Observing a Candle

Objective

In this lab you will light a candle and perform various tests to discover some things about burning and about candles. You will learn the practice of scientific observation. Observation is not the same as seeing. If five people see the same movie and then are each asked to tell about it you will hear five different stories. Some of the people are more observant than others or have better memories. You can make your memory better by carefully writing down your observations and you can become more observant by practicing. In fact, start by watching this video together as a class: https://www.youtube.com/watch?v=IGQmdoK_ZfY.

When you complete your work on this lab you will be able to do the following:

1. Explain, based on your own observations, how we know the candle needs fresh air to burn.
2. Explain, based on your own observations, why a glass held over a burning candle collects condensation on its inside.
3. Quantify how a candle changes as it burns based either on changes in mass, length, or both.
4. Describe, based on your own observation, the molecular make-up of the parts of a flame.
5. Explain, based on your own observations, why the ‘smoke’ of a recently extinguished candle is flammable.
6. Write a complete story for what happens when a candle burns. You will account for all of the above and:
   1. the process of igniting the wax.
   2. how wax is transformed into a vapor and then into soot.
   3. how a candle produces light.
   4. how wax is broken down, combines with oxygen, and ultimately forms carbon dioxide and water.
   5. why the mass of the candle decreases as it burns.

Materials

Background

You will be observing a candle and what happens when you light one. To understand what you are observing a little background would help. We need to know what candles are made of. Typically, a candle is made of paraffin wax, a kind of hydrocarbon derived from petroleum. Hydrocarbons are compounds of the elements hydrogen and carbon. Other hydrocarbons you may have heard of are gasoline, kerosene, diesel fuel, propane, and butane. Some candles are made of beeswax, which bees make from the materials they collect from flowers. But whether paraffin or beeswax, a candle is made of large molecules with long chains of carbon atoms.

We also need to know about the about the air we breathe. Air is mostly nitrogen gas (chemical formula, N₂) at 78%. Oxygen gas (O₂) makes up 21% of air. Nitrogen gas alone would put out a fire but pure oxygen gas would make it burn extremely hot and fast. A fire that depends on oxygen from the air requires that oxygen makes up at least 17% of the air. Anything less than that and the fire will go out.

As a fire burns it makes other gases: carbon dioxide (CO₂) and water vapor (H₂O, but you knew that). These gases make sense because the hydrocarbon, made of carbon and hydrogen, has been broken to bits and combined with oxygen. Chemical reactions are true transformations. The materials you start with are taken apart, the atoms are rearranged, and new substances form. Think of a molecule like a LEGO® model. It’s made of bricks (the atoms) that can be taken apart and put together in different ways. The carbon bricks combine with two of the oxygen bricks to make carbon dioxide and two of the hydrogen bricks combine with an oxygen brick to make water. You take apart one model and combine it with another to make different substances. In chemistry we use symbols and equations to make it absolutely clear what we are talking about. For example, here is a chemical equation for the burning, or combustion, of methane (CH₄). Methane is the simplest hydrocarbon:

The equation shows that for each molecule of methane that is consumed, two molecules of oxygen are used up. In turn, one molecule of carbon dioxide and two molecules of water are made per molecule of methane. Try drawing a picture of these molecules to see how this works. Your pictures need to have different colors for different kinds of atoms and you need to connect them together, like LEGO® bricks) to make molecules. Notice that even though they are rearranged, the same number of carbon atoms, hydrogen atoms, and oxygen atoms can be found both before and after the chemical change.

Now it would be helpful to write a similar equation to describe how a candle burns. Unfortunately, it is hard to write an absolutely correct equation for the burning of candle wax. This is because the wax is not just one compound. Wax is made up of a variety of compounds with varying numbers of carbon atoms, usually somewhere from 20 - 40 carbon atoms. And not all of them are simple long chains: some branch like twigs and others like trees. Because of this, a generic formula for wax is used: C_nH_2n+2. This formula specifies that only carbon and hydrogen are part of the compound and that however many carbon atoms there are, the number of hydrogen atoms is two times the number of carbon atoms, plus two. Since we do not know the exact formula it just means we can’t easily calculate the number of oxygen, carbon dioxide, or water molecules involved in burning wax. For now, this is not a problem because in this lab you will be exploring things you can see with your own eyes. And though you will eventually learn to count them, molecules are too small to see.

Safety

Tie back loose hair and clothing. In order to participate in the lab you must wear long pants without holes and closed-toe, closed-heel shoes. Wear safety goggles throughout the activity; doing so helps you to get used to how they feel and keeping them in place will help you to stop noticing they are there.

