For years I taught about density as well as the properties of carbon dioxide, though never at the same time. Why it took so long to put the two together, I'll never know. Here then, are a few demonstrations involving the density of carbon dioxide gas and a flame. These should be done by adults or under careful adult supervision as there is a potential of fire hazard.

I suggest starting by showing that air and carbon dioxide have different characteristics. Hold a lit candle in a glass for a few moments. It keeps burning. Now place a small amount of baking soda in the glass and pour in a bit of vinegar. What do the kids see happening (bubbles) and what do they think is happening (gas is produced)? Now lower the lit candle into the glass and it will go out instantly (the gas does not support the burning of the candle).

For the next part of the demonstration you will need several candles of different height. You will want as many different heights as possible so some candles may be broken, some placed on supports such as candlestick holders, inverted coffee cups, etc. I always hold these candles in place by lighting one and allowing the hot wax to drip onto the surface where I want a candle to be placed. I then place the bottom end of the candle in the wax and allow the wax to harden. The candle will be "glued" in place by the cooling wax. I have the candles / supports placed in the upright aquarium so that one corner remains open.

Now, light all of the candles. The effect should be quite pretty, but that is not the purpose. Place one cup of baking soda in the free corner of the aquarium. To this add two cups of vinegar. The familiar bubbling will occur. By watching the candles, something intriguing will be seen. They will go out, one by one, from the candle closest to the aquarium bottom to the uppermost candle. The reason? The carbon dioxide is more dense than air, therefore it rests below the air in the aquarium. Since it is continually produced, it pushes the air out of the aquarium so that it can remain below it. If enough vinegar / baking soda are added, the aquarium will become brim full of carbon dioxide. To prove this just light another candle and slowly lower it into the aquarium. It will extinguish itself once it gets below the aquarium's opening. In fact, the carbon dioxide will rest in the aquarium for quite some time, especially if the room is still.

Another demonstration to try is to make a long trough out of V-shaped aluminum foil and use it as a guide for pouring carbon dioxide. Place a candle in a candlestick holder and light it. Place some baking soda and vinegar in a large bottle and let the bottle fill with carbon dioxide. Now, hold the aluminum foil trough so that one end is just above the candle flame. The other end is held higher. If water was added to the trough, it would pour down and extinguish the candle. We are going to do this with the carbon dioxide instead. "Pour" the carbon dioxide from the bottle into the higher end of the trough. The candle flame will flicker and usually extinguish at the lower end of the trough. Why? (The carbon dioxide, being more dense than air, actually does pour down the trough. Once it reaches the other end, it pours out of the trough and over the candle flame. Since the gas does not support combustion, the flame flickers and goes out.) This should re-instill the idea that carbon dioxide is more dense than air, as well as the fact that carbon dioxide does not support combustion.

A final thought... if you have easy access to affordable dry ice, it can be used in place of the baking soda and vinegar. It really is just carbon dioxide gas cooled and compressed into solid form. If you handle it, be sure to use gloves and tongs as it can quickly and easily cause frostbite.

Until next time... keep having fun with science!

Thomas Smith owns and operates Wonder Workshops, an online supply mecca of retro toys, puzzles, puppets, magic tricks, books, novelties, and DVDs, all based in science, math, and problem solving. Tom also presents hands-on, mind-on science workshops for teachers and parents around the country. To find out more about the workshops and store, be sure to visit http://www.wonderworkshops.com You'll be glad you did!