A spontaneous exothermic reaction

In this experiment, students observe what happens when glycerol (propane-1,2,3-triol) is mixed together with potassium manganate(VII). The mixture bursts into flame, giving off clouds of steam, after a short time lag.

This reaction can be used as a fun demonstration to show a spontaneous reaction, or as an example of the redox reaction between a fuel and a powerful oxidising agent.

The time lag illustrates the speeding up of an initially slow exothermic reaction as the energy given out raises the temperature of the mixture.

Equipment

Apparatus

• Eye protection (including goggles or a face shield for the teacher)
• Safety screens
• Clean metal lid from a tin can or jar (see note 5 below)
• Heat resistant mat

Chemicals

• Potassium manganate(Vll) (OXIDISING, HARMFUL, DANGEROUS FOR THE ENVIRONMENT), 2–3 g in the form of fine crystals
• Glycerol (propane-1,2,3-triol), about 1 cm³ in a test tube

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout (teacher and students). The teacher/demonstrator should wear goggles or a face shield and use safety screens. 
• Potassium manganate(VII), KMnO₄(s) (OXIDISING, HARMFUL, DANGEROUS FOR THE ENVIRONMENT) – see CLEAPSS Hazcard HC081. Fine crystals of potassium manganate(VII) work much better than larger ones. Use a pestle to grind large crystals in a clean mortar, if necessary.
• Glycerol (propane-1,2,3-triol), CH₂OHCH(OH)CH₂OH(l) – see CLEAPSS Hazcard HC037A. Old samples of glycerol are sometimes ineffective, possibly because of absorbed water from the air. It is sometimes better if the glycerol is gently warmed just before use.
• If the lid from a jar has a plastic lining, this should be scraped off. Alternatively, use a small foil cake case which has been cleaned and dried.

Procedure

1. Put 2–3 g of potassium manganate(VII) in a small pile on the tin lid standing on the heatproof mat. Make a small hollow in the centre of the pile.
2. Pour about 1 cm³ of glycerol into the hollow in the pile of potassium manganate(VII). After about 20 seconds (but beware – it can be much longer), the mixture starts to give off steam. The glycerol in the mixture then ignites, burning with a bright, pinkish (lilac) flame for a few seconds more, leaving a dark brown or black residue.

Teaching notes

Eye protection and safety screens are essential. Small particles of potassium manganate(VII) may fly out.

A white background is useful. The reaction is even more spectacular in a darkened room.

Point out that the pink (lilac) colour of the flame is characteristic of potassium salts.

Redox chemistry

At advanced level, the redox nature of the reaction can be explored. Do this by allowing the residue to cool down and then dissolving it in water. This produces a green solution suggesting the presence of a Mn(Vl) species, as well as a brown solid, manganese(lV) oxide. This confirms the reduction of the manganate(Vll) ion; the glycerol has been oxidised to water (hence the steam) and carbon dioxide.