Cold Can

If you've never seen a compressed air can before, take a look at Figure 1. Compressed air cans are used for a variety of things, ranging from cleaning laptop keyboards to vinyl discs. When the can's trigger is pulled, air is shot out of the nozzle at a very high speed. If the can is in your hand while you hold on to the trigger, it usually gets so cold that you may need to put the can down. Today's article will explain this effect.

The first thing to know is that the "compressed air can" is not actually composed of compressed air. Really, it is mostly filled with a chemical called 1-1 Difluoroethane. Even more surprising, most of the difluoroethane in the can is liquid. So, how is it possible that the can is filled with a liquid but the air that shoots out is a gas? The explanation is in the definition of solids, liquids, and gases. The difference between these states of matter is the distance between atoms (Figure 2). For example, atoms in a gas are much farther apart than the atoms in a liquid. However, if we push the atoms in a gas closer and closer together, the atoms can eventually form a liquid.

This is the exact process that happens inside the compressed air cans. In the can, the difluoroethane is at very high pressure. For context, 1 atm (atmospheric pressure unit) is the pressure from all of the weight of the air above you, summing up to more than 2000 pounds per square foot. It requires 5.3 times that pressure (5.3 atm) to compress difluoroethane from a gas into a liquid. The can has this exact pressure of 5.3 atm, filled with both liquid and gaseous forms of difluoroethane.

When the trigger is pulled, a valve to the nozzle opens and the pressure from inside the can cause the existing gas at the top to shoot out. As the existing difluoroethane gas gets depleted, more liquid needs to transition to gas to exit the can. So how does liquid usually turn into gas? The first example that comes to mind is evaporation. Water is a liquid until heat energy is transferred to it from sunlight, allowing the transition to gaseous water vapor.

The same concept applies here. For a transition from liquid to gas, heat energy is required. In what is called the Joule-Thomson effect, the surrounding heat (from the can, your hand, the straw, etc...) is used to make the transition of the difluoroethane from liquid to gas. This continuously happens as you hold the trigger, causing more and more heat energy to be taken away from your hand and the can and into the transition process. As a result of heat energy being taken away, it feels cold!