Vacuum in a mercury barometer?

Question

Hi. How hard is the vacuum over the mercury in a barometer. Unless the mercury goes into some level of vapor phase I would think it was nearly perfect.

Answer 1

But the liquid mercury does vaporize at room temperature. The vacuum formed is responsible for "vacuum distillation" from the upper surface of the liquid mercury.

Even with this, the vacuum above the mercury in a barometer is one of the best that can be obtained with a minimum of technology.

Answer 2

The vapor pressure of Mercury (the pressure the Mercury molecules which have transitioned into the vapor phase exert on the liquid Hg) is very low compared to most other substances.

At 42 degrees C, Mercury has a vapor pressure of about 1 Pascal.
At 120 degrees C, Hg has a vapor pressure of about 100 Pa.

By comparison, water has a vapor pressure of 7.3814 kPa at 40 degrees C and its vapor pressure equals atmospheric pressure at its boiling point (100 degrees C, vp = 101.325 kPa).

1000 Pa = 1 kPa
101.3 KPa = 1 atm
1 atm = 760 mmHg

So even taking the vapor pressure of Mercury into account, the vacuum over a barometer is still quite good.

Answer 3

A barometer is suppossed to degree the rigidity of the air. Measuring something is finished with the aid of evaluating something with a time-honored volume. contained on the topic of a barometer, in maximum situations, the air rigidity is in comparison with the rigidity of a vacuum (which does no longer have any rigidity as there is not any air in it.) yet you ought to do it in yet in any different case to boot, making use of no longer a vacuum. yet an fairly stable reason to apply a vacuum for it extremely is that the airpressure of a vacuum would not rely on temperature. So a vacuum prevents the want of an fairly careful temperature controlled and calibrated pressurized 'pattern of air' to evaluate with.

Answer 4

Perfect to the engineering standards