Observation of combustion is a way to introduce the subject of air pollution and review the idea that matter cannot be created or destroyed, but is merely changed in form or location. If students understand what is happening when a candle burns, they are well on their way to understanding air quality issues, which are associated with matter (gases and particles) that we often cannot see.

Over 75% of the Earth’s air is found in the troposphere. Air is a mixture of gases and particles with over 99% nitrogen (N₂) and oxygen (O₂) by volume. Gases are individual atoms or molecules while particles are aggregates of atoms or molecules. Both are parts of the air pollution story. The levels of certain gases and particles beyond the background amounts contribute to air pollution in the troposphere. Chemical processes such as combustion (burning) and physical processes such as evaporation contribute to the degradation of air quality.  We are most concerned with the impact humans have on air pollution.

A burning candle illustrates basic air pollution concepts. It is easy to observe that there is less candle wax and wick after burning than before burning, but how does this happen and where does the wax go? The initial heat of the flame melts the wax (physical change). The liquid wax molecules travel up the wick and vaporize (physical change). The wax molecules react with the oxygen in the air (chemical reaction). As the wax oxidizes (burns), it produces mainly water and carbon dioxide, which dissipate in the air around the candle. This exothermic reaction yields light and heat. Candle wax (paraffin) consists of chains of connected carbon atoms surrounded by hydrogen atoms (hydrocarbons). If the wax continues to melt near the flame, it can burn completely and leave no ash or waxy residue. However, incomplete combustion results in excess smoke, soot (mainly carbon), and perhaps toxic chemicals.

Complete combustion of hydrocarbons, such as the methane (CH₄) found in natural gas, produces water vapor and carbon dioxide. Carbon dioxide is one of the major “greenhouse gases” that contribute to global climate change. Water vapor is a “natural” greenhouse gas, and it is certainly not a pollutant. Water vapor in the atmosphere varies from 0.001% by volume at the poles to 5% in humid areas near the equator.

Gasoline and diesel fuels are mixtures of hydrocarbons, which are compounds that contain hydrogen and carbon atoms such as octane (C₈H₁₈). These fuels are burned in internal combustion engines (automobiles, trucks). If the combustion in a vehicle engine was “perfect,” oxygen in the air would combine with all the hydrogen in the fuel to form water and with all the carbon in the fuel to form carbon dioxide.

Nitrogen in the air would remain unaffected. In reality, the combustion process is not “perfect,” and engines emit several types of pollutants. These combustion byproducts are nitrogen oxides (NO_X), partially or unburned hydrocarbons, carbon monoxide (CO), and carbon dioxide (CO₂). The physical process of evaporation of fuel releases hydrocarbons into the air. Organic (carbon-containing) compounds that readily evaporate are called volatile organic compounds (VOCs).

Incomplete combustion associated with burning gasoline and diesel fuel, burn barrels, incinerators, wood stoves, barbeque grills, wild fires, biomass burning to clear land, and coal in power plants all contribute to outdoor air pollution. Fires and open burning of leaves produce particulate matter and hydrocarbons, which contain a number of toxic, irritant, and carcinogenic (cancer-causing) compounds. Smoke from burning leaves contains carbon monoxide, a major air pollutant. Some things that go into the trash release toxic air pollutants if burned. Coal burning in power plants releases carbon monoxide, carbon dioxide, sulfur dioxide

(SO₂), nitrogen oxides (NO and NO₂), particles (fly ash), and toxic metals.

It is estimated that most people spend about 90% of the time indoors. Sources of combustion pollutants found indoors include tobacco smoke, car exhaust in garages, soldering, and some hobby activities such as welding. Combustion pollutants can come from vented or unvented combustion appliances. These appliances include space heaters, gas ranges and ovens, furnaces, gas water heaters, gas clothes dryers, wood or coal-burning stoves, and fireplaces.

Sources:

Pimentel, G. (1963). Chemistry: An Experimental Science.  San Francisco: W.H. Freeman.

