'Cannon' fire

Demonstration

Potassium manganate(VII) powder is sprinkled on to a burning mixture of hydrogen peroxide solution and ethanol. In the exothermic reaction which follows, a series of loud, but harmless, bangs are heard as the oxygen that is evolved increases the rate of burning.
 

Lesson organisation

The demonstration is spectacularly noisy, producing sharp, crackling bursts of noise and flame which sometimes seem to build rhythmically. A trial run is recommended for those trying it for the first time.

The demonstration can also be rather messy as splashes of the mixture in the basin are ejected. It is advisable to protect the bench area around the heat resistant mat with some non-flammable, washable material to aid clearing up afterwards.

The demonstration takes about 3 minutes.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear eye protection. Use safety screens throughout to protect the audience and the demonstrator.  (See CLEAPSS Supplementary Risk Assessment SRA05 on the CLEAPSS website). 

Potassium manganate(VII), KMnO₄(s), (OXIDISING, HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.

Hydrogen peroxide solution, H₂O₂(aq), (IRRITANT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Ethanol (IDA, Industrial Denatured Alcohol), C₂H₅OH(l), (HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard. 

Procedure

a Put the evaporating dish on a large heat resistant mat (or use several smaller mats) to protect the bench. Place a safety screen between the mat and the audience.

b Add 30 cm³ of 20-volume hydrogen peroxide solution and 20 cm³ of ethanol to the dish.

c Light the mixture with a burning splint. The ethanol vapour should ignite and burn with an almost invisible flame.

d Now, at arm’s length, sprinkle about 0.5 g of potassium manganate(VII) crystals into the dish.

e Immediately there will be a series of loud bangs, giving the effect of a volley of gunshot. and the flames will leap up. The noise will subside into crackling, which can last for up to a minute.

f When the noisy reaction is over, extinguish the burning ethanol (if still alight) by placing a heat resistant mat or tile over the evaporating basin.

g A residue of brown, solid manganese dioxide will be seen in the evaporating dish – and possibly in the area immediately surrounding it.

Teaching notes

Coarse potassium manganate(VII) crystals give fewer but louder bangs, while fine crystals give more but smaller ones.

It is strongly recommended not to use more concentrated solutions of hydrogen peroxide or to alter the other quantities in any other way.

The reaction between manganate(VII) ions and hydrogen peroxide is as follows:

2MnO₄^–(aq) + 3H₂O₂(aq) → 2MnO₂(s) + 2H₂O(l) + 3O₂(g) + 2OH^–(aq)

The localized evolution of oxygen accelerates the burning of the ethanol, and this produces pockets of evolved energy which manifest themselves in the mini-explosions produced.

Health & Safety checked August 2008