Hot air balloons (sjlayne, iStockphoto)
Hot air balloons (sjlayne, 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. Charles’ LawJacques Charles, a French physicist, discovered in the 1780s that heating a gas will cause it to expand by a certain fraction. The image below shows how adding heat makes molecules move faster and hit the sides and lid with greater force, thus moving the lid up as the gas expands. Charles’ Law in action (©2020 Let’s Talk Science).Charles’ Law in Everyday LifeIn order to make a hot air balloon rise, heat is added to the air inside the balloon. Adding heat causes the molecules to move further away from each other. Burners heat up the air inside a hot air balloon (Source: Sunridin [CC BY] via Wikimedia Commons).In everyday language, we would say that the air inside expands. When this happens, the total density (mass per unit of volume) of the balloon and the air inside it decreases. When the density of the balloon decreases to be less than the density of the outside air, the balloon rises. Conversely, the volume of a gas will shrink if its temperature decreases. Below you can see liquid nitrogen being poured over a green balloon. The cold liquid nitrogen cools the air inside the balloon. As a result the molecules of air slow down causing the volume of the balloon to decrease. Liquid nitrogen poured over a balloon (Source: © UW Madison Department of Chemistry. Used with Permission).During the holidays, someone you know may have used a turkey thermometer. A turkey thermometer is stuck into the turkey while it cooks and then pops up when the meat is cooked enough. How does this wondrous piece of technology work? It has to do with Charles’s Law, of course! Inside the turkey thermometer is a small amount of air. As the temperature rises inside the turkey, the air inside the turkey thermometer expands. Once it reaches a certain volume, the top pops, telling the chef that the turkey is properly cooked. Turkey thermometer (white pin stuck in the turkey breast) (Source: Patrick Fitzgerald from Atlanta, GA, USA [CC BY] via Wikimedia Commons).Gay-Lussac’s LawJoseph Louis Gay-Lussac was a French chemist and physicist who discovered in 1802 that if you keep the volume of a gas constant (such as in a closed container), and you apply heat, the pressure of the gas will increase. This is because the gases have more kinetic energy, causing them to hit the walls of the container with more force (resulting in greater pressure). Gay-Lussac’s Law in Everyday LifeInside a pressure cooker the food that you want to cook sits in water. As the temperature of the liquid water is increased, water vapour (water in its gas state) is produced. This vapour cannot escape the pressure cooker – meaning the volume is not changing. The pressure of the water vapour keeps rising until the temperature of the water and the water vapour exceed the normal boiling point of water (100 °C). At this higher temperature food can be cooked much faster. Tough meat also comes out much more tender after being cooked in a pressure cooker. Pressure cooker (LucaLuca [CC BY-SA] via Wikimedia Commons).Did you know that the air pressure on the inside of car tires changes when the car is driven? After driving, the air pressure in a car’s tires goes up. This is because friction (a contact force) between the tires and road causes the air inside the tires to heat up. The air cannot expand because the tires are essentially a fixed-volume container, so the pressure increases – this is Gay-Lussac’s Law! Measuring tire air pressure with a pressure gauge (Source: (U.S. Air Force photo by Airman Frank Snider) [Public domain] via Wikimedia Commons).Bring Science Home A pressure-filled science project from Science Buddies
Key Concepts Physics Gas Pressure Volume Boyle's Law Introduction Background You can observe a real-life application of Boyle's Law when you fill your bike tires with air. When you pump air into a tire, the gas molecules inside the tire get compressed and packed closer together. This increases the pressure of the gas, and it starts to push against the walls of the tire. You can feel how the tire becomes pressurized and tighter. Another example is a soda bottle. To get carbon dioxide gas into the liquid, the whole bottle is usually pressurized with gas. As long as the bottle is closed, it is very hard to squeeze, as the gas is confined to a small space and pushes against the bottle's walls. When you remove the cap, however, the available volume increases and some of the gas escapes. At the same time its pressure decreases. One important demonstration of Boyle's law is our own breathing. Inhaling and exhaling basically means increasing and decreasing the volume of our chest cavity. This creates low pressure and high pressure in our lungs, resulting in air getting sucked into our lungs and leaving our lungs. In this activity you will create your own demonstration of Boyle's law. Materials
Observations and Results The results look different with the water-filled balloon. Although you are compressing the air inside the syringe when pressing on the plunger, the water inside the balloon does not get compressed. The balloon stays the same size. The water balloon also keeps its shape when pulling out the plunger while closing the tip of the syringe. In contrast to gases, liquids are not compressible as their particles are already very close together. Boyle's law only applies to gases. If you filled the syringe with water as well, you should still have seen the air-filled balloon shrinking while pushing the plunger into the syringe. The air-filled balloon also should have expanded when pulling the plunger out while the tip of the syringe was closed. You might have noticed, though, that you were not able to push and pull the plunger in and out as far as you could with the air-filled syringe. This is again because of the fact that liquids cannot be compressed like gases. You should have observed that also when trying to push the plunger in or pull it back in the water-filled syringe with the water-filled balloon. It was probably impossible to move the plunger in and out! More to Explore This activity brought to you in partnership with Science Buddies Discover world-changing science. Explore our digital archive back to 1845, including articles by more than 150 Nobel Prize winners. Subscribe Now! |