What is the definition of vapor pressure?

Vapor pressure is defined as the pressure exerted by the vapor of a substance when it is in dynamic equilibrium with its liquid phase. This means that at a given temperature, the rate of evaporation of the liquid equals the rate of condensation of the vapor, resulting in a stable pressure of the vapor above the liquid. This concept is critical in understanding how liquids behave under varying temperatures and how they transition between states.

In this definition, the system is closed, allowing the vapor to build up until saturation is reached, creating a specific vapor pressure unique to each substance at a particular temperature. This thermodynamic equilibrium is crucial for applications in organic chemistry, such as distillation and other separation techniques.

The other options do not accurately describe vapor pressure. For instance, the notion of pressure in an open container does not reflect the equilibrium concept necessary for defining vapor pressure. Similarly, the descriptions regarding gas at maximum temperature and gas being compressed to a liquid state do not encapsulate the equilibrium condition that is essential for understanding vapor pressure.