“Cave of dogs” experiment
How to blow out a candle using carbon dioxide
“Cave of Dogs” – a simple experiment with a tragic history.
Wear protective gloves, eyewear, and a mask. Perform this experiment in a well-ventilated area. Observe safety precautions when working with fire.
Reagents and equipment
- 500 g sodium bicarbonate;
- 20% hydrochloric acid solution;
- glass tank (such as an aquarium);
- 2 L conical flask;
- stopper with 2 tubes;
- candle holders of varying heights;
- lighter or matches.
Arrange the candles in the candle holders and place them in the glass tank. (Make sure that the candle holders are of different heights, but not higher than the walls of the tank.) Light the candles. Pour 500 g sodium bicarbonate into the 2 L conical flask and close it with the stopper. Attach the syringe to one of the stopper tubes and use it to add the 20% hydrochloric acid solution to the sodium bicarbonate. Put the second tube into the glass tank. Carbon dioxide gas will begin to fill the tank. Watch as the candles gradually go out. Acid must be added until the last candle goes out.
Just outside Naples, Italy is a cave that was once a popular tourist attraction. Carbon dioxide gas of volcanic origin accumulates at the bottom of the cave. Local guides would arrange horrific “presentations,” lowering dogs into the carbon dioxide gas until they lost consciousness from lack of oxygen. The dogs were then revived in a nearby lake. The attraction gradually lost popularity, and the cave was eventually closed. This cave is often described in literature, and demonstrates the characteristics of carbon dioxide gas, namely its toxicity and density greater than that of normal air. The above experiment can serve as a demonstration of these same characteristics. The candles go out, from shortest to tallest, as the heavier carbon dioxide fills the tank and forces the lighter, regular air up and out, depriving the candles of the fuel they need to burn, or “breathe.”
NaHCO₃ + HCl = NaCl + CO₂ + H₂O