December 2002
Tips and techniques for creative teaching
Inherent in the first months of school
are opportunities to connect with students and set a comfortable tone in the
science classroom. This lesson plan integrates several scientific disciplines
and can be used in a range of science
courses. The activity focuses on what
occurs as a candle burns (for example,
rapid oxidation, chemical changes, and
physical changes). Students see
candles all the time, but very few
know how a candle actually works.
To get their attention, I light a
candle as students enter the room and
proclaim, “we are here to learn, and
when we burn, we learn! Science is
about making sense of the world, so
let’s see if we can figure out how something as simple as a candle works. Who
knows what’s burning in a candle?”
Students commonly answer that the
wick is burning. To demonstrate to
students that the wick is not the fuel for
the candle’s flame, teachers can light
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T h e S c i e n c e Te a c h e r
the bottom end of a 30 cm length of
string hanging from a ring stand (Figure 1). The flame rapidly moves up the
string and runs out of fuel in mere seconds. This shows that the wick is not
what’s burning in a candle’s flame.
Other students guess that wax, the
other primary material in a candle, is
what is burning. By holding a burning match next to the solid wax of a
candle, the teacher shows that neither
the solid or liquid wax burns (Figure
2, page 55). In a burning candle, liquid wax is drawn up the wick by capillary action (also known as capillarity). When the wax molecules get
close enough to the flame, they are
heated and vaporize. These vapors
(hot gases) are the fuel for the
candle’s flame.
To demonstrate that hot gases combust to fuel the candle’s flame, the
teacher should light a second candle,
blow out the first candle, and quickly
bring the flame of the second candle
into the smoke rising from
the extinguished wick
(within 2–3 cm of the wick).
The vapors ignite and relight the wick of the first
candle (Figure 3, page 55).
After the demonstration, students can begin experimenting. Students should be cautioned about the dangers of
clothing, hair, and other
combustibles near an open
flame. Students must wear
lab aprons and goggles in
case wax splatters.
After student groups are
given petri dishes containing
birthday candles held in place
with clay, they use two candles
to experiment with variations
of the teacher demonstration
by putting a burning wick at
FIGURE 1
Wick demonstration.
The wick is not the fuel for the candle’s
flame. To demonstrate this, teachers can
light the bottom end of a 30 cm length
of string hanging from a ring stand. The
flame extinguishes in mere seconds.
BILL NIXON
Tricks with Wicks
different distances along the trail of
smoke from an extinguished candle. A
“master candle” should be kept burning
to eliminate the need for lighting multiple matches. After students have ample
time to explore this phenomenon, the
teacher should lead a brainstorming discussion on how a candle works by summarizing that:
1. First the candlewick is lighted.
As it burns, it melts the wax at
its base.
2. This liquid wax is drawn up the
wick by capillary action (in the
same way water is drawn up a
paper towel).
3. When the liquid wax gets closer
to the flame, it becomes so hot it
vaporizes. These vapors are
what are burning in the
candle’s flame—these hot gases
are the fuel.
FIGURE 2
FIGURE 3
Wax demonstration.
Vapors demonstration.
Students assume that wax makes a candle
burn. To demonstrate this is not the case,
teachers can hold a burning match next
to the solid wax of a candle and show
that neither the solid or liquid wax burns.
To demonstrate that hot gases combust
to fuel the candle’s flame, teachers can
light a second candle, blow out the first
candle, and bring the flame of the second
candle into the smoke rising from the
extinguished wick. The vapors ignite and
relight the first candle.
The end of the year also provides
great opportunities to make lasting
impressions on students. Because we
started the year with burning candles,
I like to end the year the same way to
come full circle. Students are given a
small-diameter candle stuck in clay
in a petri dish. After adding 20 mL of
water to the dish, students are instructed to quickly place an inverted
test tube over the burning candle and
into the water. When the flame goes
out, water will go about one-fifth of
the way up into the inverted tube.
Fifty years ago, people claimed this
was proof that air is 20% oxygen, and
this misconception is still seen in books
today. (Editor’s Note: Neil Glickstein
addressed this misconception in his article “Seeing Isn’t Always Believing,”
which appeared in the October 2002 issue of The Science Teacher.)
However, if the water molecules
were actually taking the place of con-
sumed oxygen molecules (ignoring the
conservation of mass in a chemical reaction and in a closed system), the water
level would rise steadily as the candle
burned until it extinguished. That is not
the case. The air inside the tube is rapidly heated by the candle’s flame. Only
when the flame goes out due to lack of
oxygen and carbon dioxide build up
does the air inside the tube cools rapidly.
The teacher should explain these
ideas to the students after they have a
few minutes to experiment with this
phenomenon. They will want to do
the experiment over and over again,
so the teacher should again keep a
“master candle” on the front table to
relight the group candles. Once
again, students see that careful observations and careful thinking help to
make sense of the world.
Bill Nixon is a science teacher at
Renbrook School, 2865 Albany Avenue,
West Hartford, CT 06117; e-mail:
[email protected].
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