Sublimation and Deposition

Directors of concerts and plays sometimes use dry ice to create a fog-like special effect. Dry ice is the common name for the solid form of carbon dioxide. At room temperature, dry ice can directly change from a solid to a colorless gas. Sublimation is the phase change in which a substance changes from a solid to a gas or vapor without changing to a liquid first. Sublimation is an endothermic change. As dry ice sublimes, the cold carbon dioxide vapor causes water vapor in the air to condense and form clouds.

Where does the name dry ice come from? Solid carbon dioxide does not form a liquid as its temperature rises. Suppose 100 steaks are shipped from Omaha, Nebraska, to a supermarket in Miami, Florida. The steaks will spoil unless they are kept cold during the trip. If regular ice is used, water collects in the shipping container as the ice melts. If the steaks are shipped in dry ice, the container and the steaks stay dry during the journey. Figure 24 shows another use of dry ice.

When a gas or vapor changes directly into a solid without first changing to a liquid, the phase change is called deposition. This exothermic phase change is the reverse of sublimation. Deposition causes frost to form on windows. When water vapor in the air comes in contact with cold window glass, the water vapor loses enough kinetic energy to change directly from a gas to a solid.

Figure 24 A technician at Tinker Air Force Base in Oklahoma hangs a mosquito trap. The trap is baited with dry ice because mosquitoes are attracted to carbon dioxide.

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