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Observing a Candle

Note to Teachers: Here is how to do the Jumping Flame trick:
• 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.
• 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.
• This works because the stream of smoke is not non-combustible ash but combustible fuel. The candle vaporizes the wax before burning it and this vapor continues to be produced for a short time after extinguishing the flame. A source of ignition brought close to this fuel stream will re-light the candle.
Limewater

Make limewater by mixing about 2 g of Ca(OH)2 into 1.0 L of hot, de-gassed water. Degas the water by boiling for about five to ten minutes. The actual maximum amount of Ca(OH)2 that will dissolve is around 0.8 g/L according to the Ksp of the dissociation: 5.02 × 10-6. Filter any remaining solid before using the solution in the lab.

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 answer the following questions based on your observations:

1. What happens to the candle when you light it?
2. Can you prove that the candle needs oxygen in order to burn?
3. Can you prove that the candle produces carbon dioxide when it burns?
4. Can you prove that the candle produces water when it burns?
5. What happens when you hold a piece of glass in different parts of the flame? What do these results say about the process of burning wax in a candle?
6. Is it possible to light a candle without touching the flame directly to the wick? Why or why not?
You can see a sample lab report for this lab (Word doc).

Materials

candle
matches
safety goggles
lab notebook & pen
ruler
watch glass
400 mL beaker
50 mL beaker (for indicator)
at front of room: supply of CO2 indicator solution
(e.g., limewater: Ca(OH)2 in water)

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. When you light a candle you initiate a type of chemical reaction called a combustion reaction. This reaction can be written in chemical shorthand as:

hydrocarbons + oxygen (O2) carbon dioxide (CO2) + water (H2O) + heat/light
or
CnH2n+2 + O2 CO2 + H2O + heat/light

You can tell that a chemical reaction is occurring because of that heat and light: a sure sign. Combustion reactions require three things: fuel (hydrocarbons), oxygen, and a source of ignition. Hydrocarbons are molecules made up of hydrogen and carbon and are in fact what make up such things as gasoline, fuel oil and propane. Candles are made of hydrocarbon wax. Oxygen is supplied by the atmosphere and you supply the ignition (a match). One point of some importance is that different phases of matter burn at different rates. Solids burn more slowly than liquids and liquids burn more slowly than gases.

Combustion can be an imperfect process. That is, some of the hydrocarbons may not burn completely. When that happens several carbon-containing products can form besides carbon dioxide. First, carbon monoxide (a highly toxic gas) can form. This is only dangerous in cases of burning charcoal indoors or using a gas-powered generator in a closed space. Second, incomplete combustion can result in pure carbon: the hydrogen is burned away (it combines with oxygen to become water) and the carbon stays behind unburnt.

It may seem odd to think that burning the candle produces water but it is a fact even so. Water is a compound of hydrogen and oxygen and when those two elements combine the most common compound is H2O. It is hard to see the water that results from burning the candle because it is a gas (steam) and it is invisible.

Carbon dioxide is a colorless, odorless gas. As it forms in a combustion reaction it is very hot and therefore has a lower density than air. This causes it to rise. When water containing calcium hydroxide (Ca(OH)2) is exposed to CO2 the carbon dioxide reacts with the Ca(OH)2 to form insoluble calcium carbonate (CaCO3, also called limestone). This substance is white and when the reaction occurs it makes the water turn cloudy. Water with Ca(OH)2 dissolved in it is called limewater.

In this lab you will make a series of observations. The purpose of doing so is to hone your observation skills for the labs you will do in the future and to learn something about an object you may have taken for granted.

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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.

Limewater is a saturated solution of calcium hydroxide, a strong alkali. The pH of the solution is about 12. Treat it with care and caution. Wash hands after handling. Rinse immediately with plenty of fresh water if the solution gets on the skin or in the eyes.

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.

