adding more atoms to it? Increasing the temperature or the speed of atoms moving inside the balloon will increase the number of collisions the atoms will have with balloon. This image shows the model when there are only 16 atoms inside the balloon. It had a chance to run for about a minute after the atoms were removed. 2. Add atoms inside the balloon until there are more than 60 atoms. Run the model for a minute, take a snapshot, and drag the image into the box below. Page 3: Point 1 Volume: 3 Pressure: 0.33 Point 2 Volume: 4 Pressure: 0.23 Point 3 Volume: 5 Pressure: 0.16 Point 4 Volume: 2 Pressure: 0.52 9. According to your data, which of the four graphs above best represents the relationship between pressure and volume of a gas at constant temperature? (a) This image was taken after there were 65 atoms inside the balloon and the model was allowed to run for about a minute. 10. Which of the following best describes what occurred as the volume increased? (b) 11. In your own words, describe the relationship between pressure and volume. Making the container smaller (decreasing the volume) causes the pressure exerted by the gas molecules to increase. Making the container larger (increasing the volume) makes the pressure decrease. Conclusion: As volume increases, pressure decreases and vice versa. Page 4: 1. Why does the piston move when the temperature changes? Include in your answer how molecules colliding with the piston can account for the movement of the piston. As temperature increases, the molecules are moving faster and colliding more frequently with a greater force, thus moving the piston to the right and increasing the volume. The opposite is true as temperature is decreased. 2. Use the slider to select a lower temperature, run the model for a minute, take a snapshot and describe what happens to the volume of the gas. This image was taken when temperature was raised. Here, you can see that the volume has increased. 4. In your own words, describe the relationship between temperature and volume in the ideal gas law. As temperature increases the volume increases. As temperature decreases so does the volume. Page 5: 1. Which of the following best describes what occurred as the temperature increased? (a) 2. How does changing the temperature of the gas affect the pressure? Decreasing the temperature of the gas decreases the pressure exerted by the gas molecules. Increasing the temperature of the gas increases the pressure exerted as the gas molecules are in more rapid motion with more numerous collisions. 3. In your own words, describe the relationship between pressure and temperature. Temperature and pressure are directly proportional. As temperature increases in a closed system, so does pressure. As pressure increases, so does temperature. This image was taken when the temperature was lowered. When the temperature is lowered, the volume also decreases. 3. Use the slider to select a higher temperature, run the model for a minute, take a snapshot, and describe what happens to the volume of the gas . Page 6: Volume Measurements Taken from Model: 1. 120 molecules - 31 2. 100 molecules - 28 3. 80 molecules - 22 4. 60 molecules - 16.8 5. 40 molecules - 11 6. 20 molecules - 6 7. Which of the four graphs below best represents the relationship between the number of particles and volume of a gas (at constant pressure and temperature)? (d) 8. Which of the following best explains what occurred as the number of particles decreased (check all that apply)? (d) 9. In your own words, describe the relationship between volume and number of molecules. The relationship between volume and number of molecules is directly proportional. As the number of particles increases, so does the volume or the space that these particles occupy. Page 7: 1. Take a snapshot that illustrates that you figured out the reason why the soda can collapsed. of the gas decreases the pressure until the can implodes. Page 8: 1. Which relationship of the following best explains this experiment? (b) 2. Explain why the balloon shrank and then expanded in this experiment. As the temperature of the balloon decreased drastically in the presence of liquid nitrogen, the balloon deflated. That is, its volume decreased as well. As the liquid nitrogen evaporates, the temperature begins to rise and the gas molecules expand again to re-inflate the balloon. As we learned through the page 4 model, as temperature increases, volume does as well. 3. List two ways you can increase the pressure in a container with a fixed volume and explain why this causes a pressure increase. An increase in the number of particles in the container or the temperature of the particles in the container would both lead to an increase in pressure. In both cases, the molecules encounter more collisions. The particles inside the soda can get colder as it is plunged into the ice water. As temperature decreases, so does the pressure inside the can compared to the pressure outside the can. 2. Use your understanding to explain why the coke can on the first page collapsed. Assume the can was sealed when it was placed in the ice water. The water inside the can is first heated on the hot plate to create steam. The steam is then drastically cooled when the can is plunged into the ice water. As the temperature decreases, the steam condenses into liquid water. Pressure outside the can is much higher than that inside the can. The can collapses because of the pressure difference. Decreasing the temperature 4. SCUBA tanks have to be built to withstand very high pressure because many gas molecules are put inside them. Why would putting more molecules inside cause high pressure? Molecules of a gas are in constant, random motion. When more molecules are present inside a fixed volume (such as a SCUBA tank), there are more opportunities for collisions between molecules, which leads to an increase in pressure inside the tank. SAM HOMEWORK QUESTIONS Gas Laws Directions: After completing the unit, answer the following questions to review. 1. The Ideal Gas Law states that PV = nRT. Identify each of the variables in the equation in the space below. P: _____________V: _____________ n: ______________ R: ___________ T: ______________ 2. What is the relationship between pressure and volume in a gas? 3. When pressure is kept constant, what is the relationship between temperature and volume in a gas? 4. What is the relationship between temperature and pressure in a gas? 5. BEFORE – Volume Reading = 6.0 AFTER- – Volume Reading = 32.0 Look at the BEFORE and AFTER pictures above. What is the effect of adding atoms on the volume of a gas? Why does this happen? SAM HOMEWORK QUESTIONS Gas Laws – With Suggested Answers for Teachers Directions: After completing the unit, answer the following questions to review. 1. The Ideal Gas Law states that PV = nRT. Identify each of the variables in the equation in the space below. P: pressure V: volume n: number of particles R: constant T: temperature 2. What is the relationship between pressure and volume in a gas? As explained by Boyle’s Law, pressure and volume are inversely proportional. As volume increases, pressure decreases. As volume decreases, pressure increases. 3. When pressure is kept constant, what is the relationship between temperature and volume in a gas? As explained by Charles’ Law, temperature and volume are proportional in a closed system. Therefore, as temperature increase, volume also increases. As temperature decreases, volume decreases. 4. What is the relationship between temperature and pressure in a gas? As explained by Gay-Lussac’s Law, temperature and pressure are proportional in a closed system. When temperature increases, pressure increases. When temperature decreases, pressure decreases. 5. BEFORE – Volume Reading = 6.0 AFTER – Volume Reading = 32.0 Look at the BEFORE and AFTER pictures above. What is the effect of adding atoms on the volume of a gas? Why does this happen? As the number of particles increases, so does the volume. Number of particles and volume are proportional in a closed system according to Avogadro’s Law. Adding more particles causes more collisions between particles and the need for particles to spread out to maintain a constant pressure.