Burning Questions About a Candle: Teacher's Notes
Activity: Observations of a Burning Candle
Safety: First and Foremost
- If you choose to do any of the testing as a demonstration or as hands-on activities, it's essential to follow all safety precautions to the utmost. Students may need to be reminded that burns are painful and can be disfiguring, and trying any of these experiments (especially the cellulose nitrate, even as commercial flash paper sold in magic stores) on a larger scale can be fatal. It is our hope that in providing video footage of the more dangerous demonstrations, the need for individuals to do these in person will be nil, and thus students will be able to see the chemistry and learn from it without any risk whatsoever.
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- Wear safety goggles at all times.
- Provide a safety shield for protection of the students and of you.
- Perform all experiments and demonstrations in a well-ventilated area, in open, still air. (That is, if you do this outdoors, a breezy day can be unpredictable and hazardous.)
- Never ignite anything in a sealed or closed container.
- To intoduce the students to the observations and critical thinking
- To have the students develop a hypothesis and to develop an experiment to test the hypothesis
National Science Education Standards: Content Standards
This activity fulfills the following within the Content Standards: 9-12
- Content Standard A: Science as Inquiry
Safety must be an integral component of their investigations.
- Content Standard B: Physical Science
- Structure and Properties of Matter
- Chemical Reactions
- Lighter or Matches
- Notebooks or Paper for Each Student to Record Data
- Writing Utensil
- Small pan or non-flammable surface to collect dripping wax
Observations to make before you light the candle
Answer the following questions concisely. That is, use short sentences or sentence fragments, and only say what really needs to be recorded. You may want to work in pairs or small groups, each writing down observations, then periodically comparing notes and asking each other questions. If you do it this way, just remember: don't make critical comments! Be critical in your thinking, in the sense of always asking yourself "What does that mean anyway?" and not in the sense of "That's clearly wrong" or "What a dumb observation." Be positive in your approach, focus on the candle, and keep asking yourself questions in your own mind, especially questions that lead to further observations and more questions.
Give a physical description of the candle and its components. What are the individual parts and what are they made of?
How is the candle shaped? Why?
At what angle do you plan to burn the candle? Why?
Do some physical "tests" on the candle using what you have on you or laying around the room. What do these tests tell you about its properties?
Do any of these tests give you an indication of the nature of the candle components at the molecular level? What do you know about the chemical nature and/or composition of these components?
Hypothesize what will happen when you light the candle and start it burning
Write down what you think will happen in terms of the observations you made above, especially with respect to the molecular level. Start a new page and be complete but concise.
Now, light the candle and watch what happens
In your notebook, record all the observations you can make regarding the lighting and burning of the candle. For this first set of observations, don't worry about asking questions so much as just making as many quick notes as you can. You should plan on spending a fair amount of time on this (more than a minute and less than an hour or two).
It is important that you record your observations as quickly as you can and then come back to some of them later for further thought and extended observation.
As you look at the candle, try to separate the components into the individual elements. The flame is different from the vapor which is different from the wick and the solid and liquid wax.
There are processes in dynamic equilibrium occurring during the burning process. Think about these processes and ask yourself questions. There are physical changes occurring as well as chemical reactions. Try to imagine what these are, list them, and ask questions about them.
Your "observations" should also contain conjecture and hypotheses about what is occurring or why it is occurring. It is crucial that you have some of these extensions of the observation process recorded.
Try to make at least 10 distinct and separate observations initially. (Be aware that individual observations numbering over 100 have been recorded by others.)
It may take you a while to begin to think about what is happening. Be patient and work at the process. Once you begin to see how to ask questions related to what you are seeing, a snowball effect will occur that should open up whole new areas of observation for your critical consideration. Be sure to write down all of your thoughts and observations as they occur. Don't try to be critical of the material you write down, but simply put it down as quickly as possible. You will have plenty of opportunity later for evaluation and grouping of your data.
