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AIR POLLUTION IN the Community

Activity 2 (Explore): What Happens When Things Burn?

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Activity Overview

Activity Summary

In this multi-part activity, students begin to investigate combustion by observing a candle burn and looking at the soot it produces. They read about particulate matter and experience a variety of different demonstrations to understand the difference between PM 10 and PM 2.5.

Activity Objectives & Materials

Approximate Time: 2 class periods (90-120 minutes)



  • Students will know that particulate matter (PM) is produced by combustion of fuels.

  • Students will have an intuitive and a cognitive understanding of what PM 10 and PM 2.5 are.



  • Candle & lighter/ matches (you may want 2 candles, a bigger one for the first demonstration and a smaller one for the oxygen demonstration)

  • Glass jar &/or metal can with label removed

    • Word wall words (soot, combustion, incomplete combustion, particulate matter (PM), particle)

  • Flour & flashlight

  • Orange (or match)

  • Glass jar, aluminum foil, ice, paper, match (optional)



  • Notes sheet: What happens when things burn?

Standards Connection

SEP: Constructing Explanations

CCC: Cause and Effect



Light the candle that you are going to use for the demonstration, and put it in a place where all students can see it. Have students describe what they see.

  • Possible responses: It gives off light, there’s something coming from the top, it’s melting, it’s burning, etc.

  • The purpose of this warmup is to help prepare students for making observations that they will do later in the activity. Ask students what makes for a good observation (ex. it’s detailed, clear and fully describes what you’re observing)

Safety First: Whenever you plan to burn something in the classroom, be sure to have safety measures in place such as a fire extinguisher and fire blanket.


1. Frame the Activity

Display the truck picture(s) from the phenomenon (Activity 1) while the candle is still burning. Tell students that the candle and the trucks have something very important in common. See if any students volunteer the idea that they both burn things. If not, ask them what they see coming from both the candle and the truck. Use questioning to help them focus on the smoke/exhaust, and the idea these both come from burning things. Both the truck (diesel fuel or gasoline) and the candle (wax) are burning things. Tell students that to understand what’s happening in our truck pictures, we need to understand what happens when we burn things, so they are going to study burning today.

Truck with Smoke Cropped
Rolling coal truck.jpg
Step 1

2. Candle Burning Observations

Pass out the notes sheet “What Happens When Things Burn?” and tell students to record observations of everything they are about to see on the left side of the graphic organizer (Candle observations) with one observation per block. Relight the candle if necessary, and hold the glass jar or can above it so that it collects some soot (you may need to hold it very close to the flame). After you have collected enough soot, remove the jar/can and show it to students. You can also wipe some of the soot onto a paper towel and pass it around so students can see it. Make sure that students also observe the wick (is it getting shorter?)

Teacher Tip: Make sure to try out the candle demonstration in advance. Some candles produce more soot than others, so you may want to try out a few different candles to find one that work well.

3. Candle Burning Discussion

Once students have finished making observations, have them turn to a partner and share their observations. Encourage them to add each other’s observations to their own sheets. Afterwards, have pairs share observations with the class, and keep a record using the board or chart paper. Common observations: the wax melted, black stuff came from the flame and got stuck to the jar, the candle wick stayed the same.

  • If students have missed key observations (see below) use questioning to focus their attention on these events

  • Ask students if they know what the black stuff is that they observed. If anyone uses the word soot, then put this word up on the word wall, and have them add the word on their observation sheets wherever appropriate.

Step 4

4. Constructing Explanations

In pairs or small groups, have students look at their observations, and think about what could explain each of the things that they see. Use an example to get them started: I observed the wax melting because the fire was hot and the heat caused the wax to melt. Using the structure, “I observed <blank> because <explanation>” can help students with their thinking process. Have them write their explanations on the right side of the graphic organizer next to the observation. Afterwards, have students share their explanations and add them to the chart of the observations. After each explanation, see if there is consensus among the class about whether the explanation is true. Support students in reaching accurate conclusions, but if they are stuck on something, tell them that they will come back to it later when they have learned more.

Candle observations explanations.jpg

5. Building Vocabulary

Tell students that scientists use special vocabulary to talk about burning. Put the words “combustion” and “incomplete combustion” on the word wall and ask if any students have heard the word combustion before. Explain that combustion is the scientific word for burning, and incomplete combustion is when something doesn’t burn completely. Next put the words “particulate matter (PM)” on the word wall next to where soot is, and tell students that soot is a kind of particulate matter. It is called particulate matter because it is matter made up of small particles and it is abbreviated PM. Have students write definitions for these three terms on their notes sheets.

6. Oxygen and Combustion

Light a small candle that you can put a glass jar over. Tell students that you are going to put the jar over the candle. Have them makes observations of what they see, then put the jar over the candle until it goes out. Like before, have students develop an explanation for this observation. Use questioning to help them reach the conclusion that combustion requires air/oxygen. Consider using the analogy of what would happen to a person if they were in a sealed room – eventually they would pass out from lack of oxygen.

