Which of the following defines vapor pressure?

Vapor pressure is specifically defined as the pressure exerted by water vapor in the air when the water is in equilibrium with its liquid form. This concept arises from the molecules of water in a liquid state evaporating into the air until a balance is achieved between the number of molecules returning to the liquid phase and those entering the vapor phase. The vapor pressure thus reflects the tendency of the molecules to escape from the liquid into the gaseous state, and it is influenced by temperature; as temperature increases, the vapor pressure typically increases as well.

This definition is critical in understanding various respiratory care topics, as vapor pressure is important in processes such as humidification and gas exchange in the lungs. Furthermore, the unit of measurement for vapor pressure is typically expressed in millimeters of mercury (mmHg) or pascals (Pa), indicating its relationship to atmospheric pressure conditions.

The other choices reflect concepts related to vapor but do not accurately capture the essence of vapor pressure. For example, the maximum amount water can hold in vapor form refers to saturation and varies with temperature but doesn’t define vapor pressure itself. Total atmospheric pressure describes the weight of air in the atmosphere and is not specific to water vapor. The pressure needed to boil a liquid refers to the conditions under which