Grading

Write a formal lab report. Each individual student must write her/his own report. You will turn your report in digitally on the Classroom page. There is a sample lab report you can use to help you to write yours. It is a Google Doc and can be accessed from the online version of this lab handout.

1. Your Introduction should give the scientific background of the lab and should cover the nature of combustion reactions and how a burning candle is an example of a combustion reaction. You must include a chemical equation illustrating combustion in general.
2. Your Procedure should give a brief description of the actions you took in the lab along with the observations you made. The discussion questions included with each objective should guide what you write about in your Procedure. Include photos as appropriate to illustrate your observations. Your Procedure must include photos to illustrate your observations. If you do not have appropriate photos then they can be provided for you by your teacher.
3. Include a Data Table which gives the quantitative information you collected about the candle. For example, length, diameter, mass, change in mass, etc.
4. Your Analysis section must follow the guide in the instructions for Objective Six in paragraph form, not question and answer format. See the end of the lab handout for this guide. Write at least one paragraph per question. Use objective six to guide what you write about in the Analysis.
5. Your Conclusion section should reflect on the experience of carrying out the lab. What did you learn, what surprised you? How effective was the lab as an educational experience?

Procedure

For each objective follow the guidelines given below to invent your own procedure to investigate each of the objective questions. Each student must record their own observations in their own notebook.

Objective One

Observations of the Candle

1. If you have a lab balance, weigh the candle and record its mass to the nearest 0.1 or 0.01 grams.
2. Use a ruler to measure the length of the candle in centimeters to the nearest 0.1 cm. Also, measure its diameter and the length of the wick. Record these data in a table that you create for the purpose. Discuss with your classmates, lab partner, or teacher what this table should look like.
3. Write down a description of the candle: what color is it? Is it scented? Is it new or old? Is it smooth, carved, or covered with hardened dripping wax? Be thoughtful and try to observe as much as you can.
4. Take a picture of your candle to include in your report. Try to take a picture that puts the attention on the candle and excludes a distracting background. Selfies are nice but this is for science.
5. Get ready to light the candle. When everyone in your group is focused on the wick, light the very topmost end of the wick with a lighter or match. Watch very, very closely. First the end of the wick burns, then more and more of it, and… Try to get every detail of what happens written down. Discuss it with your lab partner or group.
6. Try to think of the photographs you would need to illustrate the process of lighting a candle. Discuss this with your group. Take some appropriate photos.

Candles and Fresh Air

1. Relight your candle if it has gone out. Obtain a large beaker (a 400 or 600 mL beaker is best).
2. Hold the beaker upside-down and place it over the candle flame so that the bottom of the beaker is about 2 cm below the top of the wax. Hold it there and watch the flame carefully.
3. If you are holding it far enough down then eventually the flame will go out. Turn over your beaker and blow into it to clear it out. Do the experiment again but this time use a stopwatch (or app) to see how long it takes for the flame to go out. Repeat the timing process for a total of three times so you can get an average time. Calculate the average time and record all of these numbers in a data table. Discuss what this table should look like with your group.
4. Now, repeat the experiment but hold the beaker farther down so that the flame gets close to the upside-down bottom of the glass container without touching it. Time how long it takes for the flame to go out, being sure to repeat your experiment so that you can get an average time. Record your data in the same data table.
5. Take a photo of the candle right after it goes out. There is a dramatic stream of ‘smoke’ that might be worth trying to capture.
6. When you finish your measurements relight your candle and leave it burning. Discuss these questions with your lab group and write down your own answers:
   1. Why does the candle go out despite the fact that it is not sealed off completely from the surrouding air?
   2. Why is the time it takes for the candle to go out different in your two experiments?
   3. Are your results consistent with the hypothesis that the candle fills up the beaker with gases lacking in fresh air as it burns? What evidence do you have for this?

Objective Two

Condensation

1. Think back to the last objective. What did you observe collecting on the glass as you performed your repeated experiments?
2. Light your candle, if you have let it go out, and hold your beaker over the burning flame again. Look more closely at the fog on the glass. Is it on the inside or the outside? Does it feel waxy or wet?
3. Take photos to document what you observe. Use the questions below to help you figure out what to photograph.
4. Discuss these questions with your lab group and write down your own answers:
   1. Why is there a clear circle of glass with no fog on it at the upside-down bottom of your beaker?
   2. Why does this area grow in size the longer you hold it there?
   3. If the fog on the glass is water, where did it come from? Refer back to the introduction of the lab for some help if you need it.
   4. Water vapor has mass. Where did the mass of the condensation on the glass come from?

Objective Three

Candles Shrink

Many people think candles shrink because the wax melts and flows away.

1. If you have a lab balance, weigh your candle again and record its mass.
2. Measure the length of your candle in centimeters to the nearest 0.1 cm.
3. Discuss these questions with your lab group and write down your own answers:
   1. If you have a lab balance, describe what happened to the mass of the candle.
   2. Compare the length of the candle at the beginning of your experiments to its length now.
   3. Explain why the mass and the length of the candle decreased. Some wax is missing, where did it go?

Objective Four

The Parts of a Flame

You will use a watch glass to investigate the parts of a candle flame. Be careful not to touch any glass that has been in or close to the flame as it will be very hot!

1. While holding one edge, place the opposite edge of the watch glass into the clear part of the flame. Be careful not to touch the pool of liquid wax just underneath it! Hold it in the flame for 30 - 60 sec. Be careful not to let the glass touch the bright yellow part of the flame.
2. Note what happens to the flame as a whole when you do this.
3. Pull the glass out of the flame and let it cool. Look closely at the glass and write down observations of what you see. For this part of the procedure there should be no black material. What is the substance that has collected on the glass?
4. Allow it to cool slowly and watch what happens. Once it is cool enough to touch, touch it and observe what you think it is.
5. Next, put your glass into the bright part of the flame. The flame will flicker and smoke and a black material should collect on the glass.
6. Once you are satisfied with the amount of material you have collected, pull the glass out of the flame. Let it cool. Once it is cool you may touch the material. Describe it in your written record of this lab.
7. Take some pictures you can use to illustrate your report. Take a photo that shows the parts of a flame, which you can add labels to later. Take a picture of your evidence for what is found in each part of the flame.
8. Discuss these questions with your lab group and write down your own answers:
   1. What is the substance that collects on the glass in the clear part of the flame?
   2. How can you explain how it got there?
   3. The black substance is a material called soot. It is made of pure carbon, just like charcoal. Where did the carbon atoms come from that make up this material?

Objective Five

Flammable ‘Smoke’ and Jumping Flames

Watch your teacher perform the Jumping Flame Trick. Then do it yourself.

1. Light the candle again if it not still lit. Allow it to burn at least until there is a nice pool of melted wax around the wick: about 15 s.
2. Light the match you just used again or light another match. Blow out the candle and before the smoke stops rising place the lit match in the stream of rising smoke. Make sure it is close to the wick but not touching it.
3. The flame should re-light even though you do not touch the flame directly to the wick. Try it again.
4. Can you do it again immediately or do you have to wait a bit and let the candle burn for a while before you can repeat the trick? Do some experiments so you can answer this question.
5. Try to take a picture of the re-lighting process. It can really help to have a partner for this.
6. Discuss these questions with your lab group and write down your own answers:
   1. Remember the clear part of the flame and what was found there. What important phase change is happening to the liquid wax as it is heated by the flame?
   2. Based on the fact that wax is flammable, what is the probable identity of the material in the smoke?
   3. Why does the Jumping Flame Trick work?

Objective Six

The Story of a Candle

This part of the lab is mostly about making an outline of a story about how a candle actually works. Use the parts of the objective as a guide for writing your story and feel free to do additional experiments with your candle as you work on your story. For each one try to think of specific evidence from your own observations that is relevant. This writing will effectively be the Analysis section of your lab report. Use the following outline to help organize your writing.

1. The wick burns first but eventually it is the wax that fuels the flame. How does this get started?
2. Describe, based on your own observation, the molecular make-up of the parts of a flame.
3. The wax vaporizes from the surface of the wick due to the heat of the flame. What stage happens next in its transformation? Based on the soot you found in the bright part of the flame, how is the hydrocarbon wax being transformed?
4. Hot solids will glow if they are hot enough and soot is a solid. Eventually, the soot burns, too. For a while, though, it is in the bright part of the flame. How does the soot make it possible for the candle flame to produce light?
5. Explain, based on your own observations, how we know the candle needs fresh air to burn.
6. Explain, based on your own observations, why a glass held over a burning candle collects condensation on its inside.
7. Quantify how a candle changes as it burns based either on changes in mass, length, or both.
8. Why does the mass of the candle decrease as it burns? What does this have to do with the fact that both carbon dioxide and water vapor form?
9. Explain, based on your own observations, why the ‘smoke’ of a recently extinguished candle is flammable.