# What pressure not increases if I seal the container?

CASE1:
A container with some water is open to atmosphere, pressure is definitely atmospheric, no somehow I put cover over it and sealed it also. Pressure would increase because new equilibrium will establish between water and its vapour so pressure will increase in the closed container.

CASE2:
There is an empty container that means only air, now I cover it. Pressure will not increase; but my doubt is; as atmospheric pressure is defined as the weight of atmosphere over it divided by the surface area. As I close the container the weight is acting on the cover that load will be taken by the whole container so how the particles inside the container will feel the atmospheric pressure? Cover is not a fluid that means no pascals law; so how pressure reaches inside the container?

jbriggs444
Homework Helper
CASE1:
A container with some water is open to atmosphere, pressure is definitely atmospheric, no somehow I put cover over it and sealed it also. Pressure would increase because new equilibrium will establish between water and its vapour so pressure will increase in the closed container.
Before any hypothetical vapor is emitted from the surface of the sealed-in water, that water is under atmospheric pressure. It was at atmospheric pressure before it was sealed in so it will be at atmospheric pressure immediately after.

If any vapor were to come out, it would be under at least one atmosphere of pressure. But under one atmosphere of pressure, that vapor would be condensing. So no vapor can come out. So pressure remains at one atmosphere.
CASE2:
There is an empty container that means only air, now I cover it. Pressure will not increase; but my doubt is; as atmospheric pressure is defined as the weight of atmosphere over it divided by the surface area. As I close the container the weight is acting on the cover that load will be taken by the whole container so how the particles inside the container will feel the atmospheric pressure? Cover is not a fluid that means no pascals law; so how pressure reaches inside the container?
Pressure is not defined as the weight of atmosphere divided by surface area. It is defined as force per unit area regardless of what is exerting that force.

In first case; evaporation rate will become slower and more number of vapor will be above water surface. As diffusion of vapor is now stopped.

In second case; I don't understand why pressure is same. What is the new force that come into play for creating same pressure.

jbriggs444
Homework Helper
In first case; evaporation rate will become slower and more number of vapor will be above water surface. As diffusion of vapor is now stopped.
Did you seal it with zero head space or with some air-filled head space?

Did you seal it with zero head space or with some air-filled head space?

jbriggs444
Homework Helper
Oh, that makes sense then. Yes. If the head space is dry air then vapor can evaporate into the head space and increase the pressure within the container.

Ravi Singh choudhary
Help me in the second case

jbriggs444
Homework Helper
Help me in the second case
Boyle's law. If you start with enough gas in the container to maintain atmospheric pressure at its current temperature then closing the lid will not change that.

Boyle's law. If you start with enough gas in the container to maintain atmospheric pressure at its current temperature then closing the lid will not change that.
Didn't get you; sorry

jbriggs444
Homework Helper
Judging by the rapidity of your response, you did not Google Boyle's law. Accordingly, I will stop helping you.

Actually Sir, I know Boyle's law as follows; Pressure is inversely proportional to the volume, if temperature remains contant. But I was not able to relate this concept here

jbriggs444
Homework Helper
The volume of the chamber does not change just because you close the lid. Accordingly, the pressure does not change just because you close the lid.

Ravi Singh choudhary
I always used to relate atmospheric pressure due to weight of the air. What I understand atmosphere is hydrostatic that means each part of it is at 1atm; neglect pressure change due to height as density is low.

But something hollow solid container is placed in that sea of atmosphere and as solid will not follow pascal's law it, air inside the container will not experience the same pressure as outside. But here thermodynamics come into play. Due to isothermal condition. I think I am almost convinced but something is missing in my conceptualization.

jbriggs444