Level: Elementary to H.S.
Time involvement: 10 to 15 minutes.
Large measuring cup, baking soda, vinegar, sheet of typing paper, matches, and a small candle and bowl. Because of the use of fire adult supervision is required. A large measuring cup is preferred because it will “trap” more CO2 and enhance the effect. Begin by placing several tablespoons of the baking soda in the cup. Place a short, lighted candle (votive candle is perfect) in the bowl. The bowl will serve two purposes: keeping wax off the demonstration area and causing the CO2 to “puddle” up in the bowl. Fold the sheet of typing paper in half the long way. Because of the slight risk of fire when the paper is held near the candle aluminum foil may be substituted.
Get ready for action! Begin pouring the vinegar into the cup with the baking soda. It will immediately begin foaming and create CO2 gas. The exact amount of vinegar is not critical. Too much will not produce more CO2. Be sparing but if the action seems to be stopping add a bit more vinegar. Gently pour the invisible gas onto the lighted candle. It will extinguish almost immediately. Be careful not to pour the vinegar out too. Relight the candle. If too much CO2 remains in the cup it may prevent the relighting. Pour or blow the unwanted CO2 away. Hold the folded paper (foil) at about a 45 degree angle above the candle. There should still be enough CO2 in the mixing cup. If the next step fails it may be necessary to “reload” the cup and activate it with more vinegar. Again, gently pour the invisible gas into the top of the paper, which will be serving as a delivery trough. The CO2 will move slowly down the trough so it will take a few seconds for the effect to be observed. The candle will be extinguished again. Take a bow! This is an ever-popular demonstration kids love.
Discussion: Baking soda is bicarbonate of soda, NaHCO3. Vinegar is weak acetic acid CH3COOH by combining the two, Carbon Dioxide CO2 gas is produced. CO2 is used in fire extinguishers because it robs the fire of the Oxygen needed to support combustion. This is a fun demonstration not just because it involves lots of action and fire but because the CO2 is invisible but does the “trick” of putting out the fire.
Notes: This is not a hard demonstration but is improved by practice to get the timing right. Because the CO2 is only slightly more dense than air, it is important to avoid doing the demonstration near fans or circulating air.
Learn more: The traditional method of fire fighting is with water. Unfortunately, for some industrial fires involving alkali metals such as sodium, potassium or lithium the addition of water will accelerate the fire. Firefighters are trained to recognize the fire and use the appropriate remedy.