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.
You can see a sample lab
report for this lab if you would like (Word doc).
400 mL beaker
at front of room: supply of CO2 indicator solution
(e.g., bromothymol blue)
water in a 200 mL beaker
50 mL beaker (for indicator)
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
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, sugar, starch, and wood. 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.
In this lab you will make a series of observations. The purpose of doing so is to hone your observational skills for the labs you will do in the future and to learn something about an object you may have taken for granted.
Tie back loose hair and clothing. Before lighting a match don the safety goggles and keep them on during all activities performed while the candle is lit. Before handling the indicator solution don the gloves provided. Treat chemicals you don’t know anything about with as much caution as you treat bleach or gasoline. If you are not wearing shoes with closed toes and heels then you should be!
Get your lab notebook and pen out and open it. When writing in this book use only pen and never obliterate: just cross out lightly anything that needs to be crossed out. Your notebook is a permanent record of your work in the lab. At the top of a new page in your specially dedicated lab notebook write down the name of this lab and the date. List the materials you are using. Write a one-sentence description of the lab. For each step of this procedure write down the actions you take and the results of those actions. Make observations of the unexpected…and the expected.
Note: when writing in your lab notebook do not answer questions directly. Rather, please write in a narrative or story form. Use the pronouns I, me, my, mine, and we, us, our(s). Use the present tense. Use the active voice (“I lit the candle,” is good. “The candle was lit,” is bad.
Before lighting the candle write down some notes about its physical appearance. Note things like color, size, shape, the condition of the wick, and the texture. Use all of your senses except taste. Never put anything in the chemistry lab into your mouth.
Light the candle. Blow out the match and wet the hot end of it before throwing it into the trash. Observe the burning candle carefully. What burns first? What happens next? What parts of the burning candle are in which states of matter (solid, liquid, gas)? What is burning: the wick, the wax (solid or liquid), or a gas?
Flames and the Need for Oxygen (O2)
This part will help you to observe the elusive fact that a candle needs oxygen to burn. Place the 400 mL beaker upside-down over the candle. What happens? Why? How long does it take?
Relight the candle and do it again. Observation sometimes requires you to see something twice to get all the details. Write down everything that seems important. Write down some things that don’t seem important!
‘Seeing’ Carbon Dioxide (CO2)
Put on the rubber gloves provided by your teacher and obtain about 25 mL of indicator solution in your 50 mL beaker. Put the beaker on a piece of white paper. What color is it?
Blow up the balloon and insert the straw in the opening.
Put the end of the straw in the indicator solution you collected and allow your breath to slowly bubble through it. What color does the indicator solution turn when you do this? The indicator solution changes from blue to yellow in the presence of an acid. It just so happens that CO2 is an acid (carbonic acid, H2CO3) when dissolved in water (CO2 + H2O --> H2CO3). You already know that you breathe out CO2. The indicator solution makes an effect of the CO2 visible and proves that it is present.
Now you will test for CO2 in the product gases that the candle produces. Put the 400 mL beaker over the candle again and let the candle go out. According to the chemical equation above there should be CO2 in the beaker now. To find out, turn the beaker over carefully and quickly and pour in the indicator solution. Cover the top of the beaker with your gloved hand and swirl the indicator around. Put the beaker down on a white piece of paper and observe the color. What does the color change tell you about the candle?
Once you are all done with the indicator solution and have disposed of it in the sink (it is dilute enough not to be dangerous) and have cleaned the glassware you may take off your gloves. But not before!
To see whether water is one of the products of the combustion reaction put some water in a beaker (200 mL is fine) and hold it overbut not in the flame.
Take a close look at the bottom of the beaker where it is close to the flame. What do you see? Where did it come from?
Now, holding one extreme edge of a watch glass in your fingers put the part of the glass farthest from your fingers into the middle of the flame, above the wick, in the dark part. Make it so that the whole flame is under the glass.
Pull it out after 15 - 20 s. What do you see? Where did the deposit come from? What do you think it is made of? What shape is it?
Wipe the watch glass clean before you put it away.
The last thing you will do is find out just what phase of matter is really burning.
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 20 s. The pool of wax may already be there since you have had the candle lit (on and off) for a while now already.
Light the match you just used again or light another match. Blow out the candle and before the smoke stops rising bring the lit match close to the wick but not touching it. Make sure the lit match is placed in the stream of rising smoke.
What happened? Do it again…or several times. It’s a fun trick. Why does the flame jump ‘through the air’? Is it really jumping through plain air or is something else involved? What does this tell you about what phase of matter is burning once a candle has been burning for a little while? Go back to the beginning again in your mind for a minute: what burns when you light a candle for the first time? What’s burning after the flame has melted a bit of the wax? What allows the flame to jump from the lit match to the wick when you hold the match in the smoke from a just-extinguished candle? Finally, could you get the wax to burn if you turned the candle upside-down? In other words, is it the liquid wax burning?