Changes Around Us: Physical and Chemical
Ignition temperature
Find out why paper does not catch fire on its own, even though it can burn.
Paper burns easily, yet a newspaper can lie in the air all day without catching fire. So fuel and oxygen are not enough. What else is needed?
Activity 5.6 (with an adult). Hold a piece of paper with tongs and bring a lit match to it: it catches fire at once. Now take another piece and use a magnifying glass to focus sunlight into the smallest, brightest spot on it. Hold it steady. Soon it smokes, then catches fire. No flame was needed!
The focused sunlight heated the paper until it was hot enough to burn. The lowest temperature at which a substance catches fire is its ignition temperature. A lit match is already hotter than paper's ignition temperature, which is why the paper caught fire at once.
So combustion needs three things together, drawn as the fire triangle:
- a fuel (a combustible substance)
- oxygen
- enough heat to bring the fuel to its ignition temperature
Take away any one and the fire cannot start, or it goes out.
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