Vapour pressure is a measure of the tendency of a material to change into the gaseous or vapour state, it increases with temperature. boiling point..
Vapour pressure, pressure exerted by a vapour when the vapour is in equilibrium with the liquid or solid form, or both, of the same substance—i.e., when conditions are such that the substance can exist in both or in all three phases. Vapour pressure is a measure of the tendency of a material to change into the gaseous or vapour state, and it increases with temperature. The temperature at which the vapour pressure at the surface of a liquid becomes equal to the pressure exerted by the surroundings is called the boiling point of the liquid.
A vapor refers to a gas-phase material that that normally exists as a liquid or solid under a given set of conditions. As long as the temperature is below a certain point (the critical temperature; this varies for each substance), the vapor can be condensed into a liquid or solid with the application of pressure. The most common example of a vapor is water vapor (gas phase water) at room temperature and one atmosphere of pressure. Do not confuse a vapor with a dust, a fine suspension of solid particles or a mist, a fine suspension of liquid droplets. A good synonym (alternate word) for vapor is gas. When a substance turns changes from a solid or liquid into a gas, the process is called vaporization. The material is said to vaporize or evaporate.
In the Macroscopic View, the vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. Examples:
The vapor pressure of a liquid varies with its temperature, as the following graph shows for water. The line on the graph shows the boiling temperature for water.
As the temperature of a liquid or solid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases. The vapor pressure of a liquid can be measured in a variety of ways. A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer.
The vapor pressure in the Microscopic View, as following
Factors That Affect Vapor Pressure as following:
In the Macroscopic View, the vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. Examples:
substance | vapor pressure at 25 Deg C |
---|---|
diethyl ether | 0.7 atm |
bromine | 0.3 atm |
ethyl alcohol | 0.08 atm |
water | 0.03 atm |
The vapor pressure of a liquid varies with its temperature, as the following graph shows for water. The line on the graph shows the boiling temperature for water.
As the temperature of a liquid or solid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases. The vapor pressure of a liquid can be measured in a variety of ways. A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer.
The vapor pressure in the Microscopic View, as following
- When a solid or a liquid evaporates to a gas in a closed container, the molecules cannot escape
- Some of the gas molecules will eventually strike the condensed phase and condense back into it
- When the rate of condensation of the gas becomes equal to the rate of evaporation of the liquid or solid, the amount of gas, liquid and/or solid no longer changes
- The gas in the container is in equilibrium with the liquid or solid
- The pressure exerted by the gas in equilibrium with a solid or liquid in a closed container at a given temperature is called the vapor pressure
Microscopic equilibrium between gas and liquid. Note that the rate of evaporation of the liquid is equal to the rate of condensation of the gas. | Microscopic equilibrium between gas and solid. Note that the rate of evaporation of the solid is equal to the rate of condensation of the gas. |
Factors That Affect Vapor Pressure as following:
- Surface Area: the surface area of the solid or liquid in contact with the gas has no effect on the vapor pressure
- Types of Molecules: the types of molecules that make up a solid or liquid determine its vapor pressure. If the intermolecular forces between molecules are: relatively strong the vapor pressure will be relatively low, and relatively weak the vapor pressure will be relatively high.
ethyl ether (C4H10O)
Pvapor (25oC) = 520 torr
The relatively weak dipole-dipole forces and London dispersion forces between molecules results in a much higher vapor pressure compared to ethyl alcoholethyl alcohol (C2H6O)
Pvapor (25oC) = 75 torr
Although dipole-dipole forces and London dispersion forces also exist between ethyl alcohol molecules, the strong hydrogen bonding interactions are responsible for the much lower vapor pressure compared to ethyl ether. - Temperature: at a higher temperature, more molecules have enough energy to escape from the liquid or solid. At a lower temperature, fewer molecules have sufficient energy to escape from the liquid or solid.
Microscopic equilibrium between gas and liquid at low temperature. Note the small number of particles in the gas | Microscopic equilibrium between gas and liquid at high temperature. Note the large number of particles in the gas |
- [accordion]
- 1. Vapor Pressure
-
- 2. Evaporation, Vapor Pressure, and Boiling
-
- 3. Physical Properties of Hydrocarbons Vol 1 by Robert
- 4. Distillation and Vapor Pressure Measurement by Rey G.
- 5. Handbook of Vapor Pressure, 2nd Edition by Antoine
- 1. Vapor Pressure
- 2. Evaporation, Vapor Pressure, and Boiling
- 3. Physical Properties of Hydrocarbons Vol 1 by Robert
- 4. Distillation and Vapor Pressure Measurement by Rey G.
- 5. Handbook of Vapor Pressure, 2nd Edition by Antoine