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Q & A: Soda Explosions!

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Most recent answer: 10/22/2007
Q:
Does tapping on the top of an unopened soda can or bottle before you drink it reduce the risk of having a "soda explosion"? If it does why does it do it?
- Henry (age 16)
A:
Henry -

You probably know that soda is fizzy because it has lots of dissolved carbon dioxide in it. Normally, though, carbon dioxide is a gas. And if you were to take all of the carbon dioxide out of a can of soda and just let it be a gas, you’d find that it would take up a lot more space than just the can, if it were at ordinary atmospheric pressure. (So when it's dissolved, it takes up less space than when it's a gas.)

If you shake a can of soda, you end up with tiny bubbles of carbon dioxide gas that stick to the inside surface of the can. Remember, though, that these bubbles are under  high pressure, so they’ll expand if the can is opened and the pressure falls to ordinary atmospheric levels. If you open the can, the bubbles expand a lot and they push the soda right out with them, causing a "soda explosion."

But if you tap on the bottom of the can, the bubbles get knocked loose of the side of the can and float to the top. So all of the little bubbles form one big bubble at the top. That way the gas still expands when you open the can, but since there’s no soda in the way, the soda doesn’t get pushed out too. So the answer to your question is yes: tapping on the bottom of the can or bottle should help to prevent a "soda explosion."

-Tamara and Mike

(published on 10/22/2007)

Follow-Up #1: tapping cans

Q:
So if tapping the can release the bubbles of carbon dixoide to the top and form one giant bubble. Wouldn’t create enough pressure to break the can or bottle open?
- kenneth (age 13)
alhambra, ca, usa
A:
Actually, the pressure hardly changes when the little bubbles shake loose and join the big bubble at the top. Except for a small effect of surface tension, they take up the same volume either way. The cans and bottles are, of course, designed to be strong enough for the pressure.

Mike W.

Lee H

(published on 10/22/2007)

Follow-Up #2: Yet More Soda Explosions!

Q:
In your section called Soda Explosions! the question asks about tapping on the TOP of a soda can. But in your answer you talk about tapping the BOTTOM of the can. Was there a typo somewhere? Are people supposed to tap only the bottom of the can, or can one tap the top or the bottom? Is it the tapping that matters or where the tapping is done?
- Elizabeth Lund
The Christian Science Monitor, Boston, MA, USA
A:
Elizabeth -

The reason for tapping on the can is that there are tiny bubbles of compressed carbon dioxide gas stuck to the sides (and bottom and top) of the can. When you tap on the can, you knock these bubbles loose so they all float to the top. If the bubbles are all at the top of the can, then they can’t push soda out with them when you open the can. How successful you are at knocking the bubbles loose may depend in part on where you tap the can, but it depends more on how you are holding the can, how much you tap it, etc. So when all is said and done, it doesn’t really matter where you tap on the can - just how good you are at knocking bubbles loose.

-Tamara

(published on 10/22/2007)

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