The Snuffed Out Candles - A Science Inquiry
Demonstration of Density
By Thomas Smith
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!
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Trick or Treating - Keep
Your Kids Safe Halloween is a holiday loved by kids everywhere. It’s a fun time
to dress up like silly or scary characters and go trick-or-treating in the
neighborhood. Be sure your kids are safe while trick-or-treating with these
quick tips..
