What happens to the volume of a balloon when it is taken outside on a cold day explain why that occurs?

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 inside 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).

Now we are in a position to have a quick look at the forces that can cause parcels of air to rise or sink.

Basically it comes down to this - there are two forces acting on a parcel of air in the atmosphere:
1. Gravity pulls downward.  The strength of the gravity force depends on the mass of the air inside the parcel.  This force is just the weight of the parcel
2. There is an upward pointing pressure difference force.  This force is caused by the air outside the parcel (air surrounding the parcel).  Pressure decreases with increasing altitude.  The pressure of the air at the bottom of a parcel pushing upward is slightly stronger than the pressure of the air at the top of the balloon that is pushing downward.  The overall effect is an upward pointing force.

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 rise.  Air that is more dense (colder) than the air around it will sink.

There's a colorful demonstration of how small differences in density can determine whether an object floats or sinks.

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.

Bring Science Home

A chemistry challenge from Science Buddies

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Key concepts Chemistry States of matter Gases Energy Temperature  

Introduction

Have 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.  

Background

Everything 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.  

Materials

• Freezer with some empty space
• Two latex balloons that will inflate to approximately nine to 12 inches
• Piece of string, at least 20 inches long
• Permanent marker
• Cloth tape measure. (A regular tape measure or ruler can also work, but a cloth tape measure is preferable.)
• Scrap piece of paper and a pen or pencil
• Clock or timer
• A helper

 
Preparation

• Make sure your freezer has enough space to easily fit an inflated balloon inside. The balloon should not be smushed or squeezed at all. If you need to move food to make space, be sure to get permission from anybody who stores food in the freezer. Also make sure to avoid any pointy objects or parts of the freezer.
• Blow up a balloon until it is mostly—but not completely—full. Then carefully tie it off with a knot. With your helper assisting you, measure the circumference of the widest part of the balloon using a cloth tape measure or a piece of string (and then measure the string against a tape measure). What is the balloon’s circumference?
• Inflate another balloon so it looks about the same size as the first balloon, but don’t tie it off yet. Pinch the opening closed between your thumb and finger so the air cannot escape. Have your helper measure the circumference of the balloon, then adjust the amount of air inside until it is within about half an inch or less (plus or minus) of the first balloon’s circumference (by blowing in more air, or letting a little escape). Then tie off the second balloon.

 
Procedure

• Turn one of the balloons so you can look at the top of it. At the very top it should have a slightly darker spot. Using the permanent marker, carefully make a small spot in the center of the darker spot.
• Then take a cloth tape measure (or use a piece of string and a regular tape measure or ruler) and carefully make two small lines with the permanent marker at the top of the balloon that are two and one half inches away from one another, with the darker spot as the midpoint. To do this you can center the tape measure so that its one-and-one-quarter-inch mark is on the small spot you made and then make a line at the zero and two-and-one-half-inch points.
• Repeat this with the other balloon so that it also has lines that are two and one half inches apart on its top.
• Somewhere on one balloon write the number “1” and on the other balloon write the number “2.”
• Because it can be difficult to draw exact lines on a balloon with a thick permanent marker, now measure the exact distance between the two lines you drew on each balloon, measuring from the outside of both lines. (For example, the distance might be two and three eighths inches or two and five eighths inches.) Write this down for each balloon (with the balloon’s number) on a scrap piece of paper. Why do you think it’s important to be so exact when measuring the distances?
• Put balloon number 1 in the freezer in the area you cleared out for it. Leave it in the freezer for 45 minutes. Do not disturb it or open the freezer during this time. How do you think the size of the balloon will change from being in the freezer?
• During this time, leave balloon number 2 somewhere out at room temperature (not in direct sunlight or near a hot lamp).
• After balloon number 1 has been in the freezer for 45 minutes, bring your cloth tape measure (or piece of string and regular tape measure) to the freezer and, with the balloon still in the freezer (but with the freezer door open to let you access the balloon), quickly measure the distance between the two lines as you did before. Did the distance between the two lines change? If so, how did it change? What does this tell you about whether the size of the balloon changed? Why do you think this is?
• Then measure the distance between the two lines on balloon number 2, which stayed at room temperature. Did the distance between the two lines change? If so, how did it change? How did the balloon’s size change? Why do you think this is?
• Overall, how did the balloon change size when placed in the freezer? What do your results tell you about how gases expand and contract as temperature changes?
• Extra: After taking balloon number 1 out of the freezer leave it at room temperature for at least 45 minutes to let it warm up. Then remeasure the distance between the lines. How has the balloon changed size after warming up, if it changed at all?
• Extra: Try this activity again but instead of putting balloon number 1 in the freezer, put it in a hot place for 45 minutes, such as outdoors on a hot day or inside a car on a warm day. (Just make sure the balloon is not in direct sunlight or near a hot lamp, as this can deflate the balloon by letting the gas escape.) Does the balloon change size when put in a hot place? If so, how?
• Extra: In this activity you used air from your lungs but other gases might behave differently. You could try this activity again but this time fill the balloons with helium. How does using helium affect how the balloon changes size when placed in a freezer?

 
Observations and results Did balloon number 1, which was placed in the freezer, shrink a little compared with balloon number 2, which stayed at room temperature?   You should have seen that when you put the balloon in the freezer, the distance between the lines decreased a little, from about two and a half inches to two and a quarter (or by a quarter inch, about 10 percent). The balloon shrank! The distance between the lines on the balloon kept at room temperature should have pretty much stayed the same (or decreased very slightly), meaning that the balloon shouldn’t have really changed size. The frozen balloon shrank because the average kinetic energy of the gas molecules in a balloon decreases when the temperature decreases. This makes the molecules move more slowly and have less frequent and weaker collisions with the inside wall of the balloon, which causes the balloon to shrink a little. But if you let the frozen balloon warm up, you would find that it gets bigger again, as big as the balloon that you left at room temperature the whole time. This is because the average kinetic energy would increase due to the warmer temperature, making the molecules move faster and hit the inside of the balloon harder and more frequently again.  

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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