The Christmas tree, that staple of holiday merriment, seems like it would be a prime source of home fires. For instance, Christmas trees contain some flammable compounds, as well as plenty of organic compounds (such as cellulose) that can serve as fuel for a fire; their needles provide lots of surface area that can come in contact with oxygen (which is necessary for fires—see “Combustion Review,” below); and they’re often placed near sources of heat and electrical energy, such as electrical outlets, holiday lights, and festive votive candles, which can ignite a fire.
But according to the National Fire Prevention Association, Christmas tree fires only accounted for 0.1 percent of reported home fires from 2007 to 2011. However, during those same years, the fires that did involve Christmas trees were nearly four times as likely to cause deaths, and were more likely to occur after Christmas than before.1
So why don’t more Christmas trees catch fire, and why are they more likely to catch fire after Christmas? Find out the answers by experimenting with the flammability of Christmas tree needles using the protocol below. Explore how the tree’s age, its dryness, and the heat source affect whether or not needles burn and how quickly, and how big the flame is.
Topics: Combustion, ignition, oxidation, organic compounds
Time required: 30-60 minutes
Fire is generally the result of a type of chemical reaction called combustion. Combustion can occur when fuel in the form of an organic substance (that is, one that contains hydrogen and carbon) encounters an energy source (such as heat) in the presence of oxygen. If there is sufficient fuel, oxygen, and energy to set off a chain reaction, combustion will occur, producing heat, light, carbon dioxide, and water as a result. Here is the unbalanced, general combustion reaction:
* If you don’t have any old, dry sprigs on hand, you can make some by placing new sprigs on a metal tray to dry naturally or by placing the tray in a traditional convection oven set to low (<170°F) for a couple hours. Check back frequently; you’ll know the sprigs are dry when the needles pop off when touched.
This experiment should be performed under adult supervision, in a well-ventilated area with access to running water. All experimenters should wear safety goggles and tie back hair and loose clothing before starting this activity. Start with the basic flammability protocol before making changes:
Place the cookie sheet firmly on a stable, fire- and water-resistant surface.
- Pull off 10 individual needles from your fresh sprig.
Form a tidy pile with the needles on the covered cookie sheet.
- Kids should hold the container of water ready so that they can put out any flames or smoke when the experiment is finished.
- Have an adult strike a match, count to five, then rest it across the pile of needles. Watch what happens as the flame reaches the needles.
- If needed, pour some water over any lasting flames or embers to extinguish them.
- Use needles from an old sprig.
- Use two matches instead of one.
- Use large and small needles from different types of tree.
A comparison of dry versus watered Christmas trees exposed to a flame:
Different substances require different amounts of energy, usually in the form of heat, to ignite (or “activate”) a combustion reaction. Substances that are considered fire hazards are usually made out of material that requires less energy to activate a combustion reaction. With enough energy, most organic substances can be ignited.
If more fuel is available, a fire can burn longer and produce more heat. Forest fire experts often try to remove fuel by tilling, cutting, or harvesting trees and shrubs from surrounding areas. Limiting the amount of fuel available reduces the hotness, spread, and rate of a forest fire. Here’s an example from a SciFri video of one unique way forest fire managers remove fuel: