'Cannon' fire

Read our standard health & safety guidance

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.

Apparatus and chemicals

For each demonstration:

Eye protection
Evaporating dish (10 cm diameter)
Safety screens
Heat resistant mat
Wooden splint

Potassium manganate(VII) (potassium permanganate) (Oxidising, Harmful), 0.5 g
Hydrogen peroxide, 20-volume (Irritant at this concentration), 30 cm³
Ethanol (IDA, Industrial Denatured Alcohol) (Highly flammable, Harmful), 20 cm³

Technical notes

Potassium manganate(VII) (Oxidising and Harmful) Refer to CLEAPSS Hazcard 81
Hydrogen peroxide, ‘20-volume’ (about 1.7 mol dm^-3) (Irritant at this concentration) Refer to CLEAPSS Hazcard 50 and Recipe card 32
Ethanol (IDA, Industrial Denatured Alcohol) (Highly flammable) Refer to CLEAPSS Hazcard 40A

Procedure

HEALTH & SAFETY: Use safety screens throughout to protect the audience and the demonstrator. Wear eye protection.
(See CLEAPSS Supplementary Risk Assessment 5 on the CLEAPSS website or the Science Publications CD-ROM)

a Put the evaporating dish on a large heat resistant mat to protect the bench. Place a safety screen between the mat and the class.

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 molecules 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