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CSAV Hawaii: Best Science Magic Trick Candle & Water

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Published on Dec 14, 2010

Amazing magic trick you can do at home! Make water defy gravity, as it fills a drinking glass that is upside down. Ingredients: Plate, candle, lighter, glass of water, food dye. Text in the video explains scientific principles. CSAV uses this demonstration to teach school children about science; it is also an excellent trick you can perform at birthday parties. NOTE: This is very similar to the old "CSAV Hawaii: Best Science Magic Trick with Water," except the newer version has more precise details about what causes the formation of the partial vacuum inside the glass. Viewers have been requesting more details on how this magic trick works, so if you want more details, keep reading.

First, you need to recognize that air has weight. At sea level, the air piled up above you puts 14.7 pounds of pressure onto every square inch (PSI) of surface area it contacts. Air is composed of several gases, including oxygen, and fire needs oxygen to burn. You can demonstrate this by lighting a small candle and then placing a drinking glass over it; the candle will burn up the oxygen trapped in the glass, and then the flame will go out, since fresh oxygen from outside the glass is unable to replace it. But as the candle burns up the oxygen, it produces carbon dioxide and water vapor, because the candle is made up of paraffin, a hydrocarbon (C25H52). The formula for this reaction is:
2 (C25H52) + 38(O2) = 25(CO2) + 26(H2O)
You know that the drinking glass is cold, from room temperature, but that the candle-created gases are warm. When the candle-created warm water vapor touches the cold drinking glass, it condenses into a liquid. There's not very much liquid water visible on the surface of the glass, but the change in volume is astonishing: the conversion from vapor to liquid is about 1,000 times smaller. When the volume inside the drinking glass shrinks, it creates a partial vacuum—a space where there is very little gas. But a vacuum is invisible. The best way to show this vacuum to schoolchildren is to repeat the demonstration, but with a moat of water (and food dye) surrounding the candle. Have the students predict what will happen; kids usually guess that the candle flame will draw in fresh oxygen from the water (formula H2O includes O), so you'll need to explain why O is not easily separated from H2O—a lucky thing for firefighters using water to extinguish a blaze. In this second demonstration, as the partial vacuum shrinks inside the glass, the outside air (14.7 PSI), which has been pushing down on the moat of water all along, will now push the water right under the rim of the glass, causing it to "defy gravity" and begin to fill the glass. Note that the water doesn't fill the glass completely; it reaches equilibrium. Most students immediately begin analyzing how to get more water to rise in the glass: a larger candle, a taller glass, colder water? Have fun!
Bonus question: What causes the bubbles at 1:01?

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