What is autoignition?

• A. Lowest temperature which a substance will spontaneously ignite.
• B. The temperature at which a liquid boils.
• C. The temperature at which vapors ignite only with a spark.
• D. The temperature at which a fuel combusts completely.

Answer: (A)

Autoignition is the temperature at which a material will ignite by itself in air, without any external flame, spark, or heater. At that temperature, the heat produced by the material’s own slow chemical reactions causes ignition to occur. The statement describing this as the lowest temperature at which a substance will spontaneously ignite captures that self-ignition threshold. It’s not about boiling, which is a phase change to vapor; it’s not about vapors needing a spark to ignite, since autoignition happens without one; and it’s not about burning completely, which is about how thoroughly the fuel combusts after ignition.