Vapor Pressure is defined as the upward pressure exerted by the substance on the atmosphere.

Vapor Pressure is defined as the upward pressure exerted by the substance on the atmosphere.

• A. The downward pressure exerted by the atmosphere on the liquid
• B. The upward pressure exerted by the substance on the atmosphere
• C. The internal pressure required to boil
• D. The external pressure needed to compress gas

Answer: (B)

Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid, i.e., the gas phase above the liquid. When molecules escape the liquid surface, they form a vapor that collides with the surrounding air and container walls, pushing upward on the atmosphere. This pressure increases as temperature rises because more molecules have enough energy to evaporate, increasing the amount of vapor in the gas phase. Saying it is the upward pressure exerted by the substance on the atmosphere matches this idea: the liquid’s vapor itself pushes on the air above it. Boiling occurs when this vapor pressure equals the external pressure, but the pressure itself is the pressure of the vapor above the liquid, not the pressure of the liquid on the atmosphere or any external compression requirement.

Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid, i.e., the gas phase above the liquid. When molecules escape the liquid surface, they form a vapor that collides with the surrounding air and container walls, pushing upward on the atmosphere. This pressure increases as temperature rises because more molecules have enough energy to evaporate, increasing the amount of vapor in the gas phase. Saying it is the upward pressure exerted by the substance on the atmosphere matches this idea: the liquid’s vapor itself pushes on the air above it. Boiling occurs when this vapor pressure equals the external pressure, but the pressure itself is the pressure of the vapor above the liquid, not the pressure of the liquid on the atmosphere or any external compression requirement.