The balloon shrinks down to
practically zero volume when
pulled from the liquid nitrogen. It is filled with very cold high
density air at that point. As
the balloon warms the balloon expands and the density of the air
the balloon decreases. The volume and temperature kept changing
in a way that kept pressure constant. Eventually the balloon ends
up back at room temperature (unless it pops).
When the air inside a parcel is exactly the same as the air outside, the two forces are equal in strength and cancel out. The parcel is neutrally bouyant and doesn't rise or sink.
If you replace the air inside the balloon with warm low density air, it won't weigh as much. The gravity force is weaker. The upward pressure difference force doesn't change, because it is determined by the air outside the balloon which hasn't changed, and ends up stronger than the gravity force. The balloon will rise.
Conversely if the air inside is cold high density air, it weighs more. Gravity is stronger than the upward pressure difference force and the balloon sinks.
We can modify the demonstration that we did earlier to demonstrate Charles' Law. In this case we use balloons filled with helium (or hydrogen). Helium is less dense than air even when the helium has the same temperature as the surrounding air. A helium-filled balloon doesn't need to warmed up in order to rise.
We dunk the helium-filled balloon into some liquid nitrogen to cool it and to cause the density of the helium to increase. When removed from the liquid nitrogen the balloon doesn't rise, the cold helium gas is denser than the surrounding air (the purple and blue balloons in the figure above). As the balloon warms and expands its density of the helium decreases. The balloon at some point has the same density as the air around it (green above) and is neutrally bouyant. Eventually the balloon becomes less dense that the surrounding air (yellow) and floats up to the ceiling.
Something like this happens in the atmosphere.
At (1) sunlight reaching the ground is absorbed and warms the ground. This in turns warms air in contact with the ground (2) Once this air becomes warm and its density is low enough, small "blobs" of air separate from the air layer at the ground and begin to rise. These are called "thermals." (3) Rising air expands and cools (this is something we haven't covered yet). If it cools enough (to the dew point) a cloud will become visible as shown at Point 4. This whole process is called free convection. Many of southern Arizona's summer thunderstorms start this way.
The relative strengths of the downward graviational force and the upward pressure difference force determine whether a parcel of air will rise or sink. Archimedes Law is another way of trying to understand this topic.
A gallon of water weighs about 8 pounds (lbs).
If you submerge a 1 gallon jug of water in a swimming pool, the jug becomes, for all intents and purposes, weightless. Archimedes' Law (see figure below, from p. 53a in the photocopied ClassNotes) explains why this is true.
The upward bouyant force is really just another name for the pressure difference force covered earlier today (higher pressure pushing up on the bottle and low pressure at the top pushing down, resulting in a net upward force). A 1 gallon bottle will displace 1 gallon of pool water. One gallon of pool water weighs 8 pounds. The upward bouyant force will be 8 pounds, the same as the downward force on the jug due to gravity. The two forces are equal and opposite.
Now we imagine pouring out all the water and filling the 1 gallon jug with air. Air is about 1000 times less dense than water;compared to water, the jug will weigh practically nothing.
If you submerge the jug in a pool it will displace 1 gallon of water and experience an 8 pound upward bouyant force again. Since there is no downward force the jug will float.
One gallon of sand (which is about 1.5 times denser than water) jug will weigh 12 pounds.
The jug of sand will sink because the downward force is greater than the upward force.
You can sum all of this up by saying anything that is less dense than water will float in water, anything that is more dense than water will float in water.
The same reasoning applies to air in the atmosphere.
Air that is less dense (warmer)
than the air around it will
Air that is more dense (colder) than the air around it will sink.
Cans of both regular and Diet Pepsi are placed in beakers filled with water (Coke and Diet Coke can also be used). Both cans are made of aluminum which has a density almost three times higher than water. The drink itself is largely water. The regular Pepsi also has a lot of high-fructose corn syrup, the Diet Pepsi doesn't. The mixture of water and corn syrup has a density greater than plain water. There is also a little air (or perhaps carbon dioxide gas) in each can. The average density of the can of regular Pepsi (water & corn syrup + aluminum + air) ends up being slightly greater than the density of water. The average density of the can of diet Pepsi (water + aluminum + air) is slightly less than the density of water.
In some respects people in swimming pools are like cans of regular and diet soda. Some people float (they're a little less dense than water), other people sink (slightly more dense than water).
Many people can fill their lungs with air and make themselves float, or they can empty their lungs and make themselves sink. People must have a density that is about the same as water.
A chemistry challenge from Science Buddies
Key concepts Chemistry States of matter Gases Energy Temperature Introduction Background Materials
Key concepts Chemistry States of matter Gases Energy Temperature
IntroductionHave you ever baked—or purchased—a loaf of bread, muffins or cupcakes and admired the fluffy final product? If so, you have appreciated the work of expanding gases! They are everywhere—from the kitchen to the cosmos. You’ve sampled their pleasures every time you’ve eaten a slice of bread, bitten into a cookie or sipped a soda. In this science activity you’ll capture a gas in a stretchy container you’re probably pretty familiar with—a balloon! This will let you to observe how gases expand and contract as the temperature changes.
BackgroundEverything in the world around you is made up of matter, including an inflated balloon and what’s inside of it. Matter comes in four different forms, known as states, which go (generally) from lowest to highest energy. They are: solids, liquids, gases and plasmas. Gases, such as the air or helium inside a balloon, take the shape of the containers they’re in. They spread out so that the space is filled up evenly with gas molecules. The gas molecules are not connected. They move in a straight line until they bounce into another gas molecule or hit the container’s wall, and then they rebound and continue in another direction until they hit something else. The combined motion energy of all of the gas molecules in a container is called the average kinetic energy. This average kinetic (motional) energy changes in response to temperature. When gas molecules are warmed, their average kinetic energy also increases. This means they move faster and have more frequent and harder collisions inside of the balloon. When cooled, the kinetic energy of the gas molecules decreases, meaning they move more slowly and have less frequent and weaker collisions.
More to explore
Looking for a Gas, from Rader’s Chem4Kids.com
Gases around Us, from BBC
Balloon Morphing: How Gases Contract and Expand, from Science Buddies
Racing to Win That Checkered Flag: How Do Gases Help?, from Science Buddies
This activity brought to you in partnership with Science Buddies
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