Vapor+Pressure

Ethan Bennis
 * Vapor Pressure**

Vapor Pressure can be measured when molecules in a liquid state have enough kinetic energy to escape into the gas phase through vaporization or evaporation. As more molecules escape from the liquid phase into gas phase, some molecules run back into the liquid phase. Eventually as larger amounts of molecules are in the gas state the rate of molecules passing back into the liquid phase increases to equal the rate of new molecules becoming gaseous, and the vapor pressure around the liquid becomes constant. This condition is known as dynamic equilibrium. It may appear that nothing is occurring, but this is just an illusion since the many molecules are passing between each state at an equal rate.



If a liquid evaporates in an open environment (outside of a container), then the vapors that arise from the liquid can move away from the liquid, and thus are most likely never recaptured into the liquid phase. The process may continue until the liquid completely evaporates. Volatility is how well a liquid can evaporate under standard conditions. An example of a highly volatile liquid would be acetone, which evaporates almost instantly.



Another important thing to mention is that vapor pressure increases as temperature increases, as you would expect with any typical gas. Since a higher temperature of a liquid also means more kinetic engery from molecular movement, it is easier for molecules to break away from the liquid phase at a higher temperature. Because of this, liquids will evaporate quicker at a higher temperature than at a lower temperature, even if they aren't at boiling point.



Liquids boil when the vapor pressure is high enough that it equals the pressure on the surface of the liquid. Since it is no longer much easier for molecules to escape on the surface of the liquid, molecules can then vaporize throughout the entire liquid. The higher the external pressure on the liquid, the higher the boiling point (Normal boiling point is, of course, at 1 atm). Pressure change can thus manipulate the boiling point of water, like how at high altitudes the boiling points are lower than at sea level.



Sources: Image 1: http://www.unit5.org/christjs/Vapor_Pressure_Boiling_Point.htm Image 2: http://library.thinkquest.org/C0126220/environment/evaporation4_e.htm Image 3: http://www.mikeblaber.org/oldwine/chm1045/notes/Forces/Vapor/Forces05.htm Image 4: http://apollo.lsc.vsc.edu/classes/met130/notes/chapter4/es_boil.html [|Chemistry: The Central Science (9th Edition)]