Sublimation occurs when an object changes from solid to liquid or liquid to solid while "skipping" the liquid phase in between. Sublimation usually takes place when a material with a very low boiling point such as dry ice is placed into a much warmer environment such as hot water. In this case a reaction produces fog which is from water vapor in the room condensing around the dry ice-chilled air. Sublimation projects provide good visual reinforcement of phase changes and they are not difficult.
Sublimation Singing Spoon
One attention-getting activity is to make a spoon sing. It can be used to show both sublimation and gas pressure. An ordinary metal spoon is warmed above a flame for a minute or two. The spoon is then pressed firmly against a chunk of dry ice which is held by someone wearing gloves and using insulated tongs.
The dry ice quickly sublimates when touching the warm spoon --- the carbon dioxide bubbles out but is blocked by the spoon surface. The gas pressure will force the spoon away a slight amount allowing some gas to escape, then the pressure of the person's hand pulls it back onto the ice. This process repeats rapidly, so the spoon vibrates and makes noise much like a tuning fork or teapot.
Sublimation in Freezer
An ordinary frost-free freezer and a little patience offer a very inexpensive sublimation project. Weigh a plastic bowl filled with some ordinary ice cubes and place it in the freezer where it will not be disturbed for three days. Remove the bowl and ice and reweigh. The "lost" ice transformed from a solid to a liquid. The dry air in the freezer allowed the molecules of water on the ice surface to escape into the surrounding air. Ice sublimation will not occur in a humid freezer.
Sublime Air Freshener
A "neat" little experiment is to heat some solid room air freshener in container surrounded by hot water. This should be done outside since concentrated air freshener vapors can be harmful. The observers will see that liquid does not appear because the material sublimes. A variation on this experiment is to heat the air freshener in a water-bath as before, but in a clear container with the lid on --- then immerse the container in some cool water after the material has disappeared into a gas. It will now condense into a solid.
Dry Ice Sublimation
A quick-and-neat demonstration uses a polystyrene foam cup, dry ice and plastic wrap. A small piece of dry ice about the size of a quarter is placed into the cup and the plastic sheet is wrapped tightly over the top and taped in place. As the ice sublimes, it will bulge and possibly rupture the plastic. This provides good visual reinforcement of the sublimation concept. Vary the experiment changing the size of the dry ice chunk. By timing each result, a graph can be constructed showing the relationship of dry ice size to gas produced.
Fire and Ice
A dry ice project can take on a "fire and ice" theme. Place a candle on a small saucer into a sauce pan and light it. Place a golf ball-sized chunk of dry ice in the bottom of the pan and wait. See how long it takes for the flame to go out. Carbon dioxide is heavier than air so as long as the air near the pan is fairly still, the flame will always go out if enough dry ice is used.
Dry ice can be purchased at many grocery stores or ice houses. Handle it carefully using insulated gloves or tongs as it has a surface temperature of above minus 80 Fahrenheit.
Alter the size of the dry ice chunk and plot its effects on the time before the candle goes out. Estimate the volume of gas produced by pouring water into the pan up to the level of the candle and then pouring into a measuring cup. This project can allow the student to show the amount of carbon dioxide gas produced per minute.
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