Show
Scuba divers (ultramarinfoto, iStockphoto)
Scuba divers (ultramarinfoto, iStockphoto)
There are four laws, known as Gas Laws, which describe how gases behave. The four laws are Boyle’s Law, Charles’s Law, Gay-Lussac’s Law and Avogadro’s Law. Boyle’s LawRobert Boyle, a famous English chemist, discovered in 1662 that if you pushed on a gas, its volume would decrease proportionately. For example, if you doubled the pressure on a gas (increase the pressure two times), its volume would decrease by half (decrease the volume two times). The opposite is also true. If you reduced the pressure on a gas by 3.5 times, then its volume would increase by 3.5 times. This law is an example of an inverse relationship - if one factor increases, the other factor decreases. 
Pulling up on the lid of a sealed container increases the volume and decreases the pressure. Pushing down on the lid of a sealed container decreases the volume and increases the pressure. Boyle’s Law in Everyday LifeHere’s a story from British Airways. Back when British Airways was called British Overseas Airways Corporation (BOAC) (before 1974), female flight attendants in the airline were finding that their uniform skirts were fitting on take-off but once they reached cruising altitude, their skirts felt too tight. This tight-skirt mystery was solved using gas laws! A spokesman for BOAC used Boyle’s Law to explain what was going on. He explained that as the pressure in the cabin decreased at the higher altitude, the pressure in the flight attendants’ stomachs also decreased, thus causing the volume of their stomachs to increase (making their stomachs bulge). Since then, female flight attendants wear adjustable skirts. The working of a syringe can also be explained using Boyle’s Law. When the plunger of a syringe is pulled out, the volume inside the barrel increases, resulting in a decrease in the pressure inside the barrel. Fluids (such as water) flow from a high pressure area to a low pressure area. This means that once the pressure inside a syringe is lower than the pressure outside the syringe, a fluid near the needle (e.g., water, medicine, etc.) will flow into the syringe. The opposite is also true. When the plunger is pushed back in, the volume decreases and the pressure increases. Once the pressure is greater than that outside the syringe, the fluid inside the barrel will flow out. The operation of your lungs also can be explained using Boyle’s Law. When you inhale (breathe in), your diaphragm (a large muscle below your lungs) lowers, which increases the volume inside your lungs. This makes the air pressure inside your lungs lower than the air pressure outside your lungs (and your body); therefore, the outside air is drawn into your lungs (much like the syringe). When you exhale (breathe out), your diaphragm pushes upwards, reducing the volume inside your lungs, increasing the pressure and forcing the air outwards. A weather balloon is a special type of high altitude balloon. These balloons can reach heights of 18 to 37 km above the Earth carrying instruments for measuring atmospheric pressure, temperature and wind among other things. When weather balloons are sent up, they are only partly filled with gas (typically with helium). Why don’t they fill them completely? Short answer – because they would pop! At higher elevations, the air pressure outside the balloon is lower than the pressure of the helium inside the balloon. As Boyle’s Law states, this causes the volume inside the balloon to increase. If the balloon was already full, this increase in volume could cause the balloon’s rubber to stretch beyond its breaking point.
Plan Investigation 14
Having established that air is matter in the last lesson, this investigation explores some properties of air, a gas. If you have added air to the tires of a car or a bicycle, you have had some experience with compressed air. It takes some effort, but it's possible to squeeze a large volume of air into a significantly smaller space. This is not true for liquids or solids, which are essentially incompressible. What is it about air, and gases in general, that allows it to be compressed? In comparison with solids and liquids, the individual particles (atoms or molecules) of any gas are quite far apart from one another, with nothing but a vacuum in between them. Therefore, when pressure is applied to a gas, the particles can be squeezed closer together. Formative Assessment
Look for evidence of students explanations in their annotated notebook entries on the page Explaining the difference in compressibility of water and air. As you interpret student work, remember, students have not yet viewed air with the Particle Magnifier. They are basing their ideas on first–hand experience compressing air and water and using the Particle Magnifier to explore particles of water and ice.
