Candle Flames

Objective: To investigate the effect of gravity on the burning rate of candles. Science Standards: Science as Inquiry Physical Science - properties of objects & materials Unifying Concepts & Processes Change, Constancy, & Measurement Mathematics Standards:

Measurement Science Process Skills: Observing Communicating Measuring Collecting Data Inferring Hypothesizing Predicting Investigating

CONTENTS

Activity Management Assessment Candle Flames Candle Flames Worksheet MATERIALS AND TOOLS Birthday candles (2 per group) Matches 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.

Activity Management:
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.

Assessment:
Discuss student observations of the candle burning and their hypotheses. Collect the student work sheets for assessment.

Extensions: 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 rate changed. 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 list.)

Candle Flames

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 shape.

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