# How does vacuum form in a barometer?

by christian0710
Tags: barometer, form, vacuum
 P: 217 Hi, here is something I can't understand: When you fill a 100% tube with Mercury, close it in one end (the top) and stick it into a bowl of mercury, vacuum forms in the top of the tube. Where does the vacuum come from if it was not there to begin with? The tube was full of mercury to begin with, so I have a hard time understanding how vacuum - empty air - can form if there was no empty air to begin with.
 Mentor P: 12,005 A vacuum is not empty air. A vacuum is a volume of space devoid of any matter, including air. In this case the weight of the mercury is enough to cause a small vacuum to form at the closed end of the tube.
P: 217
 Quote by Drakkith A vacuum is not empty air. A vacuum is a volume of space devoid of any matter, including air. In this case the weight of the mercury is enough to cause a small vacuum to form at the closed end of the tube.

So the weight of the mercury in the tube pushes the mercury out into the jar, and Volume devoid of matter just appears? Can you do this with a tube of water as well? and if vacuum forms in the top of the tube would that make water boil because vacuum exerts no pressure on water (thus lowering the boiling point?)

Mentor
P: 12,005
How does vacuum form in a barometer?

 Quote by christian0710 So the weight of the mercury in the tube pushes the mercury out into the jar, and Volume devoid of matter just appears? Can you do this with a tube of water as well? and if vacuum forms in the top of the tube would that make water boil because vacuum exerts no pressure on water (thus lowering the boiling point?)
I believe that is mostly correct. I don't think there's enough vacuum to make the water will boil, but it should fill the vacuum with water vapor. Note that water has other gasses dissolved in it, so you may not get a true vacuum at the top.
Mentor
P: 5,382
 Quote by christian0710 So the weight of the mercury in the tube pushes the mercury out into the jar, and Volume devoid of matter just appears? Can you do this with a tube of water as well? and if vacuum forms in the top of the tube would that make water boil because vacuum exerts no pressure on water (thus lowering the boiling point?)
The space above is not pure vacuum. The space above is filled with mercury vapor at the equilibrium vapor pressure corresponding to the temperature of the liquid mercury. If you did the same thing with water, the space above would be filled with water vapor at the equilibrium vapor pressure corresponding to the temperature of the liquid water. The vapors in these spaces result from the evaporation of a tiny amount of the liquid. Since the vapors are in equilibrium with the liquids, the liquids can't boil.

Chet
Emeritus