The ‘whoosh’ bottle demonstration

This demonstration requires careful preparation and is the subject of a supplementary risk assessment by CLEAPSS, SRA006. Schools are advised not to deviate from the details described in this risk assessment. If any variation is necessary, members should contact CLEAPSS for preparing a special risk assessment. Teachers should also, of course, consult their own employer’s risk assessment.

A single demonstration will take 5–10 minutes. Repeat demonstrations will require either the drying out of the reaction vessel used for the first demonstration or spare dry reaction vessels.

Equipment

Apparatus

• Reaction vessel, 1 or more (note 1)
• Rubber stopper or plastic cap (to fit the reaction vessel)
• Beaker (250 cm³), 1 for each alcohol used
• Wooden splints, as needed (note 2)
• Metre rule

Apparatus notes

1. The reaction vessel consists of a large polycarbonate bottle, as used in workplace water dispensers. These have a volume of 16–20 dm³. A clean, dry bottle is required for each demonstration. It takes time to clean and dry once it has been used for a demonstration. For this reason, up to 4 bottles may be required. The bottle must be made of polycarbonate (marked PC) and of no other material. If using empty but wet water cooler containers, stand them inverted to allow any remaining water to drain and then leave upright for several days until completely dry.
2. Attach a wooden splint to the end of the meter rule or stick using adhesive tape, angling the splint so that when the meter rule is horizontal, the splint is sloping downwards. Provide a lighter or matches well away from the alcohol bottles.

Chemicals

One or more of the following alcohols, 40 cm³ of each one used:

• Methanol (HIGHLY FLAMMABLE, TOXIC)
• Ethanol (IDA, or Industrial Denatured Alcohol) (HIGHLY FLAMMABLE, HARMFUL)
• Propan-1-ol (HIGHLY FLAMMABLE, IRRITANT)
• Propan-2-ol (HIGHLY FLAMMABLE, IRRITANT)

Health, safety and technical notes

• Read our standard health and safety guidance
• Both demonstrator and class should be wearing eye protection. Select a safe, level place for the demonstration, with at least 2.5 m clearance above the top of the vessel to the ceiling above, and no flammable materials above it. If the laboratory bench does not allow for this, four stable laboratory stools supporting a large wooden tray may give sufficient clearance and stability. Set out the bottles containing the alcohols and the beakers at least 1 m away from the demonstration. No flames within 1 m. Students at least 4 m away.
• Methanol, CH₃OH(l), (HIGHLY FLAMMABLE, TOXIC) – see CLEAPSS Hazcard HC040b.
• Ethanol (IDA, Industrial Denatured Alcohol), CH₃CH₂OH(l), (HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040a. 
• Propan-1-ol, CH₃CH₂CH₂OH(l), (HIGHLY FLAMMABLE, IRRITANT) – see CLEAPSS Hazcard HC084a. 
• Propan-2-ol, CH₃CHOHCH₃(l), (HIGHLY FLAMMABLE, IRRITANT) – see CLEAPSS Hazcard HC084a. 

Procedure

1. Pour about 40 cm³ of the selected alcohol into a beaker and then transfer into the reaction vessel.
2. Insert the rubber stopper and roll the reaction vessel on its side for 10 seconds, to and fro, allowing the alcohol to vaporise and the vapour to fill the vessel. Do not warm the alcohol.
3. Pour surplus liquid alcohol back into the beaker, draining the vessel as completely as possible, and move the beaker back to the rest of the alcohol stock, away from any risk of catching fire. Surplus liquid left in the vessel may ignite and set fire to the vessel as well.
4. Stand the reaction vessel securely inside the safety screens and remove the stopper. Light the wooden splint, and apply the lighted end of the splint to the open neck of the vessel. Do not lean over the screens to apply the ignition. It is dangerous to ignite by dropping a lighted match into the vessel when using ethanol or methanol. For both propanols, this method may be used providing the neck of the bottle is above head height.
5. The alcohol vapour should ignite with a loud ‘whoosh’, with flames shooting out of the top of the vessel.

Teaching notes

The experiment demonstrates dramatically just how much chemical energy is released from such a small quantity of fuel.

The flame colour varies with the proportion of carbon in the alcohol molecule. With methanol and ethanol there is a very quick ‘whoosh’ sound and a blue flame shoots out of the bottle. With propan-1-ol and propan-2-ol, the sound is similar but the reaction is slightly slower, easier to observe, and blue and yellow flames may be observed ‘dancing’ in the bottle. The presence of water reduces the likelihood of dancing flames.