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.
2. Your Procedure should give a brief description of the actions you took in the lab along with the observations you made. You must include a chemical equation.
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. You Analysis section must address each of the questions below in paragraph form, not question and answer format.
1. When you light a candle for the first time the flame grows, then shrinks, then grows again. The second growth coincides with the melting of the wax at the top of the candle. Explain why this occurs.
2. When a candle burns what substances are burning?
3. Candles get smaller as they burn; explain why.
4. What evidence do you have that candles require oxygen to burn?
5. What evidence do you have that candles produce carbon dioxide while burning?
6. What evidence do you have that candles produce water vapor while burning?
7. What is collected on a piece of glass in the bright part of the flame? What does this reveal about the stages of combustion of a hydrocarbon and how light is produced by the reaction?
8. What is collected on a piece of glass in the clear, blue-rimmed part of the flame? What does this reveal about the process by which the wax burns?
9. Explain the jumping flame trick and account for the fact that the smoke of a recently blown-out candle is flammable.
5. Your Conclusion section should reflect on the experience of carrying out the lab. What did you learn, what surprised you, do you think the lab was an effective educational experience?

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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

Record some quantitative observations about the candle before you light it. Record such things as length, mass, diameter, length of wick, or anything else that occurs to you. Try getting the mass of the candle at specific time intervals while burning: does its mass change over time?

Also record some qualitative observations about the candle before you light it. Record such things as wax color, color of wick, smell, new/old/damaged, or anything else that occurs to you.

Finally, record the sequence of events that occurs as you light the candle. Try to be as detailed as possible. These observations will be qualitative and should be as organized as possible. Reminder: each lab group member should write their own observations!

Objective Two

Candles Use Oxygen as They Burn

Your teacher says that in order to undergo the chemical change called combustion, candles require oxygen. Oxygen is a gas that makes up about 20% of the Earth’s atmosphere (by volume). Can you prove that oxygen from the air is required for the candle to burn? Devise and carry out an experiment.

Write down the steps of your experiment in your notebook. Perform your experiment and record your results. Be sure to record any other observations you may make while carrying out your experiment. Check in with your teacher to be sure you have not missed any important observations.

Objective Three

Candles Produce Carbon Dioxide as They Burn

Your teacher says that when hydrocarbon materials such as candle wax burns, they produce carbon dioxide (CO2). In the presence of CO2 limewater changes from clear to cloudy. (This is because insoluble particles of calcium carbonate—CaCO3—form in the water). Can you prove that candles do make CO2? Devise and carry out an experiment.

Write down the steps of your experiment in your notebook. Perform your experiment and record your results. Be sure to record any other observations you may make while carrying out your experiment. Check in with your teacher to be sure you have not missed any important observations.

Objective Four

Candles Produce Water as They Burn

Your teacher says that when hydrocarbon materials such as candle wax burns, they produce water (H2O). Burning (combustion) is a heat-releasing process and the chemical products are hot. If there is water present you will have to provide a way to cool it down in order to see it. Can you prove that candles do make H2O? Devise and carry out an experiment.

Write down the steps of your experiment in your notebook. Perform your experiment and record your results. Be sure to record any other observations you may make while carrying out your experiment. Check in with your teacher to be sure you have not missed any important observations.

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Objective Five

Flames Have Parts

Your teacher points out that there are at two distinct regions in a candle flame. One: the blue-rimmed clear region very close to the wick. Two: the dim-orange-fading-to-bright-yellow region that produces light. In the last several experiments you have investigated only the products of combustion but not the process.

Use a watch glass to find out what physical material is found in the bright, upper part of the flame. This is where combustion is occurring and the process is not instantaneous. Based on what you observe what stages do you think the hydrocarbon wax goes through as it burns?

Use a watch glass to find out what physical material is in the clear part of the flame right next to the wick. Hold the watch glass into the flame from the side, not from above. Be careful not to touch the glass to the pool of liquid wax. Once you notice something has collected pull the glass back out of the flame and allow it to cool completely while you watch. What does your result reveal about the changes the wax goes through before it burns?

Write down the steps of your experiment in your notebook. Perform your experiment and record your results. Be sure to record any other observations you may make while carrying out your experiment. Check in with your teacher to be sure you have not missed any important observations.