What defines a flammable liquid?

A flammable liquid is defined by its flash point, which is the lowest temperature at which the vapor of the liquid can ignite in the presence of an ignition source. The correct definition states that a flammable liquid has a closed-cup flash point below 100°F. This measurement is crucial in safety protocols, particularly in laboratory environments, where the risk of fire must be managed effectively. When the flash point is below this threshold, the liquid can easily ignite from common sources of ignition, making it necessary to handle such liquids with care and to implement appropriate safety measures.

The other options do not accurately capture the essential criteria for defining flammable liquids. While spontaneous ignition is a concern with certain hazardous substances, it does not pertain specifically to the classification of flammable liquids. Similarly, not all liquids that are hazardous at room temperature are considered flammable; other properties such as toxicity and reactivity may apply instead. Lastly, the production of toxic vapors when heated relates to the health hazards of a substance rather than its flammability. Thus, the closed-cup flash point is a critical factor in determining whether a liquid is classified as flammable.