Dec. 19, 2014

Christmas Tree Combustion

by Ariel Zych

Click to enlarge images
Are Christmas Trees a Holiday Hazard?

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.

Target Ages: 9-15 years with an adult
Content Areas: Chemistry, Physical Science
Topics: Combustion, ignition, oxidation, organic compounds
Time required: 30-60 minutes
NGSS: Chemical Reactions - MS-PS1-2MS-PS3-4, Stability and Change, Energy and Matter
Combustion Review:

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:

This is a simplified explanation of combustion. Indeed, there is a wide variety of suitable combinations of organic fuel, oxygen-containing substances, and energy sources that will result in combustion. Depending on the fuel used, other substances such as soot will likely be produced as well.
To learn more about how a fire starts and what a flame actually is, watch the video “What is a Flame?” (The video is the 2012 winner of the “Flame Challenge,” a challenge to scientists to answer big science questions for 11-year-olds).
A Conifer Flammability Experiment:


- One fresh sprig of fir, pine, cedar, or ash, trimmed to under five inches long
- One old, dry sprig of the same tree type, trimmed to under five inches long*
- Extra pieces of fir, pine, cedar, or ash sprigs for experimenting
- Box of matches
- Container of water
- Small cookie sheet covered with aluminum foil
- Access to a sink or other non-flammable surface (marble, slate, lab bench) that is stable and water-resistant
- Stopwatch or timer

* 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.

Safety Information

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:

Flammability Protocol:
  1. Place the cookie sheet firmly on a stable, fire- and water-resistant surface. 

  1. Pull off 10 individual needles from your fresh sprig.





  1. Form a tidy pile with the needles on the covered cookie sheet.  
  2. Kids should hold the container of water ready so that they can put out any flames or smoke when the experiment is finished.


  1. 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. 
  1. If needed, pour some water over any lasting flames or embers to extinguish them.
Repeat this experiment using one of the following modifications. Try only one at a time:
  • Use needles from an old sprig.
  • Use two matches instead of one.
  • Use large and small needles from different types of tree.
As you make your observations, apply your knowledge of combustion to try and figure out why some versions of the experiment result in a larger flame than others. Here are a few hints:
Hint #1: Water interferes with combustion
Water interferes with combustion in two ways. First, water molecules can saturate or coat the fuel, creating a physical barrier that prevents oxygen from reacting with the fuel and starting a combustion reaction.
The second, perhaps more important way is that water absorbs the energy—in this case, heat—required to ignite the fire in the first place. Water is very good at absorbing large amounts of heat, and when it does, it changes phase to steam. The steam rises, carrying heat with it and away from fuel sources—thus removing the energy needed to start a fire.   
The National Fire Prevention Agency - "Christmas Tree Fire"
A comparison of dry versus watered Christmas trees exposed to a flame:
Hint #2: The amount of energy matters
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. 
Hint #3: The amount of fuel matters
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: 
Science Friday Video - "The Goat Brigade: Preventing Wildfires in Southern California (feat. GoatPro)"

Related links
3D Up in Flames - Science Friday Video
Flaming Bubbles - Science Friday Video
About Ariel Zych

Ariel is Science Friday's education manager. She is a former teacher and scientist who spends her free time making food, watching arthropods, and being outside. You can follow her @arieloquent

The views expressed are those of the author and are not necessarily those of Science Friday.
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