Hands-on Science Carnival Activity Stations: Material Properties
Properties of Materials: Phase Transformations with Dry Ice
[Shopping List: dry ice; various plastic soda bottles; small funnels; rubber tubing; metal spoons; aluminum pie pans; fruit juice; small drinking cups; balloons; dish soap; gloves; hammer and tongs (to break up and handle the dry ice block)]
Note: For safety only adults should dispense the dry ice, and never touch it directly without gloves.
Sublimation
Place one or two pieces of dry ice in a dish or on the table top. Observe that there is no puddle of liquid as it warms like you would see with normal water ice.
Add one or two pieces of dry ice to a beaker or cup of warm water and observe the bubbles of CO2 gas forming. Also notice that the fog produced as the bubbles pop at the water surface is heavier than air, and will sink to the floor rather than rising. You can demonstrate this by slowing pouring it out of the cup. [CO2 is colorless; the white fog you see is actually tiny droplets of liquid water, condensed from water vapor in the air as it mixes and cools. Of course the carbon dioxide gas is also very cold, and thus more dense than warmer gas, so this is largely responsible for its rapid sinking in warm air.]
Crush some dry ice into powder and pour it into an empty plastic soda bottle.
Stretch a balloon over the mouth of the bottle, and observe the balloon inflate. You can speed it up by warming the bottle in warm water or with your hands.
Remove the balloon, and pour some warm water into the bottle to produce fog. Stretch another balloon over the mouth; now the balloon should inflate much faster.
You can pour some powdered dry ice into another balloon using a funnel. Tie the balloon closed and watch it inflate.
Place a smooth flat piece of dry ice on a smooth tabletop or pan (flat side down). After a few seconds you should observe the ice begin to float and drift about like a hockey puck on ice. If this doesn't happen you may need to use tongs or gloves to rub the dry ice on the tabletop until its bottom surface becomes smoother.
Fog Vortex Ring Shooter
Add some warm water to a small plastic bottle (disposable drinking water bottles work great). Add a few small pieces of dry ice.
As the fog is created, gently squeeze the sides of the soda bottle and observe fog rings shooting out of the mouth. It may take a little practice to make good vortex rings. Short and quick- but not too strong- squeezesare best.
When the fog production slows down, as you blow a fog vortex ring out of the bottle and allow the sides to return to their normal shape, you should observe a dry clear air vortex ring entering the bottle.
When the water gets too cold fog production will slow down. When this occurs dump out the cold water and add fresh warm water.
Carbonation
Remove the cap from a large plastic bottle (1/2 glallon fruit juice bottles work well) and carefully drill two small holes side by side so that a piece of clear plastic tubing will fit tightly through one of the holes. Leave the other hole open. Pull the tubing far enough to reach the bottom of the bottle.
Fill the bottle about 3/4 full of apple or other fruit juice.
Add a few chunks of dry ice to the bottle, then screw the cap on. Bubbles of dry ice will form and fog should stream out of the open hole.
Place a cup under the end of the tubing, then plug the open hole in the cap with your finger and watch fruit juice flow up the tube and into the cup. To stop the flow simply remove your finger from the hole, making sure to lift the free end of the tubing high enough for the juice inside to return to the bottle, otherwise you may create a siphon and spill juice.
Fog Bubbles (Dinosaur Eggs)
Insert a funnel in one end of some small diameter rubber tubing (~18" long).
Remove the cap from a plastic drink bottle (Gatorade bottles work well) and drill one small hole just large enough so that the rubber tubing will fit tightly through the hole. Leave 1-2" of tubing extending on the bottom side of the cap.
Fill the bottle about half full of warm water, add a few small pieces of dry ice and screw on the cap. Be sure that the tubing is not touching the water. Observe the fog escaping from the funnel.
Dip the funnel into a dish of soapy water and remove to blow bubbles. Note that the bubbles are filled with fog, which is denser than the other gases in air, so they sink rapidly to the floor where they pop and the the fog escapes. If you're careful you might be able to bounce the bubbles on the tabletop, or catch and hold them with gloves. Bubbles also often land on carpet or towels without popping.
