# Balloons and Charles' Law

## Objective

The objective of this project is to determine whether a balloon maintains the same air pressure when subject to temperature and/or pressure changes around the balloon. If the pressure of the balloon changes, how does it change in relation to the temperature change (speed of change, amount of change, etc.)?

Procedure: Easy

Concept: Medium

## Concept:

You are out on a hot summer day at a carnival and purchase a few balloons from a vendor. When you get home, you decide to leave a couple outside in the hot air and bring the rest into your air-conditioned house. You notice that the balloons inside and the balloons outside are slightly different shapes and that one group of balloons feels fuller than the other group of balloons.

What causes this effect? The major change between the two groups is the temperature, so you decide to figure out why the change in temperature causes a change in the balloon's shape, and whether there is a corresponding loss of pressure between the balloon groups.

## Materials:

• At least 15 balloons, same type and shape
• Freezing Area (freezer or refrigerator)
• Heating Element (hot water or hot day outside)
• Thermometer
• String, at least 2 feet long
• Metric Ruler

## Hypothesis

Thinking about the concept for awhile, what do you think the correlation between change in temperature, change in shape, and change in pressure of the balloon is? Can you guess a mathematical relationship to explain your hypothesis? How might you go about finding such a relationship?

## Procedure

1. Inflate all of your balloons in a room-temperature room. The more balloons you have, the more reliable your data will be.
2. Measure the circumference of each balloon by wrapping a piece of string around it, making a mark where the string overlaps with the beginning of the string, then holding the string next to a ruler.

Note: One way to easily do this is to write a number on each balloon, and then when you measure the circumference, you can write down the number of the balloon you measured. Also, record the current temperature.

3. Put at least 5 of the balloons into the refrigerator or freezer, along with a thermometer. Leave at least 5 more balloons to sit at room temperature with another thermometer as your control group. Finally, put at least 5 more balloons in a hotter area (but not too hot!) with another thermometer.
4. After 30 minutes, measure the room-temperature balloons again with the string and ruler to see how much air has leaked out. Do this again after another 30 minutes. The change in circumference should be small. Record these numbers for each balloon. Also record the temperature in the room.
5. Immediately after checking the control balloons, remove the experimental balloons one at a time from the freezer and measure them. If they have lost their spherical shape due to shrinking, try to mash them back into roundness, but don't waste too much time on this. It's important that they're still very cold when you measure them! Record these numbers carefully. Also record the temperature inside the refrigerator.

## Research

Your will enjoy this project much more if you have a knowledge of Charles's Law. Depending on your grade level, some additional math study may be required to understand Charles's Law. (See Law of Charles and Gay-Lussac for a good starting point!)

## Analysis

1. To begin your data analysis, first you must adjust some of your measurements to account for changes in room temperature and air leaking from the balloons. Take the average of the original room temperature and the final room temperature. Use this as the temperature for the control group.
2. Find the average percentage decrease in size for your control group balloons. First find the percentage lost in circumference from each balloon, then take the average of those percentages. Make sure you write this number down!
3. We'll assume that the balloons in the freezer lost about as much air due to leakage as the control group. Adjust the size of each freezer balloon by the percentage found in step 2 and record this circumference. This is the measurement you will be using in your equations. Repeat this step for the hotter balloons.
4. Using the measured circumferences for your freezer balloons, find the initial volume of each balloon. Record the initial volume for each balloon, then repeat the procedure using the adjusted final circumference from step 3 for each balloon to find the final volume. Repeat this step for the hotter balloons as well.
5. According to Charles's Law, when pressure is held constant, dividing the volume of a gas by the temperature of the gas will give a constant value, k. To prove or disprove your hypothesis, you will use this law. By dividing the initial volume by the room temperature, calculate the constant k for the experimental group of balloons.
6. Now, calculate the same constant k for each balloon by using the final values of temperature (the freezer thermometer) and volume. Do these values of k match? If not, how different are they? What do these values of k indicate? Determine the average amount of error between the initial and final calculations of k.

## Conclusion

What did you find? Based on the k values of the balloons that you calculated, how accurate would you say your experiment was. Were you correct or not, and why?

## Real-World Applications

Clearly balloons are not the only thing that would be affected by temperature changes and pressure changes. Many materials become brittle and easily breakable when their temperature changes enough. There are also materials that will explode if the pressure changes enough. Understanding the correlation between these changes and the effects that come about are crucial to studies in fields such as materials science, electrical engineering, physics, and other sciences as well.

## Extensions

Try this experiment at more temperatures or with different balloons, or find another material to substitute for the balloons that will provide equally interesting results.