Ask A Scientist© Chemistry Archive

name         Ben F.
status       other
age          30s

Question -   I was once told that if you have a half full 2L bottle of
coke, and you squeeze the sides together to create less air in the
bottle, that the soda will not go flat as fast.  I could understand how
this could work. I think it would have something to do with partial
pressure.  My instinct tells me that there may be more to it than
that.  So…  What is the best way to keep my soda from going flat?
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Ben,

Quite the contrary. Squeezing the bottle as you described and then capping it
will produce a less than 1 atmosphere pressure condition inside the bottle
because the flexible container will try to return to its original shape. The
reduced pressure inside the squeezed bottle will encourage the carbonation to
be released from solution. This will make the soda go flat sooner.

To keep the soda from losing its fizz, it should be stored capped, cold and at
an elevated pressure. I purchased a cap that enables one to re-pressurize the
bottle. One screws it in place and then operates a little pump that is part of
and affixed to the center of the cap. I purchased mine at a Wal-Mart.

Regards,
ProfHoff 334
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Keeping a higher pressure on the liquid in the 2L bottle (by squeezing the
bottle) will help to keep the carbonation in the beverage to some degree.
Another way to keep your soda from going flat is by keeping the soda as cool
as possible between uses.  The solubility of carbon dioxide in water is
higher at elevated pressures as well as lower temperatures.  A third way is
to minimize the number of times that you open and close the 2L bottle.
Every time you unscrew the cap and pour some soda into a glass you are
lowering the pressure that the dissolved CO2 in the soda has worked so hard
to maintain.  When you close the bottle and return it to the fridge the
remaining dissolved CO2 will escape again to form another equilibrium
condition governed by Henry's Law (that's another topic).  With each
successive depressurization of the bottle you lose an exponentially decaying
amount of CO2 gas until you are left w/ colored sugar water or even worse
yet colored aspartame water (diet drinks).

-Darin Wagner
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Keeping the coke cold (but not frozen) would be a big factor since the
solubility of CO2 is greater at lower temperature. Reducing the "head space"
by squeezing the bottle would also help since the smaller gas volume reduces
the amount of CO2 in the gas phase. Obviously, keeping the bottle closed is
also important.

Vince Calder
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