Relevant National Science Education Standards

This reading is meant to show the students a fundamental general chemistry concept in action. One of the phenomenon that makes a refrigerator work is the heat of vaporization. Heat is absorbed when the refrigerant vaporizes, then given off again when the refrigerant is condensed into a liquid. Also illustrated is the relationship between pressure and phase transitions. When assigning this reading you may want to bring a small dorm-sized refrigerator to class so they can feel for themselves the heat being given off by the coils in the back of the machine. (Be sure to plug in the refrigerator few hours before class time to let it get cold!) To drive further illustrate the involvement of heat in phase changes you may tell the students of the practice of spraying outdoor plants with a mist of water on cold nights. The heat of fusion given off by the water as it freezes keeps the plants from freezing. You can also illustrate the heat of boiling by placing thermometers in two beakers of boiling water, on slowly simmering and the other at a fast rolling boil. The thermometers will show both to be at the same temperature. The lesson here is that the heat of vaporization must be absorbed before the temperature can rise.

The phenomena behind refrigeration can be demonstrated using a can of electronics duster. An electronics duster is simply a liquefied hydrofluorocarbon (HFC) in a can, and can be found at any hardware store for under ten dollars. The duster sprays a stream of HFC vapor, formed by the rapid evaporation of the liquid inside the can. This evaporation absorbs a great deal of heat, and the can becomes very cold during use. The nice thing about this demonstration is that it not only shows the heat of vaporization in action, it makes use of compounds similar to the HFC refrigerants which are being used to replace CFCs, reinforcing another topic of this module. Be careful carrying out this demonstration, as the duster can may become cold enough to cause frostbite during prolonged spraying.

The heat of vaporization can be demonstrated in other ways. A drop of room-temperature water on the back of a student's hand will feel cold, due to the absorption of heat by the evaporating water. A drop of rubbing alcohol will evaporate even more quickly, and will feel even colder. You can also demonstrate heats of transition using beakers of boiling water and thermometers, showing that boiling water always has a temperature of 100°C (assuming 1 atm pressure), regardless of how rapidly it is boiling. Likewise, you can demonstrate that the temperature of ice water will remain a constant 0°C while the ice is melting, and will not increase so long as ice is present.

The reading only discusses one of the two phenomena responsible for the cooling in a refrigerator. Depending on the abilities of you students you may choose to discuss the other cooling phenomenon at play. It works like this: when the refrigerant passes through the throttle valve, in addition to vaporizing, the gas also expands. This is called Joule-Thompson expansion. The expansion occurs as the refrigerant goes from being under high pressure on one side of the throttle valve to being under very little pressure on the other. The expansion of a gas is work, and requires energy. To get this energy the refrigerant absorbs even more heat from its surroundings, cooling the inside of the refrigerator further.

This can be demonstrated using a CO₂ fire extinguisher. The nozzle of a fire extinguisher is a throttle valve, and when CO₂ passes through the nozzle it undergoes a Joule-Thompson expansion. After a discharge of the fire extinguisher, the nozzle will be very cold, and the students can feel this with their own hands.

Relevant National Science Education Standards

Unifying Concepts and Processes — the reading analyzes the workings of refrigeration systems and the chemical processes involved.

Physical Science — Interactions of energy and matter are the very core of this reading.

Science and Technology — The reading explains the technology of refrigeration while demonstrating the challenges faced in designing new refrigerants to replace CFCs.

Next: The Two Faces of Ozone