What is vapor pressure?

Vapor pressure refers specifically to the pressure exerted by the vapor molecules in equilibrium with their liquid phase at a given temperature within a closed system. This pressure is a crucial concept in understanding how liquids behave when they start to evaporate. As liquid molecules escape into the vapor phase, they exert a force on the walls of the containing vessel, which is what is referred to as vapor pressure.

Under ideal conditions, every liquid has a characteristic vapor pressure that increases with temperature. This is why the option that defines vapor pressure as the pressure a liquid exerts on its surroundings is accurate. It captures the essence of vapor pressure as the result of the dynamic equilibrium between the liquid and its vapor.

The other options describe different concepts: the temperature at which a liquid boils relates to the point where its vapor pressure equals atmospheric pressure; the atmospheric pressure at sea level is a standard measurement of the weight of the atmosphere above; and the force exerted by vapor in a closed container is not as comprehensive because it does not specifically address the relationship to the liquid phase in equilibrium. Recognizing this nuanced definition of vapor pressure is essential for understanding various applications in fluid mechanics and thermodynamics in plumbing design.