Height of mercury column?

Question

I have a rigid glass tube that is 1000 mm high.
It is dipped into a beaker of mercury.
On top of the column is attached a large syringe.
The atmospheric pressure is 760 mmHg.
I now apply a strong aspiration pressure by moving up the plunger of the syringe. The mercury column will now rise. My question (s) are: How high will the column rise ? If it rises only 760 mm Hg as i think it would, then what about the balance 240 mm ? What would occupy that volume ?
What if the tube was 3000 mm high ? What would occupy the 2240 mm of empty space ? (if the column doesn't go up all the way ).

In response to someones answer, i am not trying to get answers for my homework ! This is for some medical research and i need to get some concepts right and google hasn't helped me. Assuming i have an syringe with immense power and i have a tube that is many meters high , can the length of the vaccuum column be so much, say 3 meters ?

Answer 1

it would be a vacuum; there would be nothing there, just like in space because the pressure of the air would not push the mecury up the tube

Answer 2

Hi,

If I am correct in thinking that you have a beaker of mercury and a long tube with one end immersed and another end attatched to a syringe then provided that you do not let go, the syringe will eventually suck the mercury all the way up.

If you are askig what happens if you put an empty tube with one end closed into a beaker of mercury then it will rise to 760 mm due to atmospheric pressure. This can be verified using the approximate formula pressure=densityxgravityxheight and saying the pressure is atmospheric, density is that of mercury and use 9.81 for the gravity however you will need to use the pressure in Pa and density in Kgm^-3 for this to give you a height in meters using this value of g.

This problem is known as the Florentine well problem and was first solved many years ago by torricelli who was one of galileo's students if you are interested.

Answer 3

It seems no-one can see the wood for the trees!

The answer to your question is, "whatever was in the tube at the start of the experiment". (You didn't say whether the tube was evacuated at the start or not.) If there was air in it, then that air will push down on the mercury a little bit, so it won't quite get to 760mm.

Nothing enters the tube (apart from the mercury that's pushed into it by the outside atmosphere), and nothing exits the tube.

BTW, I don't care if it's a homework question or not - there's nothing wrong with asking people questions!

Answer 4

There is no definite answer to your question, as the height will depend upon the pressure difference, which is determined by how hard your syringe sucks.
The mercury column will only measure in what is called 'gauge' or pressure difference.
When I went to school we measured pressure in inches of mercury and later in college for small pressures - inches of water!

If you were to acheive a perfect vacumm, it wouldn't matter how long your tube was, the column of mercury would never rise above the atmospheric pressure (29.7 inches or so).

The force of air acting on the pool of mercury equals the force of air (at the top of the column) plus the downward force of mercury in the column.

Not sure about all this modern metric stuff, 39 thou (1/1000 inch)to a millimetre I think.

Answer 5

Whatever be the length of the tube mercury will rise only to a height of 760mm. It is the pressure caused by atmospheric air.

Above the mercury meniscus ( above 760 mm) there is vacuum.

Vacuum in the sense that it contains mercury vapour.

Whatever be the length over and above 760mm the space will contain mercury vapour only.

Answer 6

You can only `suck` mercury up a tube to about 31 inches, because having drawn all the air out of the tube above the mercury the air pressure outside is only able to push the mercury up that far, in other words the weight of air is equal to the weight of 31 inches of mercury, or a vacuum is equal to 31 inches of mercury. Now you know the mechanics of how it works you are on your way.

Answer 7

First, to obtain a height of 760mm of Hg, you'd need a perfect vacuum in the space above it. (The Atmospheric Pressure (760mm Hg) will be forcing the mercury to rise inside the tube).

The space above the Hg would contain a perfect vacuum. (Absolute Zero Pressure)
A syringe cannot create a perfect vacuum.

The space above the Hg would have a pressure of 760mm minus the reading on the scale. The total length of your tube would be irrelevant. e.g: if the scale reads 700mm, the pressure above it will be read as 60mm Hg Absolute.

Answer 8

at any temperature above 0 kelvin (ie any real temperature) the vacuum formed above the column would be occupied by dilute mercury gas. This would happen because mercury has a vapour pressure due to uneven velocity distributions in the liquid (ie some molecules will have enough kinetic energy to become a gas and this gas formation will equilibriate to different values depending on the temperature and the volume above the liquid).

Answer 9

Why don't you use Google to find out your homework for yourself? Read this link then you should understand the principles of the mercury barometer and you'll be able to answer the question for yourself.