Question

what is seen as vapor pressure in open vessel???

Answer 1

the pressure due to the vapor. vapor is formed by high energy liquid particles leacing the surface.

Answer 2

You can't measure vapor pressure in an open vessel. Any molecules that escape the liquid will wander away and never be seen again. That's why a glass of water will evaporate at any temperature above freezing, provided the relative humidity does not stay at 100%.

Answer 3

then hw boiling occurs in open vessel..,, beacause its explanation it is that water boils when vapor pressure equals atmospheric pressure..,water boils in open vessel ..thn frm where that vapor pressure come that equal normal 1 atm pressure????

Answer 4

environment applies pressure of 1atm on everything and when vapor pressure of heated water becomes equal to that of atm pressure it starts boiling

Answer 5

but it is understood that vapor pressure cannot be obtained in open vessel???

Answer 6

Ah, I see. Vapor pressure is defined to be the equilibrium pressure of vapor above the liquid at a given temperature in a CLOSED container. It's a measure of the escaping tendency of the molecules in the liquid: the more likely they are to escape, the higher the pressure in the vapor must be to counteract it. (The higher the pressure in the gas, the more often molecules in the gas collide with the liquid and are absorbed by it, returning to the liquid phase. The equilibrium vapor pressure is a balance between the molecules in the liquid escaping to the gas, and the gas molecules returning to the liquid by colliding with the surface.)

When the vapor pressure at a given temperature equals the external pressure applied to a CLOSED box, then of course the box will expand, reducing the density of gas over the liquid. More liquid will have to evaporate to raise the pressure back to what it should be, and the process continues until all the liquid has evaporated. That is, at equilibrium the material is ALL in the gas phase. By definition, this is the boiling point.

It's confusing applying this to an open container, however, because the pressure of the vapor above the liquid in the OPEN container has nothing to do with the vapor pressure in a closed container. Generally, it is constant, and determined by other factors. For example, the amount of water vapor above an open container of water is determined by the relative humidity that day in the atmosphere. If the humidity is low, there will always be very little vapor above an open container of water, whether it is cool or boiling, because the evaporated water just floats away and gets lost in the infinite atmosphere.

What you might want to keep in mind is that the comparison between vapor pressure in a closed container and the external pressure just defines the boiling point, and this doesn't change whether it's an open or closed container.