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Hand (touching) and Foam Decrease
Name: Teri
Status: student
Grade: 9-12
Location: UT
Country: USA
Question:
When you put your hand over a soda, the foam goes down
faster. Why?
Replies:
I have not observed this personally, so I do not know if the observation is
universally correct. But a possible mechanism is: The foam bubbles break and
send a shower of the liquid into the air. These come into contact with the
hand, which has various oils on the skin. Some droplets containing the skin
oil fall back into the soda. Skin oils are very effective at reducing the
surface tension of the foam, which acts as a defoamer, and the foam "breaks".
It does not take much skin oil for this to occur since it is a surface
phenomenon.
Vince Calder
Hi Teri,
The "fizz" in soda pop is simply carbon dioxide (CO2) that is
dissolved under pressure in the pop. Much more CO2 can dissolve in the
water when the pressure in increased. A bottle of unopened soda pop is
under high pressure, and that keeps the CO2 they put in there from
bubbling out. When the bottle is opened, the pressure is released and
the water in the pop can no longer hold all the CO2 that was dissolved
in it when the pressure was higher, so it starts to bubble out of
solution and foam.
If you block the opening (with your hand, for example), the CO2 that
is bubbling out of solution has nowhere to go, and starts to cause the
pressure to build up again (you can feel this pressure increase
against your hand). The increased pressure now allows the water in the
pop to once again be capable of keeping the remaining CO2 in solution,
so the bubbling slows down and stops... until you release the pressure
by removing your hand.
Regards,
Bob Wilson
Hi Teri,
Henry's Law states that for a given temperature, the amount of gas dissolved
in a given type and volume of liquid is directly proportional to the partial
pressure of that gas over the liquid.
In other words, the higher the pressure of the carbon dioxide over the soda,
the more the CO2 will dissolve. By placing your hand on the soda bottle mouth,
you have effectively sealed off any escaping CO2 and this will increase the CO2
partial pressure above the soda. This will allow more CO2 to be dissolved in
the soda. Since bubbling or fizzing is a function of the CO2 escaping, you have
essentially reduced this tendency and so the fizzing abates.
Greg (Roberto Gregorius)
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Update: June 2012
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