This lab is an inquiry-based lab. The idea
is for students to come up with their own procedure. They must decide what
will be held constant, what will be varied, and how to make their
measurements. The notion is to have students learn science by doing science
the way scientists do it. Students may either write narrative answers to the
questions as posed in the objective section (complete with graphs) or write a
full lab report. I expect that it will require anywhere from 1 to 3 full hours
of classroom time, depending on the level of complexity you require from the
students. Students may extend their investigations at home in order to get a
better grade. If you try it, please get in touch (my email address is on the
home page) and let me know how it worked for you.
There is a section of this web page near the end which will not print. It gives information required to make the lab successful. Be sure to read it. This section will also not print.
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.
When you complete your work on this lab you will be able to answer the
following questions based on your observations:
What happens to the candle when you light it?
Does the candle need oxygen in order to burn?
Can you prove that the candle produces carbon dioxide when it burns?
Can you prove that the candle produces water when it burns?
What happens when you hold a piece of glass in the flame and what is
the identity of the substance you can collect this way?
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 if you would like (Word doc).
watch glass
400 mL beaker
at front of room: supply of CO2 indicator solution
(e.g., limewater: Ca(OH)2 in water)
water in a 200 mL beaker
50 mL beaker (for indicator)
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 + O2CO2 + 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.
A few words about carbon dioxide. It is a gas that is more dense than air and so it can be collected from containers by pouring. When CO2 is added to water containing calcium hydroxide (Ca(OH)2) it reacts with the Ca(OH)2 to form insoluble calcium carbonate (CaCO3). 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.
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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.
Safety
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!
Procedure
The procedure for this lab is mostly up to you. After your teacher gives you an
introduction to the lab it is your decision about how to proceed. Some things to
keep in mind:
What experiments are you going to perform?
Before you set up the experiments try to decide what you think is going
to happen. Write this down and refer back to it once you are done.
In your experiments, what are you going to hold constant?
What are you going to change?
How are you going to decide whether you have answered the questions?
Tips and Pointers
Here are a few things you need to know how to do in order to be successful in answering the objective questions.
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.
Try to make measurements so that you can quantify your observations. It is easier for others to repeat your experiments and observe the same results if you can provide numerical data. At minimum use a ruler, the clock, and the volume markings on lab glassware.
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?
Try to think of a way to show the reaction of CO2 with the limewater without involving the candle.
Carbon dioxide can be collected from a burning candle by holding a container above the candle.
Carbon dioxide is denser than air.
Solids and liquids are very poor fuels. Most fuels delivered as solids or liquids need to be turned into a gas before they will burn. Is this the case for candle wax? What experiments can you devise to determine what phase of matter is really burning?
In that connection ask your teacher to show you the “Jumping Flame” trick.
Note to Teachers: Here is how to do the 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.
Limewater
Make limewater by mixing about 1 g of Ca(OH)2 into 0.5 L of degassed water. Degas the water by boiling for about 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.
Grading
Answer each of the questions in the Objectives section of this handout. In your answer to each question include:
An explanation of how it relates to the chemistry (see the Background section)
Scientific explanations require that you refer to your observations as collected during class. Scientific information is best when it can be made quantitative: length, volume, time, etc. Report these data as part of your observations and use them to support your explanations.
You will be graded on the quality of your writing, the profesionalism of your work’s appearance, the design and execution of your experiments and the degree of your understanding of the underlying science.
Comments on past student lab reports can be found here.
Some common errors in writing the report for this lab are explained here.
Last updated: Jul 07, 2008
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carbon dioxide (CO2)
+ water (H2O) + heat/light