(10) Faraday's Candle

From the Sourcebook for Teaching Science (Herr)

Introduction

Michael Faraday (1791-1867) was a British scientist who invented the first electric motor and dynamo, demonstrated the relationship between electricity and chemical bonding, and discovered the effect of magnetism on light. Faraday was not only a brilliant scientist, but also a well-known educator who brought science to the public through lectures he delivered each Christmas season at the Royal Society in London.

Faraday’s Christmas Lectures were popular because he illustrated concepts with numerous hands-on activities and experiments. Faraday knew the importance of observation in science and began his most famous lecture series by asking his audience to record as many observations as possible about a burning candle. Years later, science teachers continue to use Faraday’s activity to encourage the development of observation skills. Douglas Osheroff, the 1997 Nobel prize winner in physics (for discovery of the superfluid phases of ³H) reflected on the importance of this activity in his own intellectual development: “I remember quite well one class assignment: to record our own observations of a burning candle. I knew pretty well how a candle worked, and simply wrote down an explanation of how radiant heat from the flame melted the wax, which was then drawn up into the wick by capillary action, etc. Mr. Hock read my explanation, and then came to me and pointed out that what I had written could not possibly have been drawn from my own observations.” Osheroff had not made observations as requested, but had relied on his prior knowledge to explain what he was seeing. Mr. Hock’s comments helped Osheroff distinguish observation from inference, and this distinction ultimately helped him in his career as a scientist.

Observations of a Candle Flame

Record all observations on the data sheet (copy). Place your initials in parentheses following each set of observations. (sample)

Materials: small candle, matches, tongs, beaker, funnel, clay or putty, bromthymol blue or phenol red indicator, test tube clamp, safety glasses, dull butter knife, lamp oil (optional).

The purpose of this activity is to record as many observations of a candle as possible. Record your observations in a laboratory notebook or worksheet. Firmly plant a candle in a small clump of clay. Using beaker tongs, suspend a clean funnel over the flame and record your observations. Repeat the procedure with a funnel on which there are drops of the pH indicator methylene blue or phenol red. Does the indicator change color?

Put on safety goggles and light the candle and record all observations . Using beaker tongs, suspend a cooled beaker over the flame. What observations can you make about the inside of the beaker? Repeat the procedure with a funnel in which there are drops of bromthymol blue (figure 5.6E).

Place a dry, clean beaker over the flame and make observations as the flame is extinguished. Repeat the procedure with different sizes of glass beakers. Is there any correlation between the size of the glass beaker and the time it takes to extinguish the flame? Remove the beaker, and re-light the candle by placing a match in the smoke near the wick (figure 5.6G). Is it possible to re-light the candle simply by moving the flame into the smoke?

Using a dull knife, cut the wick free from the candle and place one end of the wick in a dish of water. Light the other end of the wick (figure 5.6H). What do you observe? Dry the wick off and place it in a dish of lamp oil and re-light it. What do you observe?

Developing Scientifically Oriented Questions (SEP-1)

In his introduction to the classic candle activity, Michael Faraday wrote: “We come here to be philosophers; and I hope you will always remember that whenever results happen, especially if it be new, you should say, 'What is the cause? Why does it occur?' and you in the course of time will find out the answer.” Write down as many questions as you can based upon the observations you made in activity 5.2.1. For example, “What is wax? What is the wick? Why is the wax soft? Why does the wick turn black? Why does the wax melt? Why does the smoke ignite? What is the substance that appears on the glass above the flame? Why does the indicator turn color when placed over the candle? Do all candles burn the same?...”

• • Write you own scientifically-oriented questions and submit them in the data table

Constructing Explanations (SEP-6) & Arguing from Evidence (SEP-7)

• Explaining the phenomena - Apply scientific ideas, principles, and evidence to provide an explanation for each of the following phenomena

• Scientific Principles - On "scientific principles" tab, describe how the observations from this activity can lead into discussions of scientific principles

• Environmental Issues - On the "environmental issues" tab discuss how combustion is related to various environmental issues.

• As a class, discuss the explanations. Each student should argue from evidence.

• Energy Balance Calculations

NGSS-based 3-course Model

• Chemistry in the Earth System