 U.S. Environmental Protection Agency. Office of Air and Radiation. Retrieved August 22, 2004, from http://www.epa.gov/air/.

Evaporative sources include paints, room fresheners, and out-gassing from building materials.

outdoor and indoor air pollution and that pollution is from both gases and particles.

4.      Use a burning candle to illustrate air pollution.

Organize the students into groups of two to four and give each person the Burning Questions student activity page and a pair of safety glasses. Provide each group with a tea or pillar candle on a piece of foil or an aluminum pan to protect the tables or desks.

What will happen when the candle is lit? Have students sketch a picture of the candle and write down their observations on the Burning Questions student activity page before the candle is lit. Spot-check the descriptions. [Look for qualitative and quantitative observations such as the candle is white, the wick is ½ inch above the top of the candle, etc.]

Light each group’s candle. For the next 5 to 10 minutes, have each student make a sketch of the lighted candle and record his or her observations. Place a 500-mL beaker over the candle to extinguish it and have the students make a final set of sketches and observations of what happens inside the beaker as the candle goes out.

Groups share their observations with the class. Point out that there are descriptive (qualitative) as well as measurement (quantitative) observations. This is a good opportunity to review chemical and physical changes and the release of energy and light by the burning of the wax. Did the students notice anything that might contribute to air pollution from the burning candle?[Smoke, soot.]

5. Use models to understand what happens in burning and what this has to do with air pollution.

Using your choice of models of molecules, guide students through the following series of activities. They will manipulate atoms to form molecules and will create reactants and products. If paper models are used, give each group (or person) an envelope with one carbon atom, four hydrogen atoms, and four oxygen atoms, which are the chemical elements involved in the burning of a candle. Alternatively, give students the Models of Atoms sheet to cut out the atoms. Tell them that each notch in the paper models represents one chemical bond.

Note: If you have an overhead projector or documentation camera, students can follow along with your demonstration. Using the template for the atoms, create a transparency and cut out the atoms.

What is needed for burning? [Fuel, heat, oxygen.] Students may be familiar with the fire triangle, which includes fuel, heat, and oxygen. Introduce the terms “reactant” 

(a substance  at the start of a reaction) and “product” (a substance  present as a result of the reaction).

What does oxygen have to do with burning? Show the model of oxygen (O₂) and have students make oxygen molecules by putting two oxygen atoms together. About 21% of the atmosphere is oxygen. Either place a beaker over a burning candle and note that the candle goes out, or remind the students that they had observed this earlier.

Why did the candle go out?There are actually at least two reasons: (1) lack of sufficient oxygen and (2) the carbon dioxide produced extinguishes the flame. However, establish that oxygen is a reactant needed for burning.

What is the composition of candle wax (the fuel/reactant)? Show a model of a candle wax (paraffin) molecule using toothpicks and gumdrops, paper atoms, or traditional wooden ball and stick models (see figure below). The wax (paraffin) is a long chain of carbon atoms with hydrogen atoms attached.

Part of a Wax Molecule

Another type of fuel is methane (CH₄), a component of natural gas. Have students construct a methane molecule using one carbon and four hydrogen atoms.

What will happen when methane burns 

(reacts with oxygen)? Take apart the reactants (oxygen and methane) and use only these atoms to make products. The rules for making products are: (1) carbon and hydrogen can combine only with oxygen, (2) the products need to be a different composition from the reactants, (3) all atoms must be used, and (4) no atoms can be added.

What is one of the products of burning that contains carbon? [Using the models, students should determine that a product of combustion is carbon dioxide (CO₂).]

To confirm this, do the following demonstration: use a straw to exhale into a small container of limewater (saturated solution of calcium hydroxide). Cloudiness in the limewater indicates the presence of carbon dioxide. Place a small container (bottle cap) of fresh limewater next to a burning candle and put a beaker over the candle and the limewater. Have students make observations. Carbon dioxide generated by combustion is a major greenhouse gas and increased levels contribute to global climate change.

