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Fire Wars

Classroom Activities

Objective
To explore the conditions needed for combustion to occur.

Demonstration I

• metal pan
• candle firmly in holder
• matches
• heat resistant glass beaker (large enough to fit over the candle)
• tongs
• safety glasses
• steel nail
• small piece of super fine steel wool

Demonstration II

• long matches

Demonstration I

1. Wear safety glasses during the entire demonstration and do not allow students to crowd around the demonstration table. Tell students they will be investigating the components necessary to allow a fire to burn. Before you do the demonstration, have students consider these questions: In a wildland area, which would burn more quickly: small trees and brush, or large trees? Why?

2. Have students describe what they know about the chemical process of fire. What do all fires have in common? What components are needed for combustion?

3. To help students understand how combustion occurs, place the candle in the metal pan, light the wick and let the candle burn for 10 seconds. Then put the glass beaker over the candle until it goes out.

4. Discuss with students what made it possible for the candle to burn. Specifically, what components were needed for fire to ignite and be sustained? (See Activity Answers for more information.) What could have been done to prolong the fire?

5. Now have students predict whether a steel nail or steel wool will burn. Following their predictions, attempt to burn the nail. (The nail may become covered with soot, but will not burn.) Next, stretch out a small piece of compacted steel wool, and use the tongs to hold the steel wool over the flame. (The steel wool will quickly light and burn.)

6. Both items are made up of the same material—steel. Why did the steel wool burn but not the nail? Revisit the question from the start of the demonstration: Based on what they saw burn in class, would small trees and brush or large trees burn more quickly? Why?

7. Ask students what might happen if all small fires were extinguished? (Suppression can lead to a buildup of fuels available for fire, which can lead to bigger fires.) What might happen if all fires were allowed to burn uncontrolled? (A loss of lives, property, and wildlands.) Point out that not all fires are detrimental; fire may be necessary to keep some wildlands healthy. Land managers and firefighters must consider a number of factors when deciding to fight a fire or recommend a prescribed burn. (See Activity Answers for more information.)

Demonstration II

1. Tell students you will be demonstrating an aspect of fire behavior. Show students the long match you will light and ask them how fast the fire will travel if the match is level, if the match is angled with the match head pointing up, and if the match is angled with the match head pointing down.

2. Once students have replied, light the match and conduct the demonstration. (If level, the fire moves along steadily; with match head up, the fire moves downward more slowly because the heat is flowing away from, rather than toward, the wood fuel source; and with match head down, the fire moves quickly upward as the wood fuel source is directly heated and engulfed by the flame.) (See Activity Answers for more information.)

Wildland fires are not limited to densely populated forests. They can occur in a number of ecosystems, including:

Major Ecosystems*

At the turn of the century, there was no agency responsible for fighting wildland fires, which at the time were believed to be unmanageable. It wasn't until 1905, when the Forest Service assumed control over the national forests, that a major debate arose over appropriate fire management strategy—whether to suppress all fires or carry out regular controlled burning. A disastrous 1910 fire season contributed to the decision to fight all forest fires. The debate resurfaced in the 1960s, when critics contended that under the right circumstances prescribed or controlled natural fires can aid natural biotic processes and help reduce the risk of extreme fires due to excess fuel accumulations. However, prescribed burns have their own drawbacks; they can saturate large pockets of air with smoke and they can escape control.

Demonstration I
Fire is a combination of heat, fuel, and oxygen. The match serves as the ignition source, which provides the heat necessary to ignite the fuel (the wax). The heat liquifies the wax, which is drawn up by the wick to fuel the fire. finally, as demonstrated by the flame going out after the beaker is placed over the candle, the fire requires some component in the air. That component is oxygen.

Scientists have refined their understanding of these three components. They know that for a material to ignite, it must first reach its ignition temperature, which depends on such factors as a material's properties, density, and surface area. While a candle flame does not provide enough heat for a nail to reach its ignition temperature (whereas a blast furnace might), it can easily cause a thin strand of steel wool to reach ignition temperature. (Soot that builds up on the nail is merely unburned carbon from the candle.) The nail is representative of a thick tree trunk, while the steel wool represents the smaller fuel sources like tinder found in forests. Both can burn, but one can be ignited more easily than the other.

In order to contain a fire, firefighters must lower the heat, remove the fuel source, or deplete the oxygen. Structural firefighters often use water to lower the heat, while wildland firefighters often try to remove the fuel source. Other factors that allow fire to start and spread include weather (wind, temperature, and relative humidity), and the topography of the affected area.

Demonstration II
The fire traveled most quickly when the match head was pointed down, demonstrating how slope affects fire behavior. Fire can blast up slopes, which is why firefighters are taught they cannot outrace an uphill blaze.

This demonstration aligns with the following National Science Education Standards:

Grades 5-8/9-12

	Science Standard B: Physical Science