Dear Hakeem Niyas,

At room temperature itself all materials give out their molecules from the surface. Let us take the case of water. At room temperature itself some water molecules go to vapor phase.This vapor exists above the surface of the water and applies a pressure on everything around it including on the surface of water. This pressure is called the vapor pressure. This additional pressure on the surface will reduce further evaporation of water. But if we blow away this vapor from the surface, again water evaporation increases. If the water container is kept inside a closed vessel the vapor pressure will increase to a maximum value which we call saturated vapor pressure appropriate for that temperature (Vapor pressure is a function of temperature). We know that at atmospheric pressure it takes 100 deg. C to vapourise water. Then how it evaporates at room temperature ?. This can be explained by M.B statistics for the energy distribution of gas molecules. Even though most of the gas molecules have energy corresponding to room temperature ( 26 meV for 300K) some will always be their which act as if there temperature(and hence energy -basically K.E) is very high.(Similarly a few atoms will be having very low energy also). The few high energy atoms will impart their K.E to the water molecules on the surface and cause their escape as vapor. Evaporation is a surface phenomena. The second part of your question; I think water may have a higher vapor pressure as it will be easy for the water to evaporate faster as its molecular weight is small compared to that of oil. There are several oils with vapor pressure as low as 10^-8 to 10^-10 mbar ( DC704 & DC 705 diffusion pump oils for example) where as water vapor pressure at RT is approximately 35-40 mbr. Hope this is helpful information to you.

Reagrds

Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed container. All liquids and solids have a tendency to evaporate into a gaseous form, and all gases have a tendency to condense back to their liquid or solid form.

When a liquid is in a confined, closed, container, an equilibrium exists between the liquid and its gaseous phase. This equilibrium exists regardless of the temperature inside the container and the temperature of the liquid. The equilibrium exists due to the fact that some of the particles in the liquid, essentially at any temperature, will always have enough energy to escape the intrinsic cohesive forces and enter the gaseous phase.When a liquid is in a confined, closed, container, an equilibrium exists between the liquid and its gaseous phase. This equilibrium exists regardless of the temperature inside the container and the temperature of the liquid. The equilibrium exists due to the fact that some of the particles in the liquid, essentially at any temperature, will always have enough energy to escape the intrinsic cohesive forces and enter the gaseous phase.

The change in vapor pressure of a pure substance as temperature changes can be described using the equation known as the Clausius-Clapeyron Equation:

lnP2P1=ΔHvapR(1T1−1T2)

Where:

P1 is the partial pressure of the liquid at T1

P2 is the partial pressure of the liquid at T2

Thanks a lot - Rajeev and Sayanthan - Your explanations have cleared my doubts