1. Oct 2, 2014

### ehabmozart

I have recently been reading about vapor pressure and my mind's already blown. I just want to listen from you guys what is vapor pressure in a simple manner. I want to know where is it's direction. Towards the liquid. Let's consider an opened bottle of water which doesn't seem to evaporate at room temperature. Does it have VAPOR PRESSURE? I'll just give away what I know and probably you can fix on that. It is the pressure exerted by the excited liquid molecules which tend to bounce off the liquid. Moreover, what is partial pressure. Thanks in advance to whoever contributes.

2. Oct 3, 2014

### Staff: Mentor

If you have a container that is partially filled with liquid water, and you evacuate the space above the liquid water (then, seal the container), part of the liquid water will evaporate to form water vapor (gas) above the liquid. When the system comes to equilibrium, the pressure of the vapor above the liquid will be equal to the pressure of the liquid. That pressure will be determined by the temperature of the liquid, and is a property of the material itself (in this case water); it is called the vapor pressure, and is a unique function of the temperature. If you change the temperature, the pressure of the vapor and of the liquid will change to a new equilibrium value.

The pressure in both the liquid and the vapor is isotropic, meaning that it is the same in all directions.

If you have an open bottle of water at room temperature, it will evaporate unless the relative humidity of the air in the room is 100%. The relative humidity is defined as the partial pressure of the water vapor in the air divided by the equilibrium vapor pressure (times 100) of the water at the room temperature. But what is partial pressure? Partial pressure is a property of an ideal gas. In an ideal gas mixture (such as water vapor in air), each species behaves as if it is the only species present, in terms of its p-v-t behavior. From this, it follows that the partial pressure of water vapor is equal to the total pressure of the gas in the room times the mole fraction of water vapor, and the partial pressure of the air (N2 and O2) is equal to the total pressure of the gas times the mole fraction of air. If the total pressure in the room is 760 mm Hg, and the relative humidity is 100 %, the partial pressure of water vapor will be equal to its equilibrium vapor pressure, and the partial pressure of the air (N2 and O2) will be equal to 760 minus the equilibrium vapor pressure of water.

Chet