“Cave of dogs” experiment

How to blow out a candle using carbon dioxide

“Cave of Dogs” – a sim­ple ex­per­i­ment with a trag­ic his­to­ry.

Safe­ty pre­cau­tions

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;
  • sy­ringe;
  • can­dles;
  • can­dle hold­ers of vary­ing heights;
  • lighter or match­es.

Step-by-step in­struc­tions

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.

Process de­scrip­tion

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