Combustion and a Jumping Flame - NSTA
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DISEQUILIBRIUM
Combustion
and a Jumping Flame
BY COLE ENTRESS
| FIGURE 1: After burning,
T
hough the candle flame is a In this Disequilibrium column,
familiar example of a com- the bottom of a candle’s we will use candles to explore
bustion reaction, neither matter and chemical reactions.
exposed wick remain un-
candles nor combustion are well The ideas explored in this Pre-
burnt (white arrow).
understood by most people (find dict-Observe-Explain (POE) les-
out by asking someone to explain This is because it is the son (in the style of Haysom and
why the bottom of the exposed evaporated wax of the Bowen 2010) are pertinent to the
wick of a partially burnt candle candle (not the wick) that teaching of MS-PS1-2 (Nation-
is unburnt; see Figure 1). Driver primarily participates in the al Research Council 2013) and
et al. (1994/2015) reported that combustion reaction. The might serve as an introduction
students generally did not under- cotton primarily exists to to combustion for students in the
stand combustion to be a chemi- “wick” melting fuel to the middle of a unit on chemical re-
cal reaction between oxygen and site of the reaction. The wick actions.
another substance. Students can, does burn to get the reaction
of course, often identify that air started, of course, and the
is needed for things to burn,
Background
top portion will be burned
but only rarely can they explain away as the wick becomes The chemistry and physics un-
how it functions in combustion. longer than needed to derlying the burning of a candle
Indeed, as many middle-level vaporize more fuel. is complex and fascinating; inter-
students are still developing an ested readers should peruse Mi-
atomic model of matter, many chael Faraday’s seminal 1860 lec-
aspects of combustion can be ture (see Resources) or the (dated
confusing, from the conservation but still good) Scientific American
of mass to whether the wax in a essay on the subject by Walker
candle burns or simply evapo- (1978). In a somewhat simpli-
rates (Driver et al. 1994/2015). fied description of the candle’s
(Note: Einstein showed us that combustion, we first add heat by
mass is not perfectly conserved. lighting the wick, usually made
However, mass is close enough to of specially treated cotton. The
being conserved that our measur- burning wick melts the hydrocar-
ing devices can’t detect the differ- bon wax near it, and that liquid
ence.) is drawn up the wick through
64
capillary action. This wax is va- What’s really happening when a dle wick. Ask students to record
porized because of the high tem- candle burns? What does burn- their predictions about the con-
perature near the top of the wick, ing mean? What does a candle ditions in which the relighting
and the vaporized hydrocarbon need to keep burning—and what would be successful. Facilitate a
reacts with oxygen, producing makes it stop? (Note: Following short discussion about students’
(through a complex series of the template of Haysom and predictions, asking each student
combustion reactions) water va- Bowen (2010), a printable work- to support their predictions with
por, carbon dioxide, and energy sheet for this activity has been evidence and reasoning.
(of both the thermal and lumines- developed for teacher use. See
cent varieties). The heat sustains Online Supplemental Materials.)
the reaction by melting more hy-
drocarbon fuel and by creating Predict Materials
a convection current that draws
Tell students that you’re going • candle
fresh air, including oxygen, up
to perform a demonstration us-
to the base of the flame. In this • long-stemmed lighter
ing the candle (adapted from
way, thermal energy, oxygen, and
Ford and Grundmeier 1993). You • drinking straw
fuel—often called the combus-
are going to blow out the candle
tion triangle—are supplied to the • (optional) chimney or
and then try to relight it with the
candle, producing a continuing other wind-screening
flame at various positions: on the
(or chain) combustion reaction. device, if your room is
candle wick, to the side of the
The overall combustion of drafty.
candle wick, and above the can-
candle wax can be represented
by a chemical reaction (see com-
bustion reaction in Online Sup-
plemental Materials), in which
atoms are rearranged into new Implementation notes
compounds with properties dif-
Relighting a candle without touching it requires you to ignite the
ferent from those of the starting
“smoke” (still-evaporating wax) that rises from the candle after it is
materials. The lesson that follows
snuffed out. Putting in a few minutes of practice the day before per-
will work best when students un-
forming the demonstration for students can greatly improve your tech-
derstand what a chemical reac-
nique. In addition, the following conditions also increase the chance of
tion is.
success:
Introduction • The candle needs to burn for a few minutes before you begin your
trials; you want plenty of hot hydrocarbons at the base of the wick.
