PHY ICS - The Institute of Physics in Ireland

The blue spiral flame
Austria
An introduction to ignition temperature/activation energy
Follow these steps
1 Clamp the tube in a spiral
(see figure).
2 Place the candle on the
heat-proof mat at the lower
end of the tube and light it.
3 Darken the room.
4 Drop about 2 ml of
propanone from the pipette
into the top end of the tube.
5 Stand clear and watch
what happens.
So what happened?
A bright-blue flame spirals up
the tube with a “whoosh”.
The propanone is a volatile
liquid, so it vaporizes and
mixes with the air as it flows
down the tube.
When the mixture reaches
the flame, the ignition
temperature is exceeded and
the propanone reacts with
the oxygen in the air. This is
an exothermic reaction, so it
spreads back through the
tube as a blue flame, which
winds its way to the top.
The air expands out of the
tube with an impressive
sound, which increases in
pitch as the length of the
resonating column of air
above the flame gets shorter.
You will need...
✔ 1–2 m of clear Perspex
tubing (5–10 cm wide)
✔ propanone (acetone)
✔ a glass pipette
✔ a night-light candle
✔ matches
✔ a heat-proof mat
What next?
If you want to do it again it
works best if you blow
through the tube first to
make sure that it is full of air
containing oxygen.
Find out more about
activation energy by looking
into how the diesel engine
works.
heat 23
Safety notes
Don’t be tempted to
use a plastic pipette to
put the propanone into
the tube. It’s a volatile
liquid so will tend to
vaporize with the heat
from your hand and
squirt out.
Don’t use too much
propanone or the liquid
will catch fire at the
end of the tube
Place the candle on the
mat. Excess propanone
may catch fire as it
drips out of the tube
but will burn safely on
the mat.
PHYSICS ON STAGE 3