Lesson

Background

Consider a hot air balloon. Gases expand when they are heated. Because the particles in the hot air move farther apart from one another than particles in the cool air, the hot air becomes less dense than the cool air. This difference in density allows the hot air balloon to rise. Jacques Charles (1746 – 1823) was a French scientist who studied gases. According to Charles’ law, the volume of a gas increases as the temperature of the gas increases, as long as the pressure remains constant. In equation form the law is written as:

V₁/T₁ = V₂/T₂

Charles’ law can be explained using the kinetic theory of matter. As a gas is heated, its particles move faster and faster and its temperature increases. Because the gas particles move faster, they begin to strike the walls of their container more often and with more force. In a hot air balloon, the walls have room to expand so instead of increasing in pressure, the volume of the balloon increases.

Since you are currently studying the gas laws, you will perform an activity that investigates Charles’ Law.

Purpose

To test Charles’ Law by constructing a hot air balloon from a plastic trash bag.

Safety

Always use caution when working around open flames.

Materials

• thin plastic trash bag
• straws
• aluminum foil
• five birthday candles
• lighter/matches
• tape
• ruler
• scissors

Procedure

1. Measure, in centimeters, the opening of the bag when laid flat on a surface. This is the diameter of the opening. Multiply it by 0.7. For example, if the bag measured 40.0 cm, the calculation is … 40.0 cm x 0.7 = 28.0 cm This will be the length of one of the straw pieces.
2. Cut off the flexible end of the straw, close to the ridges. Compress one end of the straw piece and insert it into another straw piece. Secure with a small piece of tape. Continue to combine straws until you reach the length you need. Make another straw piece of the same length.
3. Mark the center of each straw piece. Attach the straws together in a “plus sign” by lining up the marked centers. Securing the + with a piece of tape. Use tape sparingly. For the balloon to lift, it cannot be too heavy!
4. Cut a square piece of aluminum foil measuring 10.5 cm x 10. 5 cm.
5. From the center of each of the four sides, measure in 2.5 cm and make a mark. These points will be the positions of the candles.
6. Using one of the candles as your flame source, heat the bottom of another candle, allowing the wax to drip on a point marked on the foil. Press a candle into the wax and hold for 30 seconds. Repeat this process to attach all four candles to the foil. Wait a minute in between attaching each candle so the wax can harden. Be careful not to burn yourself. At this point, no candles should be lit.
7. Attach the foil and candles to the center of the straw construction. Secure each corner of the foil to a straw using a small piece of tape. Wait a few minutes to be sure the wax has hardened.
8. One person should hold the plastic trash bag open. The other person should invert the straw/candle base and attach the end of each straw to the edge of the bag by folding over a small amount and using a small piece of tape.
9. Securely tape or tie one end of a spool of fishing line/thread/string to the cross section of the straws. This tether is needed so the balloon can be retrieved if it flies very high and so you can maintain control of the balloon.
10. Be sure all pieces are secure. Now you are ready to launch!

Launching the Balloon

1. One person will hold the plastic bag vertically, from the top, while the other person carefully lights each of the candles. Do not light the plastic bag on fire! If this happens, it will shrivel, create a hole, and will not fly.
2. Lower the bag to the floor, gently hold the bag vertically. Be patient as the bag will begin to expand. The bag may begin to move horizontally across the floor at first but will eventually rise. When you start to get lift, let the balloon go on its own. Give the balloon plenty of slack as it rises. Guide the balloon as it flies to keep it away from any flammable materials and in the designated area.
3. Do not jerk the tether as it will cause the wax to roll off of the foil. After a few minutes of observing your creation, begin to gently reel in the balloon. The birthday candles burn quickly, and you do not want the candles to burn down and catch the foil or straws on fire.

You are responsible for controlling the flight of your hot air balloon. Maintain control of it at all times!

Analysis

Explain in your own words how this activity exhibits Charles’ Law.