Wearing safety goggles, use a
long-stemmed lighter to light a • Do not trim the candle’s wick. In fact, slightly longer wicks are
candle on a clear demonstration better.
table at the front of the room (a • Blow the candle out precisely. I find that blowing through a
standard taper candle, as shown standard straw allows for a brief, directed puff of air.
in Figure 1, works fine). Ask stu-
dents to share their experiences • Know the airflow in your room. A drafty room can blow the smoke
with candles. Solicit a few gen- aside. If your room is very drafty, you might consider putting a
eral responses—as well as any clear windscreen around part of the candle. (Ford and Grundmeier
observations students can make 1993 suggest a glass chimney.)
at a safe distance—and then ask:
April/May 2020 65Observe
the final, counterintuitive outcome. and try to make sense of what’s
Have students observe as you
Their fascination can be deepened happening on the atomic level.
do the demonstration. After each
if you take a slow-motion video of Students are unlikely to “rein-
trial, have students record their
the relighting (see Resources for an vent” all of what we know about
observations while you relight
example) because they can watch combustion reactions, but they
the candle, giving the flame a few
the flame travel “down” the candle can do a lot of sophisticated sense-
seconds between trials to stabi-
smoke and back to the wick. This making. You might ask, for in-
lize. The trials give the following
demonstration takes a little bit of stance, for small groups to create
results (see Figure 2):
practice to get right (see Imple- diagrams on a dry-erase board of
• Directly touching the lighter mentation Notes). NEVER try a the moment the flame jumps, la-
flame to the wick—candle demonstration for the first time in beling any molecules they think
relights front of students. are important.
Eventually, you will want to
• Candle flame held to the side
introduce the notion of a combus-
of the wick—candle does not Explain tion reaction, describing the role
relight
Ask students to try explaining of fuel, oxygen, and thermal ener-
• Candle flame held a few these results. Use a Think-Write- gy. Direct students to revise their
inches above the wick— Pair-Share strategy to surface diagrams so that they explain
candle relights. some of students’ changing ideas. the “jumping” flame in terms of
Press students to incorporate the a combustion reaction, including
Students almost never predict principles of chemical reactions the products and reactants.
| FIGURE 2: By placing a lighter flame into the plume of smoke (evaporated hydrocarbons) com-
ing from a recently extinguished candle, a flame can be made to “jump” down the column of
smoke and relight the candle (full video of this process is available in Resources).
66DISEQUILIBRIUM
Extension ing project with the goal of de-
Now, pose the following question:
Why does a lit candle go out when
signing long-burning candles at
low cost. Advanced students may
Safety notes
I blow on it? Frequently, students enjoy reporting on the difference Candle flames and lighters
will inaccurately claim that it is between candle flames (which present a clear burn risk and
due to the carbon dioxide that we are called diffusion flames) and should be handled only by the
breathe out. However, exhaled air the solid blue, so-called premixed teacher. Flames should never
is only about 5% carbon dioxide flames we see in gas stovetops. be left unattended. Work in a
and nearly 15% oxygen, making clean area with no flammable
this explanation insufficient. Stu- Conclusion objects nearby to minimize
dents might also posit that your the risk of fire and ensure your
With a few low-cost, everyday
breath acts as a chilling wind that room meets all fire-prevention
materials, students can be in-
extinguishes the flame by lower- standards. No part of the ac-
spired to think deeply about the
ing the temperature. If this were tivity should be ingested.
familiar phenomenon of a can-
true, however, it would not be
dle’s flame. An understanding of
possible to relight the candle at a
candles, and combustion more
distance, as in the demonstration.
generally, can provide insight into
In fact, the puff of exhaled air
everything from car engines to cli- Walker, J. 1978. The physics and
moves the vaporized wax away
from the hot area of the wick, in-
mate change! • chemistry underlying the infinite
charm of a candle flame. Scientific
terrupting the chain reaction and American 238 (4): 154–162.
stopping the burning. REFERENCES
Driver, R., P. Rushworth, A. Squires, and RESOURCES
Michael Faraday’s lecture for children
Follow-up V. Wood-Robinson. (1994) 2015.
on the chemical history of a candle—
Making sense of secondary science:
This lesson can be extended in Research into children’s ideas www.bartleby.com/30/7.html
many ways. Additional demon- (Routledge Education Classic Edition Royal Society of Chemistry’s jumping
Series). London: Routledge. candle flame activity—www.rigb.
strations (see e.g., Walker 1978)
Ford, L., and E.W. Grundmeier. 1993. org/families/experimental/candle-
can illustrate other aspects of
Chemical magic. 2nd ed. Mineola, chemistry
the combustion triangle or help NY: Dover Publications. Slow-motion video of a candle
students isolate the products of Haysom, J., and M. Bowen. relighting—https://photos.app.goo.
a combustion reaction. Alterna- 2010. Predict, observe, explain: gl/5sBKqhp4wBucxkDb8
tively, if you want your students Activities enhancing scientific
to think about reaction rates, you understanding. Arlington, VA:
might ask students to experiment
National Science Teachers ONLINE SUPPLEMENTAL
with how candle diameter, wick
Association. MATERIALS
National Research Council. 2013. Next Combustion reaction, shown with a
material, or composition affects Generation Science Standards: For simple hydrocarbon—www.nsta.org/
the rate of combustion (see Re- states, by states. Washington, DC: scope2004
sources). Such an investigation National Academies Press. https:// Worksheet for teachers—www.nsta.org/
could be turned into an engineer- doi.org/10.17226/18290. scope2004
Cole Entress (cole.entress@gmail.com) is a doctoral student in science education at Teachers College, Columbia University in
New York City.
April/May 2020 67You can also read
