Fire: friend and enemy
Fire let early humans cook food, fire pottery, and smelt metals, but fire out of control destroys homes and forests. What does it take to make something burn, and what does it take to stop it?
The conditions for burning
Combustion燃烧, or burning, is a fast reaction of a fuel with oxygen that gives off light and heat. Burning needs all three of these at once, often drawn as the fire triangle:
- Fuel (something that can burn);
- Oxygen (or air) in contact with the fuel;
- Heat: the fuel must reach its ignition temperature.
The ignition temperature着火点 is the lowest temperature at which a fuel catches fire. White phosphorus ignites at just 40 °C, so it’s stored under water; red phosphorus ignites at 240 °C.
A classic experiment tests these conditions. A copper sheet sits on top of a beaker of 80 °C water, with white phosphorus on one end and red phosphorus on the other, and another piece of white phosphorus lies under the hot water. Change the water temperature, or bubble oxygen onto the phosphorus under water, and see which pieces burn:
| Fuel | Oxygen | At ignition temp. | Result | |
|---|---|---|---|---|
| White P on copper | ||||
| Red P on copper | ||||
| White P in water |
Ignition temperatures: white phosphorus 40 °C, red phosphorus 240 °C. The copper sheet is about as hot as the water.
Putting out fires
Take away any one side of the fire triangle and the fire goes out. Every way of fighting a fire removes one of the conditions:
| What it removes | Examples |
|---|---|
| The fuel | Cutting a firebreak in a forest; turning off the gas valve |
| Oxygen | Putting a lid on a grease fire; snuffing out an alcohol burner with its cap; smothering a fire with sand |
| Heat (cooling below the ignition temperature) | Spraying water on burning wood |
Carbon dioxide extinguishers leave no residue, so they’re used on fires near electrical equipment, documents, and delicate instruments.
Safety:
- A flammable gas or fine dust (natural gas, flour, coal dust) mixed with air can explode if it meets a flame or spark. Gas stations, flour mills, and coal mines ban open flames.
- If you smell a gas leak, turn off the gas if you can and open windows, then leave and call for help from outside. Don’t flip light switches or use electrical devices inside, since a spark could set off an explosion.
- To escape a fire, cover your nose and mouth with a damp cloth and stay low, because hot smoke and toxic gases rise and collect near the ceiling.
Fuels
Coal, petroleum, and natural gas are fossil fuels化石燃料. They formed from the remains of ancient living things over millions of years and can’t be replaced on a human timescale. Natural gas is mostly methane:
Fuels should burn completely: give them enough air and a large surface in contact with the air. Incomplete burning produces carbon monoxide and soot, wasting fuel and polluting the air. Sulfur dioxide and nitrogen dioxide from burning fossil fuels cause acid rain.
Alternative energy: hydrogen burns to produce only water and releases a lot of heat, making it the cleanest fuel, but it’s still expensive to produce and hard to store. Ethanol can be made by fermenting crops; most US gasoline contains about 10% ethanol (E10). Solar, wind, geothermal, and tidal energy are other options.
Example 1: Making sense of the phosphorus experiment
With 80 °C water: the white phosphorus on the copper burns, the red phosphorus on the copper doesn’t, and the white phosphorus under water doesn’t. When oxygen is bubbled onto the underwater white phosphorus, it burns. What does each result show?
- White vs. red phosphorus on the copper. Both are fuels, both touch air, and both are at about 80 °C. White phosphorus burns but red doesn’t, because 80 °C is above white phosphorus’s ignition temperature (40 °C) but below red phosphorus’s (240 °C). Burning needs the ignition temperature.
- White phosphorus on the copper vs. under water. Same substance, same temperature, but the piece under water has no oxygen, so it doesn’t burn. Burning needs oxygen.
- After bubbling in oxygen. The underwater phosphorus burns, confirming point 2.
- Conclusion. Fuel, oxygen, and ignition temperature: all three are required.
Example 2: A grease fire on the stove
Oil in a frying pan catches fire. Why should you cover it with a lid instead of throwing water on it?
- The lid. It separates the oil from the air, cutting off oxygen, and the flames go out.
- Why not water? Oil is less dense than water and floats on top, still burning. Water hitting very hot oil boils violently and splatters burning oil everywhere, spreading the fire.
- Conclusion. Choose the method to suit the fuel, but every method works by removing one side of the fire triangle.
- Putting a lid on a burning frying pan
The lid cuts the oil off from the air.
- Cutting a firebreak ahead of a forest fire
Removing the trees leaves nothing to burn when the fire arrives.
- Spraying water on burning wood
Water absorbs heat as it evaporates, cooling the wood below its ignition temperature.
- Capping an alcohol burner
The cap cuts off the air.
- Turning off a gas burner
No more gas (fuel) reaches the flame.
- Throwing sand on a small oil spill fire
Sand separates the burning oil from the air.
- Blowing out a birthday candle
The puff of air carries heat away, cooling the wick below its ignition temperature.
- Using a carbon dioxide extinguisher
Carbon dioxide is denser than air and blankets the fuel, keeping air away.
Common mistakes
Mistake 1: “Water lowers the fuel’s ignition temperature.” The ignition temperature is a property of the substance and doesn’t change. Water lowers the temperature of the fuel to below its ignition temperature.
Mistake 2: “Anything at its ignition temperature will burn.” All three conditions are needed. The white phosphorus under hot water is above its ignition temperature but has no oxygen, so it doesn’t burn.
Mistake 3: “If you smell gas, turn on the light to look for the leak.” Switches can make sparks and set off an explosion. Shut off the gas if you can, ventilate, and leave.
Quick check
In the 80 °C water experiment, the white phosphorus on the copper burns but the red phosphorus doesn't. This shows that burning requires
Both are fuels and both touch air. The only difference is ignition temperature: 80 °C is enough for white phosphorus but not for red.
Which statement about fuels is correct?
Fossil fuels aren't renewable, incomplete burning produces carbon monoxide and soot, and natural gas is mostly methane.
Summary
- Burning needs fuel, oxygen, and the ignition temperature, all at once. White phosphorus ignites at 40 °C, red phosphorus at 240 °C.
- Fires go out when you remove the fuel, cut off oxygen, or cool the fuel below its ignition temperature.
- Flammable gases and dusts can explode when they meet a flame or spark. With a gas leak, shut off the gas, ventilate, and don’t touch switches.
- Coal, oil, and natural gas are nonrenewable fossil fuels; hydrogen is the cleanest fuel.
On to AP (for later)
Burning is exothermic. AP topic 6.1 shows the energy released on an energy diagram, and topic 6.8 lets you calculate a fuel’s heat of combustion from enthalpies of formation. Why does a fuel need to reach its ignition temperature? Topic 5.5, the collision model, explains that molecules need enough energy to get over the activation energy before they can react. Once burning starts, the heat it releases gives neighboring molecules that energy, so the fire keeps going.