A next step might be to ask students if they think ice would be compressible or not. Today students continue to explore the properties of air. Students compare the compressibility of air and water and then develop an annotated drawing that responds to the question, "How do you explain the difference in the compressibility of air and water?" By the end of this investigation students will understand that, in contrast with liquids and solids, air is observably compressible and will make a drawing to explain their observations. Learning Goals
Materials and Preparation
Notebook PagesWhat are some properties of air? Record properties of air that students can identify. As they mention a property ask for evidence or reasoning that supports their claim.
Save the list of properties; students will add properties as they continue their investigation of air. Introduce the term compressible - able to be squeezed or pressed into a smaller size or volume. One property of a rock is that it is not visibly compressible. One property of a sponge is that it is visibly compressible. Explain to students they'll be exploring the compressibility of air and how it compares with the compressibility of water. Do you think that air and water are compressible? Have students predict and explain their reasoning in their Science Notebooks, [Predicting the compressibility of water and air]. Air in a single syringe
Distribute a 12cc syringe to each student. Direct students to:
Students only need a minute or two to explore the compressibility of air with the syringes. Collect the syringes when they are finished.
Note: As students explore the compressibility of air, they may become interested in whether or not the syringe plunger returns to the exact same spot once it has been released. It may not, due to friction between the rubber seal and the plastic barrel of the syringe, or because a tiny bit of air leaked out between the syringe and the palm of the hand. Neither of these possibilities should distract from the big idea that air can be squeezed into a smaller space, and will expand again once the plunger of the syringe is released. Water and air in a syringeHow does the compressibility of water compare with the compressibility of air?
Move from group to group with two plastic buckets each holding two capped water-filled syringes and two capped air-filled syringes. Under your supervision, give four students at a time an opportunity to compress water and air in syringes. Be sure students point the capped ends of their syringes into one of the plastic buckets before they apply pressure to the plungers. In the unlikely event that the pressure causes a cap to release, the water and cap will end up in the plastic bucket. Provide a few minutes for students to record observations on the [Observing the compressibility of water and air] page in their Science Notebooks.
Note: Liquids and solids are compressible, but only to a tiny degree in comparison with air or gases in general.
When you compressed air in the syringe, maybe you noticed that it pushed back with a little bounce. Air acts somewhat like a spring. You can compress it, or squeeze it into a smaller volume. When you push on the plunger you can feel the air pushing back. When you stop pushing, the air inside the syringe will return to its original size. There are different ways to compress air. One way is to squeeze a certain volume of air into a smaller space. That’s what you did with the syringe. Another way is to use a pump, like a bicycle pump, to squeeze extra air into something that is already full of air. That’s what you do when you pump air into a basketball, soccer ball, or bicycle tire. Why is it possible to add more air to something that is already full of air? Only because air is easily compressible — the particles can be squeezed closer together. (Remember how different it felt when you tried to compress water in the syringe?) The bouncy feeling you notice when you compress air can be very useful. When you dribble a basketball, the part of the ball that hits the floor gets pressed into the ball a little bit. It’s a little like pushing the handle of the syringe or squeezing a spring. The air inside the ball gets squeezed, and it springs back, just like the handle of the syringe did. This “springing back” makes the ball bounces back up to you. Maybe you have tried to dribble a basketball or kick a soccer ball that did not have much air in it. If you have, you know the ball does not have the same kind of springiness or bounce.
Use the Annotated Drawing poster to review the purpose and important characteristics of an annotated drawing. Students may want to review the Annotated Drawings at the back of the Science Notebook. They should make their drawings on the page [Explaining the compressibility of water and air] in their Science Notebooks. Their drawings should answer the questions below.
Note: You will need to select two drawings that offer different explanations for students to review in the next investigation. Try to find a drawing that portrays water as made of particles that are packed together (making it visible) and air as made of particles that are spread apart (making air invisible) and select it for the class to review. Select a second drawing that portrays an alternate explanation of the composition of water and air. Alternately, you can choose to use the Sample Annotated Drawings found in the References. Recap the investigationExplain that you are going to select two annotated drawings based on different ideas about why air is compressible and water is not. The class will discuss those drawings at the start of the next investigation. |
What happens to air in a syringe when you push in the plunger?
Related Posts
Copyright © 2024 themosti Inc.