Dip a finger in the soapy water and try to push it through the bubble without it popping (a dry finger will always pop the bubble). If you completely wet your hand you can even hold a bubble in your palm without it popping.
Sublimation Vibrations
Place a small piece of dry ice in an aluminum pie pan. Note- Never touch dry ice with bare skin- you should closely supervise younger children and have them wear gloves for this activity.
Press on the dry ice with a metal spoon. The spoon and pan should begin rattling or vibrating to produce eerie sounds.
Place a piece of dry ice on one spoon and observe vibrations. You can also press or squeeze the dry ice between two spoons to make even more noise.
Notice how quickly the spoons get very cold (you can carefully touch the empty spoons). For this reason you will need to change to warmer spoons often to get the biggest loudest vibrations.
What's Happening: Dry ice is solid carbon dioxide, CO2. As it warms above -110°F or -78°C (at normal atmospheric pressure) it changes state from a solid directly to a gas without melting and becoming a liquid. This phase change is called sublimation. Because the dry ice is so cold, it can quickly freeze any moisture on your hands or other bare skin, making the dry ice stick to you and continue to freeze your skin causing frost bite. This is why you should never touch dry ice with your bare skin.
The gas produced during sublimation expands to fill a volume several hundred times greater than the volume of the original solid, which is why you must never put dry ice into a closed container. This CO2 gas pressure inflates the balloon. When you add some warm water, the heat from the water makes the dry ice sublime much faster (cooling the water at the same time), and the gas produced forms bubbles in the water which quickly rise to the surface (due to buoyancy) and escape. The gas is still quite cold however, so as it mixes with the warmer moist air in the bottle water vapor in the air condenses to form very tiny water droplets- so small that they can easily float around in air currents, yet large enough to scatter light. Thus the white stuff you see is literally a cloud or fog, just like the clouds in the sky, which form much the same way as warm moist air rises through the atmosphere until it cools and water droplets condense. Note that when these tiny water droplets mix again with warmer air in the room, they evaporate and the cloud disappears (the liquid water droplets become water vapor).
When you squeeze a plastic bottle filled with dry ice fog, you force the fog inside to shoot out the open mouth very quickly, where it encounters the motionless air outside (of course the outside air is not actually motionless, but the molecules are moving in different directions so that there is no net direction or wind). As this fast moving fog stream collides with the motionless outside air friction or drag causes the fog at the edges to slow down, while the fog in the center of the stream continues to move much faster. This turbulence makes the fog at the edges start to turn or spin, and since the hole is circular, the spinning air takes the shape of a donut or vortex. A spinning vortex of air (or fog) is very stable, and can travel much greater distances than a non-spinning puff of air before losing its shape. The same thing happens in reverse to produce the clear air rings that you might spot moving back into the bottle once the new fog production inside slows enough.
Note that the gas bubbles formed when the dry ice is dropped in water are filled with carbon dioxide gas, not air. As the bubbles rise some of this CO2 gas actually dissolves and carbonates- and slightly acidifies- the water or fruit juice. This what gives soda pop- or our fruit juice-the fizzy and tangy taste. When we plug the escape hole in the cap of our juice bottle we are also using this CO2 gas pressure to push the carbonated juice through the tubing and into a cup.
When you dip and remove the funnel from the soapy water a thin soap film stretches across the open end and the gas flowing out through the tube blows a bubble filled with fog and CO2 gas. The skin of the bubble is actually a three layer "sandwich" with water molecules in the center and soap molecules inside and outside. The soap lowers the surface tension (cohesive forces) between the water molecules and allows them to spread out into a film, but it is the water, not the soap, which gives the bubble its strength. When you touch a bubble with your dry skin it quickly attracts the thin film of water and breaks the bubble, but if your skin is wet and soapy your hand simply becomes part of the bubble and doesn't pop. Bubbles can also rest on a glove or towel if the "grassy" surface area of fibers is too small to absorb the water quickly.
Finally, when the warm spoon or aluminum pan touches the dry ice the heat causes the CO2 to sublimate and produce gas very quickly. This pushes the spoon away until it escapes, the two come together once again and the cycle repeats. This happens over and over very quickly causing the spoon vibrate, and of course these vibrations lead to the loud and creepy sound waves that make this activity so much fun.