Take one observation that you recorded above and:
- Break it down into macroscopic versus molecular levels.
- See if you can coordinate what is happening between what you've physically observed (that is, the macroscopic level) and the molecular level.
- Most important, speculate on what is actually happening at each of these levels.
- Ask yourself, for both the macroscopic and molecular levels, why the behavior that you observed took place.
- What was the molecular property involved?
What changes in that property occurred during the burning process? Go into as much detail as you can.
Now, pick another observation or two and carry out the same level of in-depth analysis. Try to relate the observations you made above to those that you make in this section.
You should find that as you practice this process of observation and recording your thoughts, you become better at asking questions about what is happening, and then writing down answers. This iteration is part of "doing science."
Repeat this process several times: this repetition is also part of doing science.
Now try to "watch" yourself observing and asking yourself questions:
How can you do this better, with conscious awareness?
Can you see possible ways of getting the answers while you are asking the questions?
Test Your Hypothesis by Doing an Experiment
After you feel like you have made enough observations to give you a pretty good understanding of what is actually happening during the burning process, it is time to do an experiment. An experiment results when you ask a question and then develop a method to answer that question. The experiment itself is actually the process of getting the answer.
So, here is what you need to do:
First, try to ask a good question. Think about the observations that you made, especially those that you don't really understand, and try to formulate some kind of hypothesis (or good guess) about what is happening.
Once you have a good question, try to think of ways that you can answer that question. What kind of measurements or observations (either with your own eyes or with other methods) would help you get the answer? Think about the kind of data you'll collect - will your data be qualitative (that is, descriptive) or quantitative (with numerical measurements). What do you have available in the lab that you could use to measure changes that occur?
Once you have some idea of how to actually do the experiment, go ahead and do it.
Safety First! Make sure that you consider safety at every step. (Note: NEVER put a thermometer into the flame. The high temperature of the flame will break most thermometers.)
As you design your experiment, consider how you will present your results. A good experiment is reproducible, and so you'll need to outline your steps (procedure) very specifically. Don't get bogged down in making it perfect the first time around, just make sure that you take clear and specific notes.
Be creative - you should be able to design experiments that you can do with what you can find in the lab or that you can easily acquire with very little effort. For example, holding a piece of glass over the candle as it burns (probably at an angle would work better) can help you answer questions about what is evolved or given off during the combustion process. Think of other simple ways of investigating further what is going on and see what kind of good experiments you can come up with.
Attached here is a printable form that has most of the questions and directions below, but with space for the students to write in their answers and observations while you work.
Also, check out these references that discuss classic candle experiments:
"The Persistence of the Candle-and-Cylinder Misconception," by James P. Birk and Anton E. Lawson, Journal of Chemical Education, 1999, 76, 914-916.
This article discusses why a candle under an inverted beaker in a container of water does not indicate the percent of oxygen in the air. The following is an excerpt from this article:
"In conclusion, in every experiment that purports to measure 21% oxygen in air by a combustion process, combustion is not consuming all the oxygen. Rather, the heating of air results in partial expulsion of air. The flame is extinguished by local depletion of oxygen and buildup of carbon dioxide, not by complete combustion of oxygen in the remaining air. When the flame expires, the apparatus and enclosed air cool back to room temperature, causing a pressure differential that results in a rise in the water level."
"'Experiment with a Candle' without a Candle," by Dusan Krnel and Sasa A. Glazar,
Journal of Chemical Education, 2001, 78, 914.
This article is a follow-up to the previous article, and describes a way to prove the same point using a Bunsen burner.
Other articles that might be of interest:
"A new use for the candle and tumbler myth," by Gavin D. Peckham, J. Chem. Educ. ,1993 70 1008.
"Demonstration of the burning of a candle," by Isidor S. Hirschhorn, J. Chem. Educ. ,1941, 18, 107.
"Evolution of the candle," by G. Griffin Lewis, J. Chem. Educ. , 1934, 11, 367.