Recommended Reading: For more background information on how delivery trucks affect pollution in cities, check out this short article from Scientific American

7. Summarizing Combustion Reaction

Ask students what they think is needed for combustion. They should say air/oxygen (use this as an opportunity to clarify that combustion needs oxygen in particular, not just air). They may also say “a candle” or “wax.” Help them to think back to the truck – does it run on wax? What word can we use to describe wood, gasoline, or wax – they are all fuels. Have students write oxygen and fuel in the “materials needed for combustion” box on the bottom of their notes sheet. Then ask them what they think is made from combustion. They should say soot and/or particulate matter. Have them write these in the “products of combustion” box. Ask if they know what the gases are in the smoke they saw coming from the candle. They may or may not know that carbon dioxide is the main gas. You can tell them that carbon dioxide and carbon monoxide are the main gases during incomplete combustion. Have them add these gases to the “products” box. Finally, tell students that hydrogen and oxygen are also products of combustion. They combine together: write H2O on the board. See if students can identify this product (water) and have them write it on their sheets in the summary box.

Teacher Tip: If students bring up the wick as something needed for combustion, ask them if the wick disappeared when the candle was burning. You can use this as an opportunity to have them think about what the wick is for in a candle.

8. What Burns?

Ask students what other things burn. Have them make a list in the space on their notes sheet. They will likely say things like wood, paper, matches, etc. Use prompts to help them think of other things: what burns in a stove (natural gas?) what burns in a grill? (propane or charcoal) what burns in a car? (gasoline). Have them add all these things to their notes sheet. Remind students that all combustion requires some kind of fuel.

9. Turn and Talk: What is a Particle?

Put the word “particle” up on the word wall, and have students turn to a partner to share what they think of as a particle. Use additional prompts as necessary (how big is a particle? What are particles made of? Can you give an example of a particle?). Afterwards, have students share out. Write down key takeaways for the group:

  • A particle is a very small part of something.

  • The word particle can mean a lot of different things – a particle of dust, an atom, or parts of an atom.


After students share, point out that it’s important when we use the word “particle” to be clear on what kind of particle we’re talking about and how big they are, because there are many different kinds of particles.

Teacher Tip: The particle turn & talk is designed to activate student background knowledge. The key takeaways will be addressed in the next section.

10. PM 10 and PM 2.5

Tell students that scientists use the terms PM 2.5 and PM 10 to describe particles that are different sizes. Ask students what they think PM stands for (particulate matter) and which they think is bigger (PM 10). Have students look at the diagram on their notes sheet showing PM 10 and PM 2.5, and see what stands out to them. After they share, have them read the short paragraph below the diagram, and identify two similarities and two differences between PM 10 and PM 2.5.

  • Similarities: both are made of small particles, both are products of combustion, both are harmful to human health, both can be solid or liquid

  • Differences: PM 2.5 is smaller than PM 10, PM 2.5 can only be seen if there is a lot in once place, PM 2.5 can get into the human bloodstream, there are some differences in what PM 2.5 and PM 10 are made of (PM 10 includes dust, soot, pollen and various chemicals; PM 2.5 is mostly a chemical mix because it is usually smaller than dust and pollen)

Vocabulary Tip: Micron and micrometer are exactly the same unit of measurement. Micrometer is used in this module because it is the SI unit of measure and because students are likely more familiar with the prefix “micro-“. Some diagrams (such as the one included on the student sheet) use the term micron, so you may need to clarify this for students.

Connection to Current Events: Students may be interested in knowing how big coronavirus is compared to air pollution. One coronavirus is approximately 0.1 µm, which makes it much smaller than most PM 2.5.

11. Demonstrating PM 10

Prepare by taking a small amount of flour in your hand and turning off the lights. Have students prepare to observe, then toss the flour in the air and shine the flashlight on it. Ask students what they saw, and whether they think the flour is PM 10 or PM 2.5 (it is PM 10). Students should be able to see the flour particles.

Demonstration Safety: Since breathing in PM 2.5 and PM 10 can be harmful, students should not breathe in the flour. Breathing in the scent from the match or the orange is not significant enough to cause harm.

12. Demonstrating PM 2.5

Have students prepare to observe, then move to a corner of the room and either light a match and blow it out, or peel an orange. Have students raise their hands when they can smell the match or the orange. Ask them whether they think the scent particles from the match or the orange are     PM 10 or PM 2.5 (they are PM 2.5). Students should be able to detect that there are particles by the smell, but they are too small to see.

13. Demonstrating Smog (optional)

Prepare to create the demo by putting a piece of aluminum foil over a glass jar to make a lid. Take the lid off, and put ice on it to cool it down. In the meantime, put a small amount of water in the jar and swirl it around to cover the inside. Light a small strip of paper with a match and drop the paper and the match into the jar. Quickly cover the jar with the foil lid. Allow students to observe. (What happens is the combustion products combine with the cooled water vapor to create a hazy smog: PM2.5).

Connection to Module 2: Smog that causes a Code Red Day is the anchor phenomenon in Module 2. If you’ve taught this module with students, showing them how particulate matter and moisture can create smog (and not just ozone) may be a particularly interesting demonstration.

14. Visualizing Particulate Matter (optional)

Show students the New York Time’s visualization and article on particulate matter. Use the graph feature to compare the amount of PM 2.5 in different cities around the US and the world.

15. Formative Assessment

Show students the pictures of the trucks from the original phenomenon (either the ones below or the ones you showed them) and remind them of the questions they came up with during their first activity about the trucks and their exhaust. Have them use the information on their notes sheet to answer at least one of these questions using accurate scientific terminology (ex. “what is truck exhaust made of?”. You may want to remind students that not all products of combustion are visible.

  • Both PM 10 and PM 2.5 are coming from the truck. While the soot (PM 10) is visible, other chemicals that make up PM 2.5 are also likely present even though we can’t see them.

Truck with Smoke Cropped
Rolling coal truck.jpg
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