This experiment shows the spontaneous reaction of ethyne and chlorine, along with its bursts of yellow flame and black sooty deposit of carbon.


Lumps of calcium carbide added to dilute sulfuric acid in a beaker produce ethyne gas. As this gas is released, addition of household bleach to the acid produces chlorine gas as well. When the two gases meet they react spontaneously, giving bursts of yellow flame and leaving a black sooty deposit of carbon.

Lesson organisation

This is a quick teacher demonstration, taking about 5 minutes. It should be performed in a fume cupboard or hood to avoid exposure to chlorine in particular. In a slightly darkened room the flashes of flame that accompany the mini-explosions are more easily visible.

Apparatus Chemicals

The teacher will need:

Eye protection

Safety screen

Access to a fume cupboard

Beaker, 250 cm3

Dropping pipette


Calcium carbide, CaC2 (HIGHLY FLAMMABLE), one fresh, pea-sized lump (Note 1)

Sodium chlorate(I) solution (Sodium hypochlorite, household bleach), 5% (0.7 M) (Irritant), 5 cm3, (Note 2)

Sulfuric acid, 2 M (CORROSIVE), 50 cm3

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

This demonstration should be performed in the fume cupboard. Wear eye protection. Place a safety screen between the demonstration and the class. 

Calcium carbide (calcium dicarbide) (HIGHLY FLAMMABLE) - see CLEAPSS Hazcard.

Sodium chlorate(I) solution (Sodium hypochlorite, household bleach), 5% (0.7 M) (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book. 

Sulfuric acid, 2 M (CORROSIVE) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Small quantities of two hazardous gases are produced in this experiment:

Ethyne (Acetylene) (EXTREMELY FLAMMABLE) - see CLEAPSS Hazcard. 


1 Calcium carbide reacts with moisture in the air, leaving a residue of calcium hydroxide. In selecting calcium carbide lumps for this experiment, avoid pieces coated with significant white powdery deposit, indicating they have deteriorated in storage. Ensure that calcium carbide is always kept in a securely capped container in a dry environment.

2 Avoid using household bleaches containing detergents or thickening agents. Sodium chlorate(I) solution (sodium hypochlorite) purchased for chemical suppliers is more concentrated (about 1.5 mol dm-3) and is classed as Corrosive. It should be diluted.


a Place 50 cm3 of 2 M sulfuric acid in a 250 cm3 beaker.

b Add one pea-sized lump of calcium carbide. This will sink and react to give off bubbles of ethyne gas.

c Now add about 1 cm3 of domestic bleach to the acid using a dropping pipette. Chlorine gas is evolved.

d Within a few seconds (or possibly immediately) the two gases will react with explosive ‘pops’, mostly at the surface, giving a yellow flame and black sooty smoke. Intermittent flames will continue for about a minute.

e More bleach or calcium carbide may be added as appropriate to continue the reaction.

f If necessary, quench the reaction by pouring into a large bowl of cold water in the fume cupboard.

Teaching notes

The reaction of calcium carbide with water to form ethyne in is still used occasionally by cavers in ‘carbide lamps’ to produce a bright yellow flame by burning the gas.

CaC2(s) + 2H2O(aq) → C2H2(g) + Ca(OH)2(aq)

In the presence of dilute sulfuric acid this becomes:

CaC2(s) + H2SO4(aq) → C2H2(g) + CaSO4(s)

Chlorine is produced from bleach (a mixture of sodium chlorate(I) and sodium chloride) by the reaction:

ClO (aq) + Cl(aq) + 2H+(aq) →Cl2(g) + H2O(l)

The reaction of ethyne with chlorine can be simple addition across the double bond:

C2H2(g) + Cl2(g) →CHCl=CHCl(l)
C2H2(g) + 2Cl2(g) →CHCl2–CHCl2(l)

However, under the conditions of this experiment, chlorine reacts explosively with ethyne, removing hydrogen from the hydrocarbon and depositing soot, in what is probably a free radical reaction:

C2H2(g) + Cl2(g) →2C(s) + 2HCl(g)

The intermittent reaction seems to be caused by a buildup of ethyne followed by reaction in which it is all used up. More ethyne is evolved and it again builds up to a critical value and so on.

Health & Safety checked, August 2016


This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry


Microscale versions can be found at: Creighton University

Page last updated October 2015