Q: Where does the fire and sound in a whoosh bottle come from?

Based on some of the science demonstrations I’ve been a part of in the past, I think liking fire is an unspoken requirement to become a chemist.  It seems like chemists are always lighting stuff on fire.

These whoosh bottles in particular are quite popular for pyro-chemists and fire-loving crowds.  So how do these work?

The basics of this experiment rely on the combustion of alcohol within a confined space.  In combustion, fuel reacts with oxygen to produce heat and exhaust.  In the combustion of alcohol with oxygen, carbon dioxide and water are created.  Combustion reactions are used for generating electricity (through the burning of fuel) and aircraft and space propulsion, among other applications.

 

combust-diagram

Methanol:   2 CH3OH  +  3 O2  →  2 CO2  +  4 H2O

Ethanol:   CH3CH2OH  +  3 O2  →  2 CO2  +  3 H2O

Propanol:   2 CH3CH2CH2OH  +  9 O2  →  6 CO2  +  8 H2O

Isopropanol:   2 CH3CHOHCH3  +  9 O2  →  6 CO2  +  8 H2O

 

These reactions are exothermic and give off heat.  The heat not only produces the flames; it also causes the gas to expand.  The gas is forced out of the narrow top of the bottle, creating the “whoosh” sound.

Speaking of the bottle, demonstrators typically use large polycarbonate bottles.  Glass bottles may shatter, and broken glass is never something you want to deal with in these situations.  Polycarbonate bottles are able to resist the heat from the fast-burning fire, and are more likely to bend or crack than shatter.

Once the demonstrator has a bottle, they pour in a small amount of alcohol and cap the bottle.  The demonstrator may use methanol, ethanol, proponal, isopropanol, or a mixture of these alcohols depending on availability.  (The type of alcohol and concentration may give slight differences, but the overall effect remains the same.)  The liquid is swirled in the bottle to coat the inside, and the alcohol evaporates into the bottle’s air.  Excess liquid is poured out to prevent setting fire to the bottle.

At this point, an environment rich in alcohol and oxygen is ready for combustion.  To kick start the reaction, the demonstrator removes the cap and uses a long lighter to introduce a flame at the top of the open bottle.  The alcoholic air ignites, leading to flames and the whoosh sound.

Whoosh bottles are actually pretty easy to do, but I don’t recommend doing this unless you have some training and access to a proper space.  Fire is awesome, but safety is first!

Keep calm and science on.

Featured Image Credit

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