# vapor pressure

#### inaccensa

10+ Year Member
7+ Year Member

Vapor pressure is the pressure when there is an equilibrium between the molecules entering and leaving liquid phase. Does this mean that when the equilibrium is not established there is no VP.

Ek says that for solids, the mp is established when there is an equilibrium between the VP of solid and liquid. This is confusing, since I thought that the when the VP equals atm pressure, it leads to boiling for liquids. Can anyone please explain this

OP

#### inaccensa

10+ Year Member
7+ Year Member

Vapor pressure is the pressure when there is an equilibrium between the molecules entering and leaving liquid phase. Does this mean that when the equilibrium is not established there is no VP.

Ek says that for solids, the mp is established when there is an equilibrium between the VP of solid and liquid. This is confusing, since I thought that the when the VP equals atm pressure, it leads to boiling for liquids. Can anyone please explain this
someone?

#### ezsanche

10+ Year Member
5+ Year Member

Vapor pressure is the pressure when there is an equilibrium between the molecules entering and leaving liquid phase. Does this mean that when the equilibrium is not established there is no VP.

Ek says that for solids, the mp is established when there is an equilibrium between the VP of solid and liquid. This is confusing, since I thought that the when the VP equals atm pressure, it leads to boiling for liquids. Can anyone please explain this

Vapor pressure is the pressure that vapors (gas molecules) exerts when molecules leave a liquid phase and enter a gas phase. Imagine that you have a cup of water filled half way at room temperature and that you just put a seal or a cap on the top of the cup so that it prevents anything from leaving or coming in. Now ask yourself what is in the cup? Obviously liquid water but at the surface of the water there are invisible water molecules in gas phase (vapors) that come out of the liquid water. These gas molecules move around in all directions colliding with anything that they come into contact with (the cap, the walls of the container, other gas molecules, and even back into the water). These molecular collisions exert a force over a given area which we call vapor pressure. If there are a lot of molecules leaving the liquid phase and entering the gas phase then we say that that liquid has a high vapor pressure because more gas molecules means more collisions with their surroundings.

The reaction associated with vapor pressure is the one written below.
Where A is any molecule or compound.

A(l)<--> A(g)

Now as with any chemical reaction, the reaction proceeds until it establishes an equilibrium. So if the vapor pressure is equal to zero that means there are no molecules leaving the liquid phase and entering the gas phase. It only takes a single gas molecule to exert a force and hence a pressure.

So when a reaction is not in equilbrium there is vapor pressure but it will either be less or greater than the vapor pressure when the reaction is at equilibrium. It all depends which phase is being favored. If there are more molecules in the liquid phase then there will be less vapor pressure. If there are more molecules in the gas phase then there will be more vapor pressure.