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Spontaneous Combustion of Acetylene


1 Liter Graduated Cylinder

Large Watch Glass

Plastic 1 mL Dropper

100 mL 3M Hydrochloric Acid

Small bottle of Calcium Carbide Chunks (about 1 gram)

Bottle of Fresh Bleach

Gloves and Goggles



  • Add a few small calcium carbide chunks to 100ml 3M HCl in a 1 liter graduated cylinder to generate acetylene gas.
  • Cover the grad cylinder with a watch glass to contain the gas.
  • When the carbide is dissolved remove the watch glass and place it on the table.
  • Quickly add a pipete full of bleach to the cylinder. Chlorine gas is generated which will immediately ignite the acetylene.
Note: The bleach must be from a freshly opened bottle for this demo to work well.
          (Must use ultra bleach, because ~5% sodium hypochlorite is needed).



Calcium carbide reacts with water to yield acetylene as shown.

CaC2(s) + 2 ~ H2O(l) --> C2H2(g) + Ca2+(aq) + 2OH-(aq)


Bleach is about 5% NaOCl. When added to HCl, chlorine gas is made.

ClO-(aq) + Cl-(aq) --> Cl2(g) + H2O (l)


The tendency of Cl2 to remove hydrogen is so great that it reacts spontaneously with acetylene to produce an exothermic reaction.

C2H2(g) + Cl2(g) --> 2HCl + heat

C2H2(g) + O2(g) --> 4CO2(g) + 2H2O (g) + heat


In a limited supply of oxygen there is a smokey flame because of the formation of carbon.


Waste Management

The leftover HCl solution should be neutrailzed and then poured down the sink.