What Is Vapor Pressure?

Under certain external conditions, the liquid molecules in the liquid will evaporate into gaseous molecules, and at the same time, the gaseous molecules will hit the liquid surface and return to the liquid state. This is a two-phase change that occurs in a single-component system, and equilibrium can be reached after a certain period of time. At equilibrium, the content of gaseous molecules reaches a maximum. The pressure that these gaseous molecules can produce when they hit the liquid is referred to as the vapor pressure.

1. Molecules with higher energy in liquids have a tendency to escape from the liquid surface and enter the gas phase (escaping tendency). This is the reason for the generation of gaseous molecules and is the nature of liquids. Vapor pressure is a quantity used to measure the degree of this tendency. It is a property of liquids. When the external conditions (temperature, pressure) are constant, there is a certain value. ^[1]

When the vapor pressure of a liquid reaches an external pressure, the liquid is boiled, and the temperature at this time is the boiling point of the liquid under the external pressure.

Take water as an example. If the water temperature reaches 100 degrees Celsius at one atmospheric pressure (101.325kPa), the water vapor pressure is exactly one atmosphere pressure, and the water begins to boil. 100 degrees Celsius is the boiling point of water at one atmosphere pressure.

For another example, the phenomenon of "water boiled and cooked rice" will appear in high altitude areas. The essence of this phenomenon is that the temperature of the water will not reach 100 degrees Celsius when it boils, and it will not reach when it is heated. This phenomenon helps to understand the characteristics 3 and 4 of liquid vapor pressure. At high altitudes, the air is scarce and the external pressure is lower than one atmosphere. According to the characteristics of vapor pressure4, the vapor pressure of water will decrease, but the change will be small. According to the characteristic 3 of the vapor pressure, as the temperature increases, the vapor pressure of water increases, but compared with the low external pressure of high altitude, the water can reach the low pressure of high altitude without boiling at this time, and boiling occurs. , Causing the boiling point to decrease. This is why Takayama ca nt cook rice with water but can steam it with water. Increase the temperature to reach or exceed 100 degrees Celsius).