What does vapor pressure indicate about a liquid?

Vapor pressure is a crucial concept in understanding the behavior of liquids, particularly in terms of their ability to evaporate. It refers to the pressure exerted by a vapor in equilibrium with its liquid phase at a given temperature. A high vapor pressure indicates that a significant number of molecules are escaping from the surface of the liquid into the vapor phase, meaning the liquid evaporates quickly. This property is especially important in fire investigations, as substances with high vapor pressures can potentially create flammable atmospheres, contributing to the spread and intensity of a fire.

The other options do not accurately reflect what vapor pressure indicates. For instance, the freezing point of a liquid is determined by other factors such as its molecular structure and intermolecular forces, rather than vapor pressure. Similarly, the density of a liquid relates to its mass per unit volume and is not directly tied to its vapor pressure. Lastly, while the ease of ignition of a liquid can be influenced by its vapor pressure, it is more appropriately assessed through flash point and flammability limits rather than vapor pressure alone. Thus, vapor pressure primarily serves as an indicator of how quickly a liquid will evaporate.