Which character do you agree with?
Normally we would expect the lemonade to lose weight when the top is left off.
What science is involved?
The fizzy drink contains dissolved carbon dioxide gas. The amount of
carbon dioxide dissolved is very large, far more than would normally
dissolve in the same sized bottle of water. This is because the gas is
forced in under high pressure when the drink is manufactured, so that large
quantities of gas dissolve.
When the top is taken off the bottle the pressure is released. The gas can
only stay dissolved while it is under high pressure, so when the top is
taken off the bottle the gas immediately starts to form bubbles in the
lemonade. This is the "fizz" that you see and hear when you take the top
off the bottle. The bubbles of gas are lighter than the liquid so they
rise to the surface where they burst, and the gas comes out of the top of
the bottle as more and more bubbles rise to the surface and burst.
All gases weigh something, and carbon dioxide is quite a heavy gas -
noticeably heavier than air. As the gas leaves the bottle the drink which
is left in the bottle will weigh less.
What complicating factors might be involved?
The amount of weight lost by the fizzy drink depends on how quickly the
carbon dioxide gas leaves the bottle. If you shake the bottle then the gas
forms bubbles and leaves the bottle much more quickly, so the fizzy drink
will lose weight more quickly. The temperature also makes a difference.
More carbon dioxide can dissolve in the fizzy drink when it is cold. So if
the fizzy drink is cold then it will not lose weight as quickly because the
gas will stay dissolved for longer.
Although some air might dissolve in the fizzy drink to replace the carbon
dioxide, this will not make much difference. The carbon dioxide has been
dissolved under high pressure, so the amount of air that may come in will
be very much less than the carbon dioxide which leaves. The weight will
also be affected by evaporation. When the top is left off the bottle then
water can evaporate from the fizzy drink, and this will make it weigh less.
What can you do on your own?
If you have sensitive scales then you can weigh a bottle of lemonade or
some other fizzy drink and check to see how much the weight changes when
you leave the top off the bottle. If you don't have very sensitive scales
then you can use two identical bottles or cans on a lever balance, loosen
the top on one of them and see if the balance changes. You can investigate
how long it takes for the weight to stop changing. You can also put some
cotton wool in the top of the bottle to stop evaporation and see whether
that makes any difference to the amount of weight lost.