Demonstrations designed to capture the student's imagination
Demonstrations to capture the student's imagination, by Adrian Guy of Blundell's School. In this issue: combustion of methanol
This is a spectacular and simple demonstration to illustrate the combustion of methanol.
The methanol whoosh
Combustion of fossil fuels is a topical issue and studied across all stages of secondary education. An exciting way to introduce the topic to your students, this demonstration produces a large, blue flame and loud 'whoosh' sound.
- large, 20-litre polycarbonate plastic water bottle from an office water dispenser. Invert the bottle to allow it to drain completely. Leave for a few days to dry thoroughly
- large rubber bung to fit the opening of the water bottle
- 50ml methanol
- butyl rubber gloves
- wooden splint attached to the end of a metre rule
Wearing butyl rubber gloves, pour methanol (50ml) into the empty water bottle. Stopper the bottle with a rubber bung and roll the bottle on its side so that the methanol covers all the internal surface, most of it will evaporate to produce a methanol-air mixture. After ca 30s remove the bung carefully - it may be under some pressure and droplets of methanol may spray out. Pour any excess methanol down the sink and rinse with plenty of cold water. Put the upright bottle on a lab bench but make sure the ceiling is at least 2.5m above the top of the bottle and check there is nothing that might be easily ignited above the bottle. (If the bench is too high, put the bottle on a stool but make sure you do not lean over the mouth of the bottle.) Using a lit splint attached to a metre rule, ignite the vapours by introducing the flame to the bottle neck. A combustion will ensue producing a 'whoosh' and a large, blue jet flame extending ca 1m from the bottle neck.
On warm days methanol evaporates more readily and the demonstration is more dramatic. If it is a cold day, roll the bottle for longer to achieve a suitable concentration of methanol vapour. Do not use the same bottle to repeat the demonstration. After one use, rinse the bottle with water and allow to drain and dry, then inspect to ensure it is not damaged. After the main burn, vapours can continue to burn slowly around the bottle neck. To extinguish these cover the neck with a heat-resistant mat or replace the bung. You can use plastic water bottles of any size.
As well as a good way to capture students' attention, you can also use this demonstration to challenge younger students to identify one of the main products of combustion. After the reaction has finished get your students to inspect carefully the bottle's contents. They should notice that condensed water vapour collects in the bottle as a product of combustion - you can test this with anhydrous cobalt(II) chloride paper.
To demonstrate the effect of increasing carbon content on the incomplete combustion of fuels try substituting methanol with first ethanol and then propan-1-ol or propan-2-ol. The degree of incomplete combustion can be compared by the colour of the flame; the larger alcohols burn with increasingly luminous flames.
When igniting the bottle in the upright position ensure students are at least 5m away and you and the audience are wearing eye protection. If the bottle is placed on its side, particular care is needed to ensure that the jet wash behind and area ahead are well clear. For this demonstration, students should be 5m directly behind the bottle.
Methanol is highly flammable and toxic by inhalation, skin contact and ingestion. Butyl rubber gloves should be worn to handle methanol to prevent contact with skin. The excess methanol poured down the sink must be diluted with plenty of cold water and the demonstration performed a safe distance away to avoid accidental ignition of the vapours. Remove and dispose of rubber gloves prior to igniting the bottle.
The bottle must be made of polycarbonate - look for PC on the base. Do not use glass bottles. Check carefully that the bottle is not damaged, in particular look for crazing round the neck of bottles which have been used a few times. The experiment can be done with ethanol (industrial denatured alcohol), propan-1-ol or propan-2-ol but no other flammable liquids should be used. Never attempt to enrich the air in the bottle with oxygen.