Grades: 5 - 8
Purpose: To show how burning can release invisible and visible pollution.
- birthday candle
- lump of clay (tea light candles which have a broad, flat base may be used as a substitute for birthday candles and clay)
- heat-resistant glass cover, such as a petri dish or heat resistant microscope slide
Time: 20 - 30 minutes
Light the candle and ask students what is burning and why. Discuss why the candle does not show any smoke. Pass the glass cover over the candle to cool the flame. The glass will turn black. Ask the students what happened. Blow out the candle and observe the smoke. You can also try re-lighting the candle through its smoke trail by placing a lighted match within the trail.
CAUTION: An adult should demonstrate this activity to students. Students should be reminded to stay a safe distance away from the candle to prevent burns.
Expected results and the reasons they occur:
Mostly candle wax is burning, but a bit of the wick is burning also. The candle does not show any smoke because it burns efficiently with lots of heat/light and a little waste (i.e. soot, which is unburned fuel). Cooling the flame by passing a cool glass over it reduces efficiency, and the glass collects unburned fuel. Blowing out the candle reduces the temperature of the tiny burning ember on the candle wick, producing smoke (i.e. unburned fuel). If the candle re-lights through the smoke, it is showing that smoke and soot are unburned fuel -- the result of incomplete combustion -- and can be burned when re-ignited.
Activities adapted from the Puget Sound Air Pollution Control Agency's "Clean Air Express," and from "Where's the Air," Wisconsin Department of Natural Resources.