Five or six liters of water is placed in the bottom of a 55 gallon drum by the fountain outside of Cascade Hall. The drum is placed on a large burner and the water is boiled. After a few minutes, when the drum is filled with water vapor, the drum is sealed and cooled by throwing the drum into the pool of the fountain. After a short time the drum collapses with a boom. Please schedule this demonstration at least one week before it is to be performed. A smaller can crush using an ether can may be performed on shorter notice. This demonstration illustrates the temperature dependence of vapor pressure. 

Curriculum Notes 

This demo can be used anytime atmospheric pressure is being discussed, but it is probably most effective when the temperature dependence of vapor pressure is being discussed in a unit on the equilibrium of states of matter. This demo is performed outside, so most instructors like to schedule it at the end of class so that students can leave afterward. Allow about 10 minutes for this demo if someone is starting it early for you and the drum is about ready to cap when you dismiss the students to go outside. If you start it from the beginning and wait for the water to boil it can take about 20 minutes

One day of lead time is required for this project.

Discussion 

After the drum has boiled for a while, it is filled with water vapor at about 100^oC and ambient atmospheric pressure (about 760 Torr). When the water vapor cools inside the closed drum, it condenses out and the vapor pressure falls to about 20 Torr at room temperature. If the drum was allowed to equilibrate to room temperature, the sides of the drum would experience about 200,000 N (about 20 tons) of force. The drum presumably fails well before this pressure difference is reached.

Materials 

• large propane burner with attached propane tank
• long butane lighter
• 55-gallon steel drum with tightly fitting steel bungs and intact bung gaskets
• plastic carboy containing 4-5 L of water
• two pairs of heat resistant gloves
• groove joint pliers

Procedure 

• Turn on the gas to the burner and carefully light it with the long butane lighter. Adjust the flame to maximum.
• Remove the large bung from the drum and add the water to the drum.
• Have an assistant help you set the drum on the burner.
• The water should begin to boil in about three to five minutes. After the water begins to boil, leave the burner on for another five minutes to allow the steam to displace the air in the drum.
• Turn off the burner! Be sure that the burner is off before sealing the drum. Failing to do so could result in an explosion!
• Using the insulated gloves, screw the cap into the drumhead. Tighten with the pliers.
• Using the insulated gloves, have an assistant help you toss the drum into the fountain pool. Collapse can occur anywhere between thirty seconds to three minutes later.

Safety Precautions 

• Be careful lighting the burner. It can flash up into a fairly large flame. Wear goggles.
• Make sure that the cap is not in the drumhead when you begin to heat the drum. Boiling water in a closed drum could result in a steam explosion.
• The sides of the drum get very hot during this demonstration. Do not touch the drum when it gets hot.
• Have a fire extinguisher on hand in case of accidental fire.
• Do not leave burner unattended while lit.

Prep. Notes 

If possible, a preparator should be beginning this demo early so that the water has been boiling for a few minutes and the drum is almost ready to cap by the time the students start to arrive.