Volatility describes the tendency of a liquid to vaporize. This property is inversely related to the liquid's boiling point; a lower boiling point indicates higher volatility. The term "highly volatile" implies a substance that vaporizes with extreme ease at ambient conditions. A liquid with a boiling point below 68°F (20°C) will boil and turn into a gas at or below typical room temperature. This represents an extreme level of volatility, creating a significant vapor hazard that readily forms ignitable mixtures with air. Among the given choices, this is the most accurate description of a highly volatile liquid.

B. 70°F (21°C): This is a specific temperature, not a descriptive range, and is functionally similar to option A, but less definitive as a threshold for extreme volatility.

C. 100°F (38°C): This temperature is the upper boiling point limit for NFPA Class IA flammable liquids. While all Class IA liquids are volatile, this category includes substances that are less volatile than those that boil at room temperature.

D. Greater than 212°F (100°C): This describes liquids with low volatility, such as water and combustible liquids (e.g., motor oil), which is the opposite of highly volatile.

1. NFPA 921, Guide for Fire and Explosion Investigations, 2021 Edition.

Section 5.6.4 Volatility: "Volatility is the tendency of a substance to produce vapor... In general, liquids with low boiling points... are more volatile than liquids with high boiling points." This reference establishes the fundamental principle that lower boiling points correspond to higher volatility.

2. NFPA 30, Flammable and Combustible Liquids Code, 2021 Edition.

Section 4.3.1.1 (Class IA Liquids): This section defines the most volatile classification of flammable liquids as those having "boiling points below 100°F (37.8°C)." Option A describes a subset of these liquids at the most extreme end of the volatility scale.

3. Drysdale, D., An Introduction to Fire Dynamics, 3rd Edition, John Wiley & Sons, 2011.

Chapter 5, Section 5.2.1, "Flammable Vapour: The Flashpoint": The text discusses that liquids with low boiling points have high vapor pressures at ambient temperatures, making them volatile. It notes that substances like diethyl ether (boiling point 34.5°C) are considered very volatile. A liquid boiling below 20°C would be even more so. (Page 169).