Water from a Match: The Principles of Combustion
|Ben Stark||Illinois Institute of Technology|
|BCPS, 182 Life Sciences|
|Chicago IL 60616|
to investigate the chemistry of combustion and, to some extent, of photosynthesis; appropriate for grades 3-high school, with sophistication of explanations scaled appropriately
matches (wooden are best, but cardboard ones also work), watchglasses (about 10-15 cm in diameter) or large glass slides (about 8 by 10 cm); can be done as a class demonstration or by groups as small as two (or even by individuals); obviously suitable only as a demonstration for students younger than high school age
Hold the watchglass or slide with one hand while holding a lighted match about 2 cm under the slide with the other hand (it may help to have one person hold the slide and another light and hold the match). It may also help to move the match slowly (back and forth or around) underneath the slide. The students are asked to observe and tell/record what they see. What they should see is fog on the underside of the slide above where the match has been.
The fog is condensed water vapor. When the match burns, one of the products of this combustion is water (released initially as vapor, which is invisible). The vapor rises and is trapped under the glass, which is cool enough to cause the vapor to condense (but see also the next paragraph); we see this as the fog on the underside of the glass. Why should the burning of wood (or cardboard, which is processed wood) produce water? When plants produce the building blocks of wood (ultimately by the process of photosynthesis) the carbon, oxygen, and hydrogen atoms of carbon dioxide and water are disassembled from each other and reassembled into a more complicated arrangement; this is an energy requiring process, with sunlight providing the energy. When the wood (or cardboard) burns, this process is reversed: the carbon, oxygen, and hydrogen atoms are disassembled from each other and reassembled into simpler arrangements (carbon dioxide and water) with the release of energy as heat and light. As pointed out above the water that is released makes the fog. The carbon dioxide condenses at a temperature much colder than that of the watchglass/slide and so is not visible.
It should be noted that the fog shows up best when the watchglass or slide is cool. If the watchglass or slide gets too warm, the fog may not form (i.e., the water produced by the combustion of the match won’t be able to condense); if a slide is used a second time shortly after being used for the experiment it may still be too warm (from the heat of the match flame) and a fresh watchglass or slide should be used.
A brief lab report describing the procedures used, observations made, and an explanation of what happened (see above section) would be a good way to assess this lesson. Obviously the report should increase in sophistication and detail for students in the higher grade levels.
Any good high school biology text should have a description of the chemistry of photosynthesis; any good high school chemistry text should have a description of combustion. The biology text should also discuss "respiration" which is the biological version of combustion.