What is a flammable liquid characterized by?

A flammable liquid is characterized by having a closed-cup flash point below 100°F. This classification is critical because the flash point is the lowest temperature at which a liquid can form an ignitable mixture in air. Liquids with a flash point below this threshold can easily ignite at normal working temperatures, posing a significant fire risk.

The definition aligns with widely accepted safety standards, including those from NFPA and OSHA, that outline the thermal characteristics of liquids and influence how they are handled in various industries. Understanding this concept is essential for proper fire safety practices and risk assessment during investigations of fire incidents.

In this context, a closed-cup flash point above 100°F would categorize the liquid differently, potentially as a combustible liquid rather than a flammable one. Similarly, a vapor pressure above 2068 mm HG at 100°F would not directly indicate flammability but rather the volatility of the liquid under certain conditions. The notion of varying flash points with temperature, while pertinent in discussions of thermodynamics, doesn't define a liquid as flammable. The clear demarcation provided by the flash point below 100°F makes option B the defining characteristic of flammable liquids.