Pressure in a boiling tube

[Primarygun]

IF I invert a test tube, with it fully filled in with water, and put it above a beaker of water, just like the process of electrolysis.

What is the pressure at the bottom and at the mouth of the test tube?

I guess the pressure at the mouth is 1 atm, but where supports this relatively large value apart from the little weight of the water held?

[Sisyphus]

I'm not sure I understand the setup, but I think you've got an inverted testtube, completely filled with water with its mouth in a beaker of water, below the surface. Correct? And you're wondering what the pressure is at the bottom of the beaker and at the mouth of the test tube? Well, the bottom of the beaker is 1 atm + the pressure of the depth from the surface of the water in the beaker, and the same for the mouth of the test tube. The weight of the water in the tube doesn't matter because it is less than the weight of air above the surface.

[Primarygun]

Yes, it is the case.

Why is it 1 atm +..?

The weight of water trapped in the tube is not large enough to generate 1 atm , isn't it?

[5614]

The pressure due to the weight of a fluid is given by:

 

[math]P = \rho g h[/math]

 

P = pressure

[math]\rho[/math] = density of the fluid

g = acceleration due to gravity

h = height of fluid above the point you are measuring the pressure of

 

Does that help?

[Neil9327]

IF I invert a test tube' date=' with it fully filled in with water, and put it above a beaker of water, just like the process of electrolysis.

What is the pressure at the bottom and at the mouth of the test tube?

I guess the pressure at the mouth is 1 atm, but where supports this relatively large value apart from the little weight of the water held?[/quote']

 

The closed end of the test tube is not a vacuum - the water is pushing up against the glass, and vice versa, at slightly less than 1 atm

[Rocket Man]

at the beakers water level, the pressusre in the test tube will be 1atm, it will be less at the top but not much less, a 10cm head of water doesnt have much pressure associated.

 

to get a total vacuum at the top of the tube, you'll need roughly 10m of water. (in which case you'll probably drop the boiling point so the top will be filled with water vapour)

the pressure at the water level of the beaker will still be 1atm though, no matter how deep or high the tube extends, the mass of water is held up by the pressure against the surface of the beaker water. if it was anything else it wouldnt be in equilibrium.

 

the force holding it up comes from the mass of air above the beaker, 10m of water countering roughly 100km of air gives some indication of relative densities (after you count the pressure-height gradient of the atmosphere)

[Primarygun]

to get a total vacuum at the top of the tube, you'll need roughly 10m of water. (in which case you'll probably drop the boiling point so the top will be filled with water vapour)

If there's a vacuum at the destinated space as you described, what is the pressure of water in contact?

[Rocket Man]

AFAIK, liquid water cannot exist in a vacuum.

the pressure of water in contact will be determined by the temperature, lower temperature means less pressure untill the water will boil. the water in contact will experience a pressure equal to the atmospheric pressure minus the head of water, this will be just lower than a total vacuum, just enough for the water to be liquid at the current temperature.

 

water at low pressure is quite an interesting topic, liquid water is unstable at low pressures, but solid and gas occupy more volume so the water will collapse into a lower energy state as determined by the pressure. (it will basically sublimate at low pressures)

 

really interesting is that you can pull a wire through ice and have the water freeze above it where it is at a lower pressure, you can melt ice without heating it, i've seen it done.