Which statement correctly differentiates flash point and autoignition temperature?

Which statement correctly differentiates flash point and autoignition temperature?

• A. Autoignition temperature is the temperature at which vapors can momentarily ignite.
• B. Flash point is the ignition temperature that requires an external spark.
• C. Flash point is the lowest temperature at which vapors can momentarily ignite; autoignition is the temperature at which a material will ignite without an external ignition source.
• D. Autoignition occurs only at room temperature.

Answer: (C)

Ignition behavior hinges on whether an external ignition source is present. The flash point is the lowest temperature at which the vapors above a liquid can form an ignitable mixture with air and be ignited by a source such as a spark or flame. That ignition is typically brief—a flash rather than sustained flame—because the mixture may momentarily ignite but not continue burning once the flame source is removed. Autoignition temperature is the temperature at which the same vapor-air mixture will ignite on its own, without any external ignition source, once the material is hot enough to provide the required energy for ignition.

This distinction explains why the statement identifying the flash point as the minimum temperature for a vapors’ ignition in the presence of an ignition source, and the autoignition temperature as the point at which it will ignite without any external source, is the best description. The other options either misstate what autoignition means, imply that flash point is simply an ignition temperature in general, or claim something false about autoignition conditions (such as occurring only at room temperature).