Question 6

The Correct Answer is C **Explanation of C (Fire Point):** The fire point is the lowest temperature at which a liquid, under specified conditions, gives off vapors in sufficient quantity to ignite and sustain combustion for at least five seconds. This definition precisely matches the condition described in the question: the lowest temperature at which the vapors will ignite and cause **self-sustained combustion** (i.e., combustion that continues after the ignition source is removed). **Why A (Autoignition Temperature) is incorrect:** The autoignition temperature (or spontaneous ignition temperature) is the lowest temperature at which a substance will spontaneously ignite (without an external spark or flame) under normal atmospheric pressure. The question specifies ignition "in the presence of a spark or flame," which contradicts the definition of autoignition. **Why B (Flash Point) is incorrect:** The flash point is the lowest temperature at which a liquid gives off enough vapor to form an ignitable mixture with air near the liquid's surface. While a spark or flame will cause a brief "flash" (momentary ignition), the vapors are not produced at a rate sufficient to sustain combustion. The flash point is typically a few degrees lower than the fire point. **Why D (Vaporization Temperature) is incorrect:** Vaporization temperature is a general term describing the temperature at which a substance changes from a liquid to a gas (e.g., boiling point). While vaporization must occur for ignition, this temperature does not specify the concentration of vapors necessary for ignition or, critically, for sustained combustion.