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Name: Teri
Status: student
Grade: 9-12
Location: UT
Country: USA

When you put your hand over a soda, the foam goes down faster. Why?

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.

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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