Learning About Burning

Combustion and Respiration

or, How is a Candle Like a Cookie?

by Rich Espey

(This page is for student use. Notes for teachers can be found  here. )

When we first learned about the Carbon Cycle we discovered that carbon is found in many different forms all around us. In this series of activities you will discover the role carbon plays in some important daily activities.
 

Part 1 - Burning a Candle

You've probably seen candles burning countless times before, but have you ever stopped to consider what role carbon plays in the burning process? In this activity you will have a chance to discover the chemical reaction that takes place when a candle burns.
 

Materials

• a small candle
• a rubber stopper
• matches
• tongs
• a cold glass beaker
• a funnel
• a little clay or putty
• bromothymol blue or phenol red indicator
• test tube clamp
• safety glasses

Procedure

From these activities, can you determine what products result from the burning of the candle?

When you burn a candle you obtain___________________, _________________, and __________________.

If you're still not certain, go ahead and set up the equipment as shown in this diagram:

If you weren't sure before, now you might know a gas being produced by the candle. This gas can extinguish a flame and can make bromothymol blue turn yellow. This gas is ...... carbon dioxide!
These activities have shown that a burning candle produces water vapor and carbon dioxide. The burning reaction also produces heat as well.

Now that we know the products, what are the reactants that make this reaction occur?

Place a tall beaker over a burning candle. What happens to the candle? Why does this occur?

The flame goes out because the burning reaction uses up all available oxygen. Without oxygen as a reactant, the burning process cannot continue.
Now we can begin to write an equation which describes the burning process:
 

Of what elements is the wax composed? Since there is both carbon and hydrogen in the products, there must be both carbon and hydrogen in the reactants. Thus, the wax must contain carbon and hydrogen, and it does. One type of wax has the chemical formula C₂₅H₅₂. Materials like wax which contain hydrogen and carbon are called hydrocarbons. When hydrocarbons combine with oxygen in the process known as combustion (burning), they produce carbon dioxide, water vapor, and heat.

If you would like to learn more about how candles are produced from petroleum,  click here.
 

Part 2 - Humans Are a Lot Like Candles

Yes, that's absolutely true. In this activity you will learn just how much you are like a candle!
 

Materials

• a plastic bag (newspaper bags work well)
• a rubber band
• a drinking straw
• a tall beaker
• a little bromothymol blue or other indicator
• safety glasses

Procedure

In what way are you like a burning candle?

You've discovered that you produce carbon dioxide and water as part of your natural body processes, just as a burning candle produces carbon dioxide and water. The burning candle combined wax and oxygen to make these products. What are the reactants that react to produce carbon dioxide and water in our bodies?

Obviously we need oxygen to survive just as a candle flame needs oxygen to survive. But we don't eat wax!!! But, just as wax is made from carbon and hydrogen, most of the foods we eat contain carbon and hydrogen. An important group of these carbon and hydrogen containing foods is the carbohydrates, which includes sugars and starches. Carbon, in fact, is in all the foods we eat:
 

So we can write an equation which describes what happens when we eat food and it is "burned" in our cells to produce energy. We call this process "cellular respiration".
 
 

 So now you can see just how much humans are like candles!!

If you'd like to learn some more about how humans convert food into energy, click here . This site presents a slide show about the metabolism of food. It is pretty sophisticated, so beware!!
 If you'd like to try another activity about carbon in food, click  here .

Part 3 - An Inquiry - Who Else Makes Carbon Dioxide?

We have seen that a burning candle and a breathing human both produce carbon dioxide. Who or what else can produce carbon dioxide? Here's a chance to design an activity to answer that question.
 

• Obtain some organisms you would like to test for carbon dioxide production. You have already tested an animal, so you might want to test a plant like seaweed (Anacharis or Elodea), a fungus like yeast, or some protists living in pond water.
• You will need to devise a method to determine if these organisms are indeed producing carbon dioxide, so you will probably need a carbon dioxide indicator like phenol red or bromothymol blue.

Here are some other questions you might want to consider in your inquiry:

Which types of organisms produce the most carbon dioxide?
Which types of organisms produce carbon dioxide the fastest?
How does temperature affect the rate of carbon dioxide production?
How does adding a carbon source like sugar affect the rate of carbon dioxide production?
Do decomposers like bacteria and fungi produce carbon dioxide as part of their processes?
Are landfills a source of atmospheric carbon dioxide?
 

• Think of a question you have from the above list or from your own curiosity that you'd like to more about.
• Think of a hypothesis related to that question that you could test.
• Write down a list of materials you think you'll need and have your teacher approve the list.
• Write down a list of steps you think you'll take to answer that experimental question.
• Conduct your experiment making careful observations.
• Record your observations and data that you collect.
• Use your discoveries as you draw your own version of the carbon cycle in the next part of this activity.

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