What is a product of burning that contains hydrogen? [Water (H₂O) is the other product. Two water molecules will be formed from burning of one methane molecule.]

To demonstrate that a liquid is produced in the reaction, burn a candle and extinguish it by placing a beaker over it. Carefully sprinkle some talcum powder into the beaker. It should stick to the walls if there is enough water produced. The control for this demonstration is to follow the procedure above but put an unlit candle under the beaker.

6fg   6. Tying it all together.

Identify the reactants and the products of the burning wax. To simplify things, use methane to represent a small piece of paraffin (see figure below). In its simplest form, burning is like respiration in terms of the overall chemical reaction. Carbon compounds combine with oxygen to form water (in vapor form) and carbon dioxide. However, combustion is relatively fast whereas respiration is slow and controlled. Make sure students note that heat and light are given off in this exothermic reaction and that there is conservation of mass.

Complete Combustion Reaction

                             CH₄                                 +              2O₂                            g   CO₂                            +             2H₂O

                        Methane             +             2 Oxygen g Carbon Dioxide         +           2 Water

For advanced classes, work with students to develop the chemical shorthand (balanced equation) for the burning candle. The equation for burning of a typical wax molecule is:

                     2 C₂₈H₅₈ (s)           +                  85 O₂ (g) g 56 CO₂ (g)              +        58 H₂O (g)

(The composition of candle wax varies and the burning candle reactions are actually much more complex.)

What would be the products from burning a candle or methane (CH₄ ) if there wasn’t enough oxygen? [There can be incomplete combustion where not everything is burned.] Using the models, demonstrate making products out of one carbon, four hydrogen, and only three oxygen atoms. The results are carbon monoxide (CO) and two water molecules (H₂O). As an air pollutant, carbon monoxide binds to hemoglobin in the blood and inhibits uptake of oxygen.

Hold a beaker above a burning candle to collect soot on the bottom of the beaker. Soot (black carbon and other compounds) consists of particles that can be harmful to the respiratory system. Smoke contains partially burned carbon compounds.

Toxic chemicals can be produced by burning candles. These air toxics can be organic compounds or metals. Candles with lead in their wicks can emit unacceptable levels of lead. The student reading on Candles and Air Pollution would be appropriate at this point.

What can candles tell us about other types of air pollution?  [A burning candle mirrors what is happening in combustion of coal in power plants, gasoline in car engines, trash in burn barrels, wood in wood stoves, charcoal in barbeque grills, and fuel in combustion appliances. Incinerators, biomass burning, and wild fires contribute combustion pollutants to the atmosphere. Complete combustion does not always occur when things burn.] Point out that burning contributes to air pollution but there are other sources such as dust, volcanic eruptions, various industrial processes, and vaporization of certain compounds.

In class or for homework, have students complete the Burning Questions student activity page.  They will complete the reactants and products equation, and write a paragraph about what they have learned about air pollution by observing the burning candle.Look for evidence of identification of reactants (wax and oxygen) and products (water and carbon dioxide), gases and particles, and what burning has to do with air pollution.

7.   Homework.

For homework, ask the students to list all

Assessment Options

1.   After students have observed the burning candle and discussed the products and reactions, they are ready to extend their knowledge to what happens in forest fires to impair air quality. Have students list each reactant [trees with lots of organic molecules made of carbon-hydrogen-oxygen and O₂] and each product [heat, smoke, CO₂, and H₂O vapor] of the forest fire.

each product [heat, smoke, CO₂, and H₂O vapor] of the forest fire. of the combustion appliances, fireplaces, burn barrels, vehicles, gas-powered lawn mowers, etc., and the potential air pollution that they might cause on the Burning Questions at Home student activity page. At the bottom of the page, students put the number of carbon monoxide and smoke detectors where they live. Compile these statistics and have students calculate the total number of combustion sources per class and the average number of these items. The results of this assignment could be used in the final lesson, which addresses global climate change.

2. The rubric below (adapted from MI CLiMB) can be used for evaluating the candle observations in Burning Questions.