Fizzy drink

Science on the Underground: Fizzy drink poster

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.

Brenda Keogh & Stuart Naylor © 1998

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