Crushing Soda Cans with Air Pressure

Contributed by Pinrui Pan

Introduction:

Wanna surprise your friends with a soda can implosion and brag about your ability to harness the Force like Darth Vader? Sadly, there is no such thing as the Force in the real world, but with physics, we could still create the implosion drawing power from something invisible: the atmospheric pressure! We will see how we could crush a can in a blink of an eye with a rapid change in the air temperature inside it.

Materials

• A soda can
• A big bowl 
• Water 
• Stove in the kitchen
• Risk of scalding is involved, so make sure to have the following items as well!
  • Tongs/Baking gloves
  • Safety goggles

Procedure

Preparation:

1. First, open the tap of the soda can and empty it with the body intact. You could either drink it or dispose of the liquid by pouring it into the sink. 
2. Fill approximately one third of the can with water.
3. Prepare a good amount of cold water in the bowl. Room-temperature water is also okay.

The following steps involve some certain risks and we recommend doing them with your parents and your goggles and baking gloves on. 

4. Turn on the stove and put the soda can on it (with the water inside). Be careful when using the stove. If you need help in this step, you could also ask your parents to do it for you.
5. Wait for a while till the water is boiling and you could see a significant amount of water vapor coming out of the can opening.
6. Use the tongs to hold the can firmly.
7. Following the step 6, put the can with the hot water directly upside down into the bowl and the magic should happen.

Physics Concepts and Questions

• Let’s think about what has happened and what physical quantities have changed in the last step. When we put the soda can into the cold water, we brought it into contact with a much colder environment. As we know that energy flows from hot objects to cold objects (second law of thermodynamics!), the heat of the air molecules inside the can would be transferred to the cold water, resulting in a drop in the molecules’ temperature.
• Now we know that the temperature would decrease for the air inside the can. Next, let’s think about the composition of it. Besides normal air which includes oxygen and nitrogen, the air inside also includes a large amount of water vapor (the gas form of liquid water) from the previous hot water. When the temperature of water vapor rapidly drops as it touches the cold water, it will change its phase back to liquid (liquid water). 
• With a large amount of water vapor turning back into water, the number of air molecules in the can rapidly decreases. To see how that affects the soda can, let’s look at the famous ideal gas law:

PV=NkT

where P is Pressure, V is volume, N is the number of air molecules, T is temperature, and k is a constant called the Boltzman constant. If the volume of the can does not change, the pressure on the left side of the equation would have to decrease rapidly along with the number of air molecules on the right side of the equation so that the two sides are kept equal. With the rapid drop in pressure inside the can, the air would want to fill up the can again under the influence of atmospheric pressure, so that the pressure is the same inside and outside. However, the opening is sealed with water. Therefore, the air outside has no other option but to push the can inward, decreasing the volume on the left side of the equation so that the ideal gas law is satisfied.

Further Investigations:

Think about another case. What would happen if we change the last step to pouring out the hot water first and then press the can upside down into the water? Would the soda can still be crushed? If not, why? (Hint: think about the composition of the air molecules inside the can after pouring out the water)