Objective: To investigate the effect of gravity on the burning
rate of candles.
Science as Inquiry
- properties of objects & materials
Unifying Concepts & Processes
Change, Constancy, & Measurement
Science Process Skills:
Candle Flames Worksheet
MATERIALS AND TOOLS
- Birthday candles (2 per group)
- Balance beam scale (0.1 gm or greater sensitivity)
- Clock with second hand or stopwatch
- Wire cutter/pliers
- Wire (florist or craft)
- 20 cm square of aluminum foil
- Eye protection
The burning rate and other properties of candle flames is investigated.
This activity serves as an introduction to the candle drop activity
that follows. In both activities, students are organized into cooperative
learning groups of three. It may be useful to keep the same groups together
for both activities.
The objective of this activity is to observe candle flame properties
and prepare students to make observations of candle flames in microgravity
where observing conditions are more difficult. Before letting students
start the activity, conduct a discussion on the different observations
they can make. Make a list of terms that can be used to describe flame
shape, size, color, and brightness.
At the end of the experirnent, student groups are asked to write a hypothesis
to explain the differences observed in the burning of the two candles.
It may be helpful to discuss hypothesis writing before they get to that
part. The hypotheses should relate to gravity-induced effects. In the
case of candle 2, the wax of the candle is above the flame. Convection
currents (a gravity-driven phenomenon) deliver lots of heat to the candle
which causes more rapid melting than occurs with candle 1. Much of that
wax quickly drips off the candle (gravity pulls the wax off) so more wick
is exposed and the candle burns faster.
The wire used in this activity is a lightweight wire of the kind used
by florists and in craft work. You can find this wire in craft and hardware
stores. Do not use wire with plastic insulation. The flame of the candle
tipped at an angle of 70 degrees may reach the wire and begin burning
the insulation. Each group will need two wires about 20 centimeters long.
Precut the aluminum foil into 20 centimeter squares. One square is needed
for each group.
Provide each group with one set of student sheets. Save the student reader
for use after the activity has been completed.
Discuss student observations of the candle burning and their hypotheses.
Collect the student work sheets for assessment.
- Burn a horizontally held candle for one minute. Weigh the candle
before lighting it. As it burns, record the colors, size, and
shape of the candle flame. Weigh the candle again and calculate
how much mass was lost.
- Repeat the above experiments with the candles inside a large
sealed jar. Let the candles burn to completion. Record the time
it takes each candle to burn. Determine how and why the burning
- Burn two candles which are close together. Record the burning
rate and weigh the candles. Is the burning rate faster or slower
than each candle alone? Why?
- Investigate convection currents with a convection current demonstration
apparatus that is obtained from science supply catalogs, or construct
the apparatus as shown below.
- Obtain a copy of Michael Faraday's book, The Chemical History
of a Candle, and do the experiments described. (See reference
Candles are useful for illustrating the complicated physical and chemical
processes that take place during combustion. The candle flame surface
itself is the place where fuel (wax vapor) and oxygen mix and burn at
high temperatures, radiating heat and light. Heat from the flame is conducted
down the wick and melts the wax at the wick base. The liquid wax rises
up the wick because of capillary action. As the liquid wax nears the flame,
the flame's heat causes it to vaporize. The vapors are drawn into the
flame where they ignite. The heat produced melts more wax, and so on.
Fresh oxygen from the surrounding air is drawn into the flame primarily
because of convection currents that are created by the released heat.
Hot gases produced during burning are less dense than the cooler surrounding
air. They rise upward and, in doing so, draw the surrounding air, containing
fresh oxygen, into the flame. Solid particles of soot, that form in the
region between the wick and flame, are also carried upward by the convection
currents. They ignite and form the bright yellow tip of the flame. The
upward flow of hot gases causes the flame to stretch out in a teardrop