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Chapter 17 Properties of Gases Buoyancy and Gases Just like liquids, buoyancy is also a property of gases. How do balloons float? Answer: They displace a very large volume of air. This volume of air weighs more than the total weight of the balloon. Hot-air balloons float because they weigh less than the volume of air displaced. Charles’ Law The relationship between temperature and volume. Formula: V1/T1 = V2/T2 The volume of gas increases as temperature increases. The volume of a gas decreases with decreasing temperature. Gases and pressure Pressure is the force acting on a unit area of surface. Pressure is usually measured in units of pound per square inch (psi). Example: Bicycle tires SI unit for pressure is called pascal (Pa) One pascal = one newton of force acting on one square meter of surface area. Atmospheric pressure The air we breathe is made up of many different gases including carbon dioxide, oxygen, and nitrogen. Atmospheric Pressure is the force of the air that is acting on each square inch of the Earth’s surface. This pressure is created by the force of the Earth’s gravity pulling the air towards the surface. Air Pressure Why do my ears pop? If you've ever been at a higher altitude, you may have noticed that your ears pop and you need to breathe more often. As the number of molecules of air around you decreases, the air pressure decreases. Your ears will “pop” to balance the pressure. If you were on top of Mt. Everest you would be breathing fewer molecules of oxygen. You would need to breathe faster to help your body get more oxygen. Boyle’s Law The relationship between pressure and volume of a gas under constant temperature. Formula: P(1)V(1) = P(2)V(2) (Initial) (Final) Boyle’s Law states that you increase the pressure as the volume decreases, and you decrease the pressure as you increase the volume. Boyle’s Law in action You have a beach ball and a basketball. Obviously the beach ball is bigger than the basketball. Pump enough air into the beach ball to inflate it fully. Pump the same volume of air into the smaller basketball. The basketball will have much more pressure in it due to the fact that there are many more air molecules colliding with the smaller surface of the basketball. Thus increasing the pressure as volume decreases!
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