Level: Elementary to H.S
Time involvement: 15 – 30 minutes depending how many students.
A cube about 18 inches per side has transparent plastic sides and an open top. The bottom is closed with a sheet of plywood. A large bowl rests in the bottom and contains some hot water. Several chunks of dry ice (CO2 in solid form) are placed in the bowl of water. Immediately the dry ice is warmed and releases CO2 gas. Because the CO2 (carbon dioxide) is heavier than air it stays in the bottom of the tank. The CO2 is invisible. Students are given bubble wands and bubble solution. By blowing gently above the tank bubbles can drift down into the tank. Amazingly the bubbles do not reach the bottom because the bubbles contain room air which is less dense than CO2. The bubbles gently float on the CO2 for awhile before breaking. Occasionally the cold temperature will freeze a bubble!
Discussion: This demo is always a crowd pleasing demonstration because of the audience participation. Only 3-4 students at a time can fit around the tank, so larger groups will take more time. Alternate activities for waiting students should be provided by the teacher. The science take away is the principle of buoyancy. Not only do helium balloons and smoke from fires rise into the sky but many weather patterns are driven by this principle.
The underlying concept that explains why things sink or float is the relative density of the object compared to the fluid density. In the case of the atmosphere temperature determines density and thus whether an air mass will lift over or “wedge” under an adjacent air mass is determined by the relative temperatures. Warm air always rides over colder air and vice versa for cold air. Meteorologists thus speak in terms of warm fronts and cold fronts. Submarines too, depend on the ability to change their density to “dive” or “surface”. Dry Ice is not dangerous but must be handled with care. Touching it can cause flesh to freeze and cause permanent damage to tissues. Use gloves when handling.
Learn more: Dry Ice sublimes (passes directly from a solid to a gas without the intermediate liquid stage) at minus 109°F. It is used primarily as a refrigerant to keep food products frozen during shipment. Dense fog can be created for theatrical effects. Dry ice was first observed in 1835 by French inventor Adrien-Jean-Pierre Thilorier. It was patented in the U.S. in 1924 for commercial purposes.
For more info: http://en.wikipedia.org/wiki/Dry_ice=/