Date: Sat May 8 10:33:53 1999
Posted By: Richard Kingsley, Grad student, Bachelor of Education (Science), OISE - University of Toronto
Area of science: Other
There is plenty of physics involved in opening a bottle of beer.
Supersaturation occurs when the partial pressure of a gas above a liquid is
lower than the partial pressure of the gas dissolved in the liquid.
1) Air pressure inside and outside equalises
The air in the bottle, before you open it, is pressurised. At this point,
the partial pressures of the carbon dioxide in the air and in the liquid
are the same; however, the partial pressure of the gaseous carbon dioxide
is higher than outside the bottle. When you open the bottle, the air
inside expands and the pressure decreases (That is happening when you hear
the "whsshhh" and you will see vapour at the bottle entrance which is due
to the cooling of the air as it expands). This happens because the pressure
inside and outside of the bottle move towards equilibrium. Now the partial
pressure of the carbon dioxide is greater in the beer than in the air above
it. Therefore, the carbon dioxide in the beer becomes supersaturated.
2) Partial pressure of the gaseous and aqueous carbon dioxide
The partial pressures of the gaseous and aqueous carbon dioxide will
tend to move towards a state of equilibrium. In order to obtain this,
carbon dioxide will be forced out of solution, which then lowers the
partial pressure of the aqueous carbon dioxide. The carbon dioxide will
come out of solution at any nucleation site. This could be a
microscopic ridge or fissure on the inside of the glass which is where you
see bubbles after you have poured the beer.
When you pour the beer into the glass sideways you are doing two things.
You are increasing the surface area of the liquid which allows the carbon
dioxide to escape faster and form less foam. As beer moves down the side
of a glass it will spread out and move at a slower rate which causes less
disturbance to the stationary beer. The act of pouring straight into the
glass will force air bubbles into the beer (you can see this effect when
you pour water from one glass to another) which also act as nucleation
sites and allow more carbon dioxide to come out of solution.
I hope this answers your question.
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