Introduction
In this lab I observed the burning of a candle very closely. I found out that the candle needs oxygen to burn, that it produces carbon dioxide similar to the way that my body produces carbon dioxide, and that a candle produces water as a second waste product. I learned that if I hold an object in the flame it becomes covered with soot which is unburned carbon fuel. Finally, I learned that neither the solid wax, nor the melted wax, nor the wick burns when a candle is lit. In fact, the wax itself is burning as a vapor or gas. Objectives
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? Materials
candle
matches
safety goggles
lab handout & pen
ruler
watch glass
400 mL beaker
CO2 indicator solution (limewater)
50 mL beaker (for indicator)
Procedure
My lab partner and I performed a series of experiments to investigate the chemistry of the burning of a candle. The teacher provided the following chemical equation to describe the burning, or combustion, in the candle: Hydrocarbon fuel + O2 CO2 + H2O
The candle we used was a small, white emergency candle about 3.5 cm tall. The candle had already been used by someone else because I could see from the black wick that it had been lit before. When we lit the candle I noticed that at first it was only the wick that was burning. Eventually, after the flame had melted a small pool of wax, I noticed that the flame grew a bit brighter and the wick didn’t seem to be burning as fast as before. It looked like the fuel had changed from the wick to the wax. Because the flame grew bigger just at the moment when the wax melted leads me to conclude this. The first experiment we did was to cover the candle with a 400 mL beaker. When we covered the candle we noticed that the inside of the beaker became fogged, possibly from water. Very quickly, within about 10 sec., the candle went out. We repeated this experiment a few times to see if it might take less time for the flame to go out the second time. It did not take any less time to go out, or at least we couldn’t tell if it did.
The second experiment we did was to put the candle out with the beaker as before but then quickly turn over the beaker. My lab partner fetched some limewater solution and poured it into the container. I swirled the solution with the gases inside and the clear limewater became cloudy, as if suddenly there were tiny white particles suspended in it. The third experiment we did was to find out whether water was really a product of the combustion reaction. We held the 400 mL beaker upside-down over the candle flame and the inside of the beaker became coated with condensation. After a short while the bottom of the beaker became hot and the condensation that had collected there evaporated away, leaving a circle where there was none. The fourth experiment we did was to hold the edge of a dry watch glass right in the flame. A deposit of black soot formed on the watch glass when we held the glass in the bright orange-yellow part of the flame. When we held the glass in the bottom, clear, blue-rimmed part of the flame a clear liquid appeared on the glass. After we pulled the glass out of the flame and let it cool off the clear liquid solidified and turned out to be candle wax.
The fifth and last experiment we did was to find out whether it is the liquid wax that burns or some gas or vapor that the liquid wax becomes when it is heated. I lit the candle and let it burn until there was a pool of hot liquid wax around the wick. Then I lit a match using the flame and blew out the candle. Next I held the lit match in the stream of “smoke” without touching the flame to the wick. The flame jumped from the match to the wick. We weighed the candle before starting our experiments and found it had a mass of 12.7 g. After we finished all of our work we blew out the candle and found that it only had a mass of 10.2 g. Somehow, the candle had lost 2.5 g of its original mass. Analysis
In this lab we did a series of experiments to explore the chemical reaction: Fuel + O2 CO2 + H2O where the fuel is a candle. This reaction is a combustion reaction. We found that the candle does in fact need oxygen to burn because when we cut off the supply of fresh air with a beaker, the candle went out. This happened because the burned candle used up the oxygen inside the beaker and new oxygen from the air couldn’t get to it. We were able to prove that CO2 is a product of the reaction. When we swirled the gases produced when the candle burns with the limewater, the limewater became cloudy white. The cloudiness showed that the limewater had come into contact with carbon dioxide since that is the only thing that causes it to become cloudy. This proves that candles produce CO2 when they burn. Our experiments showed that water (H2O) is also a product of the combustion reaction. When we held the beaker over the flame, water from the flame condensed on it. The condensation came from the flame, showing that H2O is a product of the combustion reaction. One interesting thing we discovered was that you can see the carbon before it is completely burned up into CO2 by holding a piece of glass in the bright part of the flame. The black soot we collected was pure carbon—at least, that’s what the teacher said.
The soot particles get so hot in the flame that they glow, just like the heating element on an electric stove glows when it is turned high enough. At the bottom of the flame, where it is clear, we collected unburned wax. What must have happened is that the heat of the flame melted and then vaporized the wax. The clear part of the flame must be where the wax goes just before it really starts to burn in the visible flame above it. Surprisingly, we found out that the phase of matter that is burning in the candle is a gas. We know this because of the jumping flame. If the solid or liquid wax were burning the jumping flame trick would not have been possible. The “smoke” that appears when you blow out a candle is really the vaporized wax that I mentioned in the previous paragraph. This means that it is flammable and so when a lit match is brought into the stream of vaporized wax it can burn it and the flame seems to jump from the match to the candle.
I can be sure that this is the case because I can’t make the flame jump from just anywhere: it only works if you hold the match in the stream of “smoke” that comes out when you blow out the candle. Finally, the candle lost 2.5 g of its original mass over the course of our experiments. The only was to explain this is to assume that the lost mass is the amount of the candle that burned and became carbon dioxide and water. No mass can ever really be created from nothing or totally destroyed. The candle gets smaller and loses mass over time because the wax is chemically changing from a hydrocarbon to two other compounds: CO2 and H2O. Conclusion
We were able to successfully complete all of the objectives of this lab. I was surprised by the fact that candles produce water when they burn. Since water is used to put out fires I didn’t think it could be made by one! Also, I thought the jumping flame trick was amazing. It looks like magic but I know that it isn’t. The flame just burns the flammable gas from the candle and relights the wick. I certainly understand how a candle works a lot better than I did before doing this lab.
