Dry Ice Comet
This demonstration provides a stunning visual model of comets, particularly illustrating how the length and direction of a comet's tail vary in relationship to the comet's location relative to our Sun. It also helps the children gain a better understanding of the composition of comets.
Dry ice is frozen carbon dioxide. When the dry ice warms up, the carbon dioxide changes directly from a solid (the dry ice) into a gas (the carbon dioxide). It never goes through a liquid stage, as water does, hence the name "dry ice." The process of going from a solid state directly to a gaseous state is called sublimation. Carbon dioxide is colorless and invisible. The swirling white mist is actually water vapor that has condensed into a small cloud due to the cold of the dry ice. This effect is similar to the collection of moisture on the outside of a cold soda can. However, when the air is chilled by the sublimating dry ice, the water condenses in midair as a cloud.
1. Put on safety glasses and gloves. Ask the children to maintain a safe distance while still being able to see the demonstration.
2. Put the dry ice into the paper grocery bag and crush it to a fine-grained consistency using the mallet. (The finer the texture, the better.)
Do not touch the dry ice with bare hands!
3. Line the large plastic bowl with the plastic garbage bag.
4. Pour the following liquid ingredients into the garbage bag: half of the water, ammonia (warn the children about the strong smell!), alcohol, and the corn syrup. Next, add the cornstarch and the soil.
Explain that each of the materials mixed into the model mimics or represents the actual components of comets and the typical percentages.
5. Carefully add in half of the crushed dry ice and mix well with other ingredients by kneading the outside of the garbage bag.
The dry ice will create a cool, cloudy vapor that is safe to touch. This water vapor cloud represents the outgassing of the comet that forms the gas tail as a comet approaches our Sun.
6. Add in the rest of the dry ice and mix well by kneading the outside of the garbage bag.
7. Add in the rest of the water.
The water/dry ice slush will start to thicken as the dry ice freezes the water.
8. Close the garbage bag around the comet and shape it into a ball.
It may be necessary to add a bit more water if the comet ball does not stick together.
9. Carefully remove the comet ball and place it in the pie pan or tray.
10. Holding the flashlight and hairdryer "Sun" (on low or cool setting) next to each other, point them toward the comet model from about 18 inches away. Turn out the lights to create a more dramatic visual effect. Move the hairdryer closer and farther from the dry ice ball. Ask the children to identify where the comet will experience the strongest and weakest solar wind.
The flashlight represents our Sun and the hairdryer represents the pressure from sunlight and the solar wind.
Potential Misconception Alert: Make sure the children understand that our Sun does not "blow" a wind, but that the solar wind is instead a stream of particles that exert a very small pressure on matter.
11. Ask a couple of children to hold the flashlight/hairdryer "Sun." Holding the comet in the pan, walk in an ellipse around our Sun. The children will need to keep the hairdryer and flashlight aimed at the dry ice throughout your orbit, but they should stay in the same spot. Make sure to walk far enough away so there is no effect from the blow drier. The dry ice ball should then be enveloped in an even cloud of vapor.
Ask the children to focus on the changing orientation of the comet's tail relative to our Sun throughout the orbit. Also have them notice how distance from our Sun affects the length of the tail.